How to Measure a Window: Step-by-Step Guide for Blinds, Curtains & Replacements

Key Takeaways

• Use consistent tools and steps: tape measure, level, square, notepad, and photos to document each window and avoid mix-ups.
• Measure width (top/middle/bottom) and height (left/center/right) to 1/16–1/8 in, then record the narrowest width and shortest height for inside mounts.
• Verify depth and clearance for hardware; compare your recess depth to the product’s minimum to ensure brackets and headrails fit.
• For inside mounts, enter exact smallest sizes and let the manufacturer apply standard deductions; don’t self-deduct unless specified.
• For outside mounts, mark a top line and add overlap (more for blackout shades) on sides, top, and bottom to improve coverage and light control.

Measuring a window sounds simple yet a tiny mistake can throw off your whole project. I wrote this guide to make it easy and stress free. Whether you need new blinds fresh curtains or a snug replacement window I will help you get it right the first time.

I will show you the tools I use and the steps I follow for accurate width height and depth. I will also point out the common spots where people slip up so you can skip the guesswork. By the end you will feel confident taking measurements for inside mount or outside mount and you will be ready to order with zero doubt.

How To Measure A Window: The Essentials

Measure a window with consistent tools and repeatable steps. I keep the same order for every opening.

• Gather tools: tape measure 1 in wide, steel ruler, level, pencil, notepad, smartphone camera
• Check frame: square jambs, intact sill, level head, flat casing
• Clear obstructions: locks, cranks, handles, alarms, tiles

Measure for an inside mount first if the treatment sits within the frame.

• Measure width: top, middle, bottom of the window opening
• Measure height: left, center, right of the window opening
• Measure depth: head to any front obstruction like trim or tiles
• Record to 1/8 in increments, then keep the narrowest width and the shortest height for ordering inside mount products (Blinds.com https://www.blinds.com/how-to-measure)
• Avoid self deductions, then let the manufacturer take standard inside-mount deductions of about 1/4–1/2 in depending on product tolerances (Home Depot https://www.homedepot.com/c/ab/how-to-measure-for-blinds/9ba683603be9fa5395fab901f4b34cd)

Measure for an outside mount instead if the treatment covers the wall beyond the frame.

• Mark desired top line above the window, then choose left and right limits
• Add overlap per side for coverage, then confirm space for brackets beyond casing
• Use greater overlap for blackout shades and roller shades, then use modest overlap for decorative drapery panels (Hunter Douglas https://www.hunterdouglas.com/tips-and-trends/window-measuring)

Confirm depth and clearance before you order brackets or headrails.

• Verify minimum inside-mount depth on the product page, then compare to your measured depth
• Note handles and cranks on casement windows, then plan spacer blocks if clearance is tight
• Photograph each opening with notes, then label photos by room and window number

Rules for accuracy and ordering data

Task	Count	Unit	Rule	Source
Width points	3	inches	Record narrowest to 1/8 in	Blinds.com https://www.blinds.com/how-to-measure
Height points	3	inches	Record shortest to 1/8 in	Blinds.com https://www.blinds.com/how-to-measure
Depth check	1	inches	Meet or exceed product minimum	Home Depot https://www.homedepot.com/c/ab/how-to-measure-for-blinds/9ba683603be9fa5395fab901f4b34cd
Inside deduction	—	inches	Manufacturer deducts about 0.25–0.5 in	Home Depot https://www.homedepot.com/c/ab/how-to-measure-for-blinds/9ba683603be9fa5395fab901f4b34cd

Outside-mount overlap guidance

Treatment type	Top overlap	Side overlap	Bottom overlap	Note	Source
Blackout shades	3–6 in	2–3 in	2–3 in	More overlap improves light control	Hunter Douglas https://www.hunterdouglas.com/tips-and-trends/window-measuring
Light-filter shades	2–3 in	1–2 in	1–2 in	Enough overlap prevents glow at edges	Hunter Douglas https://www.hunterdouglas.com/tips-and-trends/window-measuring

Examples in context

• Measure window width inside mount: 3 points, keep 35 7/8 in if other points read 36 in and 35 15/16 in
• Measure window height inside mount: 3 points, keep 58 1/2 in if other points read 58 5/8 in and 58 9/16 in
• Measure window depth: head to handle, confirm 2 1/4 in clearance for a roller shade cassette per product specs

Common pitfalls I avoid

• Mixing units: keep everything in inches, then convert only at the end
• Rounding up: round down to 1/8 in for inside mounts, then let factory deduct
• Skipping photos: capture each opening with a sticky note label, then match to your order lines

• Measure window width
• Measure window height
• Measure window depth
• Inside mount measure window
• Outside mount measure window

Know Your Window And Frame

Know the window type and the frame condition before I record final sizes. Match the measuring approach to the construction and material.

Replacement Vs. New Construction

• Confirm the project type aligns with the opening, then select replacement for an existing frame and new construction for a rough opening (ICC IRC R609, https://codes.iccsafe.org).
• Identify replacement windows as insert units that keep the interior and exterior trim, then measure inside the existing jambs for width, height, and depth (FGIA InstallationMasters, https://fgiaonline.org).
• Identify new construction windows as units with a nailing flange and an integral frame, then measure the rough opening between studs and header and sill before any trim (ASTM E2112, https://www.astm.org/e2112).
• Measure replacement openings where the frame is square and sound, then avoid inserts if the frame shows rot or distortion that reduces clearances (FGIA, https://fgiaonline.org).
• Plan new construction where wall sheathing accepts a flange and flashing, then follow code for flashing and weather-resistive barrier integration around the window (ICC IRC R703, https://codes.iccsafe.org).

Frame Material And Condition

• Inspect wood frames for rot, swelling, and flaking paint, then probe soft spots and note any pre‑1978 paint for lead‑safe work rules (EPA RRP, https://www.epa.gov/lead/renovation-repair-and-painting-program).
• Inspect vinyl frames for bowing, UV chalking, and brittle corners, then confirm straight jambs and stable screw holding before inside mounts (FGIA, https://fgiaonline.org).
• Inspect aluminum frames for corrosion, thermal breaks, and loose corners, then account for condensation staining that can indicate air leaks at joints (DOE, https://www.energy.gov/energysaver/energy-efficient-windows).
• Inspect fiberglass frames for hairline cracks and uneven gel coat, then verify consistent dimensions across the jambs for accurate insert fit (FGIA, https://fgiaonline.org).
• Document water intrusion at sills with staining or high moisture readings, then correct flashing or drainage before measuring final sizes for any unit (ASTM E2112, https://www.astm.org/e2112).

Tools And Preparation

I set up before I measure a window. Solid prep keeps each reading precise.

Tools Checklist

• Tape measure, 25 ft, with fractional markings
• Steel ruler, 12 in, for tight jamb checks
• Level, 24 in, for sill and head
• Carpenter’s square, 7 in, for quick squareness
• Notepad, graph paper, or a measuring app
• Pencil, fine tip, plus masking tape for marks
• Straightedge, 36 in, for bow detection
• Flashlight, compact LED, for interior cavities
• Stud finder, deep scan, for rough opening checks
• Utility knife, sharp blade, for caulk scoring
• Small pry bar, flat, for trim peeks
• Vacuum, hose attachment, for track debris

Measurement target	Value	Context
Tolerance	1/16 in	Record exact to the nearest 1/16 in
Width points	3	Top, middle, bottom
Height points	3	Left, center, right
Diagonals	2	Corner to corner for square
Minimum depth	Per product spec	Inside mount clearance

Prep The Opening

• Check squareness at the jambs and sill, place the square tight to each corner
• Check level at the head and sill, place the level on clean contact points
• Check plumb on both side jambs, hold the level against unobstructed faces
• Clear obstructions from tracks and returns, remove clips brackets locks and stops
• Clear old caulk and paint ridges, score with a knife where the tape rides
• Confirm minimum depth for inside mount, compare the opening depth to the product spec
• Confirm mounting surface soundness, press on jambs to spot flex or rot
• Confirm daylight visibility, use a flashlight to scan for gaps and water stains
• Document frame material and condition, note bow warp corrosion or cracks
• Protect finishes, apply low tack tape where I mark and measure
• Mark a datum, strike a light pencil line at the planned head for outside mount
• Stage access, set a stable step stool and keep both hands free for the tape

Step-By-Step: How To Measure A Window

I follow a repeatable window measuring routine for confidence and accuracy. I capture width, height, and depth in multiple spots, then I lock in the smallest numbers for fit [Blinds.com, Measuring Guide], [Andersen Windows, Replacement Guide].

Measure Width (Top, Middle, Bottom)

• Place the tape across the inside jambs at the top.
• Read the inside edge to inside edge to 1/16 in.
• Write the top width on my sheet.
• Place the tape at the middle across the jambs.
• Read the inside edge to inside edge to 1/16 in.
• Write the middle width on my sheet.
• Place the tape at the bottom across the sill and head jamb line.
• Read the inside edge to inside edge to 1/16 in.
• Write the bottom width on my sheet.
• Confirm the sash and locks sit fully closed before each pass.

Source: Blinds.com, Measuring Guide; Andersen Windows, Replacement Guide

Measure Height (Left, Center, Right)

• Place the tape from the sill to the inside top of the opening on the left.
• Read to 1/16 in and note any bowed sill or head.
• Write the left height on my sheet.
• Place the tape from sill to head at the center.
• Read to 1/16 in and note any reveal gaps.
• Write the center height on my sheet.
• Place the tape from sill to head on the right.
• Read to 1/16 in and compare against the left and center.
• Write the right height on my sheet.

Source: Andersen Windows, Replacement Guide; Pella, Measuring for Replacement Windows

Measure Depth And Check For Square

• Place the tape from the interior face to the exterior stop to capture depth.
• Read the clear depth that a blind or insert can occupy, not the full wall thickness.
• Write the smallest unobstructed depth where rails or locks project.
• Place a carpenter square in each corner to check for gaps.
• Read any gap over 1/8 in as out of square.
• Write which corner shows the largest variance.
• Place a level on the sill and up each jamb.
• Read level and plumb to flag twist or bow that affects fit.

Source: FGIA AAMA InstallationMasters, Field Practices; The Home Depot, Window Measuring Guide

Record The Smallest Numbers

I use the tightest numbers to size inside mounts and inserts, then I apply product specific deductions if required by the manufacturer [FGIA, Product Performance]. I keep raw reads and final records separate.

Measurement log example

Metric	Point	Read (in)
Width	Top	36.125
Width	Middle	36.000
Width	Bottom	35.875
Height	Left	58.250
Height	Center	58.125
Height	Right	58.000
Depth	Clear	2.375

• Select the smallest width, then 35.875 in in this example.
• Select the shortest height, then 58.000 in in this example.
• Select the shallowest clear depth, then 2.375 in in this example.
• Save the smallest numbers as the order sizes for inside mounts.
• Add planned overlap for outside mounts after I mark the top line per room design.

• Blinds.com. Measuring Guide for Blinds and Shades. https://www.blinds.com/measuring
• Andersen Windows. How to Measure for Replacement Windows. https://www.andersenwindows.com
• Pella. Measuring for Replacement Windows. https://www.pella.com
• FGIA AAMA. InstallationMasters and Product Performance Guidance. https://fgiaonline.org
• The Home Depot. Window Measuring Guide. https://www.homedepot.com

Special Cases And Shapes

I match my measuring steps to the window geometry. I record clean numbers before any deductions.

Bay And Bow Windows

• Measure each panel at top, middle, bottom for width, then at left, center, right for height. I record the narrowest width and shortest height per panel.
• Measure the projection from the wall to the seat board nose. I note the depth for brackets and head support.
• Measure the head board and seat board overall span and thickness. I confirm level and flat with a 24 in level.
• Measure the inside corner angle at the returns. I note angles and counts with examples like 30°, 45°, 90°.
• Record frame type and mullion widths. I include examples like 1.5 in, 2.0 in, 3.5 in.
• Check operability and lock positions closed before each read. I follow FGIA InstallationMasters guidance for consistent reference points (FGIA AAMA InstallationMasters).

Arched Or Round Windows

• Measure the chord width at the spring line. I place the tape from left spring point to right spring point.
• Measure the rise from the spring line midpoint to the highest point of the arch. I keep the tape plumb.
• Compute the radius for a segment arch with R = (h ÷ 2) + (w² ÷ 8h). I use consistent units per read.
• Measure the leg heights on both sides. I confirm equal legs on symmetrical arches.
• Measure a full circle diameter for round units across multiple axes. I take 0°, 45°, 90° reads and record the smallest.
• Check template needs with the fabricator for tight arches. I follow WDMA measurement terms for accuracy (WDMA IS-6).

Term	Symbol	Example Value
Chord width	w	48 in
Rise	h	12 in
Radius (segment)	R	36 in
Leg height	—	24 in
Round diameter	D	30 in

Tilt-In And Retrofit Frames

• Measure the daylight opening between jamb liners for tilt-in sashes. I keep the sash locked and tilted fully closed.
• Measure depth from the interior stop face to the exterior stop face. I confirm minimum pocket depth per product spec and FGIA AAMA 2400 for inserts (FGIA AAMA 2400).
• Measure without removing balances or tilt latches. I note obstructions like tilt latches and pivot shoes.
• Record the meeting rail height from the sill to rail center. I align coverings with the rail if mounting inside.
• Check squareness by comparing diagonals. I flag any variance over 1⁄4 in across 60 in span per common tolerance ranges cited in manufacturer guides (Pella ProLine, Andersen Install Guide).
• Document frame condition for aluminum or vinyl retrofits. I look for bowed liners, cracked corners, and loose stops before final sizes.

Error-Proofing Your Measurements

I lock in accuracy by treating each measure of the window like a repeatable routine. I cross-check entries before I order.

Common Mistakes To Avoid

• Lock the sash, latch the locks, and seat the panels before you measure.
• Confirm the tape blade sits flat against the jambs, head, and sill for each read.
• Measure in one unit, record in one unit, and label each line, for example in inches.
• Read the tape at eye level, avoid parallax, and mark to 1/16 in.
• Record three widths and three heights, keep the smallest inside mount size.
• Check square with a 24 in steel square, verify diagonals match within 1/8 in.
• Scan for obstructions, note handles, stops, parting beads, sensors, and tiles.
• Verify minimum depth, compare to product specs, for example 1.75 in blind headrail.
• Avoid soft tapes, use a steel tape or laser, then confirm with a steel ruler.
• Photograph the frame, tag material type, note bowing, rot, corrosion, or cracks.
• Note manufacturer deductions, read the product chart, and avoid manual trimming.
• Label each opening, use room names and window IDs, for example Bed 2 North.
• Back up notes, store photos and sketches, and match them to each line item.

When To Add Or Subtract Allowances

I apply allowances based on mount type and product, then I defer to manufacturer deductions if they exist.

Numbers and typical ranges

Context	Mount Type	Action	Typical Allowance	Purpose	Source
Blinds or shades, inside mount	Inside	None by customer	0 in entered, maker deducts 0.125–0.500 in width	Fit inside jambs	WCMA, Hunter Douglas tech sheets
Shutters, inside mount	Inside	Subtract	0.125–0.250 in width, 0–0.125 in height	Hinge and panel clearance	Manufacturer install guides
Roller or cellular shades, outside mount	Outside	Add	2–3 in per side, 2–3 in above, 2 in below	Light gap control and bracket space	Manufacturer templates
Blackout drapery, outside mount	Outside	Add	3–6 in per side, 4–6 in above, 2–4 in below	Blackout coverage	Workroom standards
Aluminum or vinyl blinds, outside mount	Outside	Add	1–2 in per side, 2 in above	Valance and return coverage	Manufacturer specs
Replacement window, insert frame	Inside opening	Subtract	0.250–0.500 in total clearance	Shim space and plumb set	AAMA InstallationMasters, AWDI
New construction window, rough opening	Rough to unit	Add	Rough opening 0.500–0.750 in wider and taller than unit	Shim space per code	AAMA 2400, manufacturers

• Add overlap for outside mounts, increase side coverage for light control, then match bracket footprints.
• Subtract for insert replacements, keep a small clearance for shims, then follow AAMA guidance.
• Enter exact smallest sizes for inside-mount blinds, rely on the maker’s factory deduction, then confirm in the spec sheet.
• Increase allowances for blackout goals, expand side and head overlaps, then verify trim limits.
• Reduce height allowances near sills, leave clearance for lift rails or louver swing, then confirm movement.
• Reserve depth for hardware, check headrail or frame depth, then compare to measured recess.

• Window Covering Manufacturers Association, installation and product spec guides
• AAMA InstallationMasters, American Architectural Manufacturers Association
• AWDI, Installation Guidelines for Replacement Windows
• Hunter Douglas, Levolor, and Graber technical sheets

Converting Measurements Into An Order

I convert my field notes into clear order sizes now. I match mount type, product rules, and supplier deductions, then I format each line item for the order form.

Rounding, Units, And Labeling

• Convert: I round inside mount sizes down, I round outside mount sizes up, I keep increments at 1/8 in for inch orders, or 1 mm for metric orders, if the supplier accepts that tick size for the product line (Blinds.com, Hunter Douglas).
• Record: I enter the narrowest width and the shortest height for inside mounts, I enter the full target coverage for outside mounts, I add any planned overlap in the height if a sill is not present.
• Apply: I leave factory deductions to the supplier for inside mounts, I remove my own deductions only if the supplier states no factory deduction policy for the product line (Hunter Douglas).
• Fix: I lock units to inches or millimeters for the entire order, I avoid mixed units on the same line, I add a unit flag per line item.
• Label: I tag each window with room name, elevation, and orientation, I add mount type, product, color, control side, and lift type, I attach photos with a tape in frame for each opening.
• Map: I number openings clockwise per room, I keep the same ID on the drawing, the photo file name, and the order line, I include hand‑sketches for bays, bows, arches, and rounds.

Verifying With The Supplier

• Confirm: I check the product spec sheet for minimum depth, maximum width, maximum height, weight limits, stack or roll‑up size, and slat or fabric limitations, I adjust if any constraint conflicts with my opening.
• Clarify: I ask if inside mount deductions are factory applied, I ask for stated manufacturing tolerance, I document the answer on the order, if tolerances affect tight fits at 1/8 in margins (Hunter Douglas).
• Align: I verify bracket footprint, headrail length policy, and end cap clearance, I verify valance return sizes for outside mounts, I verify projection for obstructions like handles and cranks.
• Validate: I send my line items with IDs, sizes, mounts, controls, colors, and notes, I request a written acknowledgment that mirrors my labels and units, I compare acknowledgment to my field sheet line by line.

Item	Inside Mount practice	Outside Mount practice	Typical increment
Rounding	Round down	Round up	1/8 in or 1 mm
Deductions	Factory applied	None, add overlap	Supplier specific
Entered size basis	Daylight opening	Desired coverage	Product specific
Tolerance check	Yes at tight fits	Yes at obstacles	Per spec sheet

Conclusion

Measuring a window can feel intimidating at first but it becomes second nature with a steady routine. I trust my notes I trust my process and I double check anything that looks off. That mindset saves stress later.

Use this guide as your go to playbook and keep your own checklist handy. Snap photos label each opening and stay consistent from start to finish. If a detail raises a question I pause and verify with the supplier before I order.

You deserve a clean fit and a smooth install. Take your time and you will get results you can be proud of.

Frequently Asked Questions

What tools do I need to measure windows accurately?

Use a metal tape measure, steel ruler, level, pencil, and notepad. A straight edge and square help check frame squareness. Avoid soft sewing tapes. Have a flashlight to inspect jambs and frame conditions. Keep a calculator for unit conversions and a smartphone for labeled photos.

How do I measure a window for an inside mount?

Close the sash and locks. Measure width at the top, middle, and bottom; record the narrowest. Measure height at the left, center, and right; record the shortest. Measure depth and confirm the product’s minimum depth. Check for squareness. Don’t round up; follow product-specific deductions.

How do I measure a window for an outside mount?

Mark the desired top line and ensure level. Add overlap to width and height for full coverage, light control, and privacy (typically 1–3 inches per side, 2–4 inches above and below, depending on product). Verify mounting surface and obstructions. Record exact finished sizes you want.

How do I measure window depth?

Measure from the front of the obstruction to the nearest point that will accept brackets. Take measurements in several spots and record the smallest. Compare to the manufacturer’s minimum depth requirement to ensure the product will fit and operate correctly.

How do I check if my window is square?

Measure diagonals from top-left to bottom-right and top-right to bottom-left. If the difference exceeds 1/4 inch, the frame is out of square. Alternatively, compare width and height measurements across multiple points. Choose mount type and allowances that compensate for out-of-square frames.

What’s the difference between replacement and new construction window measurements?

For replacement windows, measure inside the existing jambs: width at top/middle/bottom and height at left/center/right, recording the smallest numbers. For new construction, measure the rough opening between studs and the sill, not the finished interior. Confirm plumb, level, and square before ordering.

How do I avoid common measuring mistakes?

Use rigid tools, keep units consistent, don’t round up, and always record the smallest inside-mount numbers. Ensure the tape blade sits flat, locks are engaged, and obstructions are identified. Label each window, take photos, and double-check entries before converting to order sizes.

How should I measure bay and bow windows?

Measure each panel’s width and height individually. Record the projection (depth from wall to front of the unit) and the total span of the head and seat boards. Note angles if available. Verify level, squareness, and any trim that could affect bracket placement or coverage.

How do I measure arched or round windows?

For arches, record chord width (straight line across), rise (center height from chord), and overall height. For round windows, measure the diameter in two directions to confirm roundness. Keep units consistent and follow product-specific templates or deduction rules from the supplier.

How do I measure tilt‑in or retrofit frames?

Measure the daylight opening: visible glass area plus the immediate frame reveal, without removing stops or sashes. Confirm depth at several points and note any tilt latches or balances that could interfere with brackets. Use the smallest measurements and follow product clearance requirements.

What allowances should I add or subtract?

Inside mounts usually require no customer-added deductions; suppliers apply standard deductions for fit and operation. Outside mounts typically need added overlap for coverage: 1–3 inches per side for blinds/shades, 3–6 inches for blackout, and extra above for hardware. Always follow the product’s rules.

How do I convert my field measurements into order sizes?

Match your mount type, product rules, and supplier deductions. For inside mounts, submit the exact smallest width and height unless told otherwise. For outside mounts, provide your finished sizes including overlaps. Use consistent units, label each opening, and round only per supplier instructions.

How do frame materials affect measurement accuracy?

Inspect wood for rot or bowing, vinyl for warping, aluminum for corrosion, and fiberglass for cracks. Structural issues can skew measurements and cause binding. Document damage, check for water intrusion, and address flashing or drainage problems before finalizing sizes or placing orders.

Should I measure in inches or millimeters?

Use one unit system consistently throughout the project. Many suppliers prefer inches to the nearest 1/8 inch or metric to the nearest millimeter. Do not mix units on the same form. Note the unit clearly on every measurement and order line to prevent errors.

When should I choose inside vs. outside mount?

Choose inside mount for a clean, built-in look when depth and squareness meet product requirements. Choose outside mount for maximum light blocking, to hide out-of-square frames, or when depth is limited. Consider obstructions, trim style, and desired coverage before deciding.