Measure window width at three points and use the narrowest for inside mount; measure height at three points and use the largest — always submit raw measurements without deductions, using a steel tape measure.
Updated for 2026 · 12 min read · Written for DIY homeowners and first-time buyers
Ordering the wrong size is the single most common — and most expensive — mistake people make when buying cellular shades online. A shade that's a quarter-inch too wide won't fit inside the window frame. One that's too short will let light leak in around the edges. The good news: measuring correctly takes about ten minutes and requires nothing more than a steel tape measure. This guide walks you through the exact process we recommend — the same one our team uses to verify every custom order for our Honeycomb Cellular Shades.
Not sure which type of shade to order yet? Start with our Honeycomb Cellular Shades Buyer's Guide or compare TDBU vs Standard operating systems before measuring. And if insulation performance matters for your climate, make sure you've reviewed our guide on single vs double vs triple cell honeycomb shades and their R-value insulation performance before placing your order.
Important: Submit your raw measurements exactly as you take them at checkout. Do not make your own deductions. We handle all necessary deductions during production. See our full Measuring & Installation Guide for additional details.
Window Measurement Reference Calculator
Use the calculator below for a quick reference estimate, then follow the full step-by-step guide to confirm your measurements before ordering.
Use this calculator to get a quick reference estimate. Keep reading below for the full step-by-step guide, or visit our Calculation Tools page for more sizing resources.
Step 1: Choose Your Mount Type Before You Measure
Decide between inside mount (clean, built-in look, requires 1.5"+ frame depth) and outside mount (covers the full frame, better for shallow frames or maximum light blockage) before taking any measurements.
Every measurement you take depends on one decision made first: will the shade mount inside the window frame, or outside it, on the wall or trim surrounding the window? You cannot measure correctly until this is decided, because the two methods measure completely different things.
Inside Mount
The shade sits inside the window recess, flush with the wall, giving a clean, built-in look that shows off attractive trim or molding. This is the most popular choice for a streamlined appearance.
Requirement: your window frame needs at least 1.5–2 inches of depth to accommodate the headrail and brackets — deeper for shades with a cordless lift mechanism. If your frame is shallower than that, outside mount is usually the better call. For families with young children, also see our guide on Child Safety & Cordless Window Blinds.
Outside Mount
The shade mounts on the wall or trim above and around the window, fully covering the frame. This is the right choice for shallow frames, windows with obstructions like a crank handle, or when you want maximum light blockage and privacy.
Rule of thumb: if you're not sure which to choose, measure your frame depth first. Anything under 1.5" almost always means outside mount. See our full Installation Guide for bracket placement diagrams.
Step 2: What You'll Need
A rigid steel tape measure is the only tool that gives accurate results — fabric or cloth tapes stretch and can introduce up to half an inch of error on a single window.
- A steel tape measure (avoid cloth or fabric tape — it stretches and introduces error)
- A pencil and paper, or your phone's notes app
- A step stool if the window is tall
- Optional but recommended: a helper to hold the tape steady on wide windows
Do not round your measurements. Record them to the nearest 1/8 inch. Rounding is the number one cause of avoidable sizing mistakes.
Step 3: Measuring for Inside Mount
For inside mount: use the narrowest of three width measurements and the largest of three height measurements — submit both as raw numbers without any deductions.
Measuring Width
- Measure the width of the window opening at the top, middle, and bottom of the frame.
- If your measurement is not an exact 1/8" increment, round down to the nearest 1/8.
- Record the narrowest of the three measurements and submit that number exactly as measured.
Do not subtract anything from your width measurement. We will make all necessary deductions during production.
Measuring Height
- Measure the height on the left side, center, and right side of the frame, from the top of the opening to the sill.
- Record the largest of the three measurements, rounded up to the nearest 1/8". If you get a 1/16" or 1/32" increment, round up to the nearest 1/8.
- Submit that number exactly as measured.
Measuring Depth
Measure how far back the frame recesses from the front edge of the trim to the window glass or blind stop. Minimum 1.5" required for standard cellular shades, up to 2.75" for double-cell or motorized versions. Check for obstructions — window cranks, locks, or handles inside the frame can force you to outside-mount even if the depth would otherwise work.
Step 4: Measuring for Outside Mount
For outside mount: use the largest of three measurements, then add a minimum of 4 inches total (2" per side) to both width and height to prevent light gaps and ensure solid bracket placement.
Outside mount is more forgiving because you're not constrained by the exact frame opening — but the goal shifts from "fits inside a hole" to "fully covers the window with no light gaps." For a visual walkthrough, see our Installation Guide.
Measuring Width
- Measure the width of the window opening at three locations (left, center, and right).
- Use the largest of the three measurements.
- Add a minimum of 4 inches to that measurement (approximately 2" per side) to prevent light leakage and provide solid bracket mounting surface.
- If privacy is a top priority, add more than 4" for additional coverage.
Measuring Height
- Measure the height at three locations (left, center, and right), from where you plan to mount the headrail down to where you want the shade to end.
- Use the largest of the three measurements.
- Add a minimum of 4 inches to your window opening height, then adjust based on your actual bracket mounting position.
Common Measuring Mistakes to Avoid
The seven most common measuring mistakes all come down to the same root cause: assuming the window is square, the tape is accurate, and the frame has no hidden obstructions.
| Mistake | Why It Happens | How to Avoid It |
|---|---|---|
| Measuring only once | Assuming the frame is perfectly square | Always measure width and height at 3 points each |
| Using a fabric tape measure | Fabric stretches, introducing up to 1/2" of error | Use a rigid steel tape measure only |
| Rounding to the nearest inch | Feels close enough | Record to the nearest 1/8" |
| Making your own deductions | Trying to pre-calculate the order size | Submit raw measurements only — we handle all deductions |
| Forgetting to check depth | Depth is not visually obvious | Always measure frame depth before choosing inside mount |
| Mixing up width/height rules | Using narrowest for height or largest for width | Inside mount: narrowest width, largest height |
| Ignoring obstructions | Cranks, locks, and handles are easy to overlook | Run your hand around the full frame interior before deciding on mount type |
Special Cases
Out-of-square windows, multiple windows in a row, and door frames each require extra care — measure every window individually and consider outside mount when frames vary by more than 1/4 inch.
Uneven or Out-of-Square Windows
If your three width or height measurements differ by more than 1/4", your window is noticeably out of square — common in older homes. In this case, strongly consider outside mount, since it's far more forgiving of an irregular opening.
Multiple Windows in a Row
Never assume windows in the same room are identical, even if they look it. Measure each window individually — two windows installed side by side can differ by 1/4" or more.
French Doors and Sliding Doors
The same width/height rules apply, but double-check depth carefully — door frames often have less usable depth than window frames, and hardware is more likely to interfere with an inside mount.
Frequently Asked Questions
Do I measure the glass or the frame?
Always measure the frame opening (the visible recessed area), not the glass itself. The glass measurement will always be smaller and will lead to an undersized order.
Should I subtract anything from my measurements before ordering?
No. Submit your raw measurements exactly as taken. We will make all necessary deductions during production. See our FAQs for more common questions.
What if my three measurements are exactly the same?
Great — your window is square. Simply use that consistent number for both width and height.
How much should I add for outside mount?
Add a minimum of 4 inches total to both width and height. This provides approximately 2" of overlap on each side, preventing light gaps and giving brackets a solid mounting surface.
Can I return shades if I measured wrong?
Because cellular shades are cut to custom dimensions, most manufacturers do not accept returns on incorrectly measured orders. Double-check your numbers before submitting — ideally with a second measurement on a different day. Review our Satisfaction Guarantee for full details on our policy.
What is the difference between opening size and order size?
Opening size is the raw number your tape measure shows. For inside mount, submit the opening size directly — we handle the rest. For outside mount, submit the opening size plus your chosen overlap (minimum 4").
Final Checklist Before You Order
Run through this checklist before submitting your order to catch the most common sizing errors before they become expensive mistakes.
- Decided on inside or outside mount, based on frame depth and obstructions
- Measured width at 3 points — recorded the narrowest (inside) or largest plus minimum 4" overlap (outside)
- Measured height at 3 points — recorded the largest (inside) or bracket-to-sill distance plus minimum 4" overlap (outside)
- Measured frame depth and confirmed it meets the minimum for your chosen shade type
- Used a steel tape measure, recorded to the nearest 1/8"
- Did NOT make any deductions — submitted raw measurements only
- Double-checked all measurements one more time before submitting
Measuring correctly is the single biggest factor in whether your new cellular shades feel like a professional, built-in upgrade — or an ill-fitting DIY project. Take the extra ten minutes. Ready to shop? Browse our full Honeycomb Cellular Shades collection.
Further Reading
- Honeycomb Cellular Shades: What to Know Before You Buy — operating systems, materials, and energy efficiency
- TDBU vs. Standard Cellular Shades — which operating system is right for your windows?
- Blackout vs Light Filtering Blinds — choose the right fabric before you order
- Cellular Shades Pros & Cons: An Honest Review — is it the right window treatment for you?