Inside-mount blinds have to clear the window frame, so the factory shaves a fixed amount off the width you send them. Measure the opening, subtract the deduction, and round to the nearest eighth — the size the plant actually cuts to.
A 36-inch opening with a half-inch deduction orders at 35.5 inches — and rounding to the eighth keeps odd measurements on a size the factory can build.
The example
Three windows measured at the opening, all with the same half-inch inside-mount deduction.
| A | B | C | D | E | F | |
|---|---|---|---|---|---|---|
| 1 | Window | Open W | Open H | Deduct | Order W | Order H |
| 2 | Kitchen | 36 | 48 | 0.5 | 35.5 | 48 |
| 3 | Bedroom | 24 | 36 | 0.5 | 23.5 | 36 |
| 4 | Slider | 72 | 80 | 0.5 | 71.5 | 80 |
The formula
The deduction comes off the width; multiplying by 8 and rounding down snaps the result to the nearest eighth of an inch:
How it works
The eighth-inch trick is the important part:
B2-D2takes the factory deduction off the opening width.*8converts inches to eighths — 35.5 inches becomes 284 eighths.ROUNDDOWN(...,0)drops any leftover so the blind never comes back too wide to fit./8converts back to inches, landing on a size the factory can actually fabricate.
Height on an inside mount is usually ordered at the opening, or a touch under, so the bottom rail clears the sill. Width is where the deduction always applies.
Try it: interactive demo
Enter the opening width and the manufacturer's inside-mount deduction.
Variations
Outside mount adds overlap
An outside mount covers the frame, so you add overlap instead of subtracting.
Round height too
Apply the same eighth-inch rounding to the height for a clean spec sheet.
Pitfalls & errors
Round down, not up. Rounding the width up on an inside mount makes the blind too wide to drop into the frame — a remake at your cost.
The deduction is the manufacturer's, not yours. It is printed in their measuring guide and varies by product; keep it in a cell so a different line just means a different number.
Practice workbook
Frequently asked questions
Why round to an eighth of an inch?
Does the deduction change by product?
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