Blind deductions explained
The most common measuring mistake is subtracting a little "just to be safe". The factory already does.
Short answer: for an inside mount, order the exact opening size. The manufacturer takes a small deduction from the width so the blind fits without rubbing.[1 · Oct 29, 2026][2 · Oct 29, 2026][3 · Oct 29, 2026] For an outside mount, there's no deduction; the blind is made to the size you order.[4 · Oct 29, 2026][5 · Oct 29, 2026][6 · Oct 29, 2026] If you take a deduction too, the blind ends up too narrow.
Typical factory deductions (inside mount)
| Product | What the maker takes off |
|---|---|
| Bali cellular shades | ⅜ in off the overall width[4 · Oct 29, 2026] |
| Levolor natural (woven) shades | Fabric up to ¼ in narrower (⅜ in motorized)[1 · Oct 29, 2026] |
| Levolor vertical blinds | Height adjusted so vanes clear the sill by about ¼ to ½ in[6 · Oct 29, 2026] |
| Blinds generally (one retailer's estimate) | Usually no more than ½ in of width[5 · Oct 29, 2026] |
The deduction leaves a slim light gap at each side. If that bothers you, Blinds.com suggests an outside mount.[5 · Oct 29, 2026]
Things that look like deductions but aren't
- Rounding. Measure to ⅛ in.[1 · Oct 29, 2026] Blinds.com and SelectBlinds say to round in-between width readings down and heights up.[3 · Oct 29, 2026][7 · Oct 29, 2026] Home Depot's guide rounds both down.[2 · Oct 29, 2026]
- Floor clearance. If a blind or shade goes to the floor, take at least ½ in off the height.[2 · Oct 29, 2026][6 · Oct 29, 2026] For vertical blinds, Levolor recommends at least ½ in of floor clearance.[8 · Oct 29, 2026]
- Overlap. On outside mounts you add width, not subtract it. See what size do I need.
- Several blinds on one headrail. Bali leaves at least ¼ in between blinds that share a headrail.[9 · Oct 29, 2026]
When you might adjust
A few brand rules do change the number you order:
- Vertical blinds, inside mount: order the shortest height, not the longest.[6 · Oct 29, 2026][2 · Oct 29, 2026]
- Vertical blinds, sheer shades and solar shades, inside mount: Blinds.com measures the width only at the top.[3 · Oct 29, 2026]
- Levolor's corded 2 in and 2½ in wood blinds with a flush valance and cord tilt: deduct ¼ in.[1 · Oct 29, 2026]
When in doubt, follow the brand's own measuring sheet for the exact product.
Affiliate links: if you buy through a link in this box we may earn a commission, at no extra cost to you. It does not change the sizes or sources on this page. How we handle affiliate links.
Sources: 1. Levolor, How to measure windows for blinds and shades. 2. The Home Depot, Measuring the right way for window treatments (PDF). 3. Blinds.com, How to measure. 4. Bali, Cellular and pleated shades measuring guide (PDF). 5. Blinds.com, How to measure for a bay window. 6. Levolor, Vertical blinds technical guide (PDF). 7. SelectBlinds, How to measure for custom blinds and shades. 8. Levolor, Vertical blinds side-stack installation instructions (PDF). 9. Bali, Wood, composite and faux wood blinds measuring guide (PDF, hosted by a retailer). Checked Oct 29, 2026. Trademarks belong to their owners; figures are restated in our own words and tables, not reproduced.