Window Blind Size Calculator
Calculate the correct blind width and drop for inside or outside mounting.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
For an inside mount, measure the narrowest opening; the estimate subtracts 0.25 in from width. For an outside mount, measure the outside frame and enter the desired overlap per side; the estimate adds that overlap to the width and adds overlap plus 2 in to the drop. These are planning dimensions, not fit approval. Follow the blind maker’s instructions for bracket placement and recess clearance.
Worked example
calculated with the default values below
Window width 36 in · Window height 60 in · Mounting Outside mount (over the frame) · Outside-mount overlap per side 2 in Blind width to order 40.00 in · Blind drop to order 64.00 in · Blind width to order 101.6 cm · Blind drop to order 162.6 cm
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
inside width = W − 0.25; inside drop = H; outside width = W + 2o; outside drop = H + o + 2
- W
- Window width (in)
- H
- Window height (in)
- o
- Overlap (in)
- outside
- Mount flag: 1 = outside, 0 = inside
- Bw
- Blind width
- Bd
- Blind drop (height)
Inside width = opening width − 0.25 in; inside drop = opening height. Outside width = frame width + 2×overlap; outside drop = frame height + overlap + 2 in. These are estimates, not a fit verdict; follow the manufacturer’s fitting guide.
Frequently asked questions
Do these dimensions confirm that a blind will fit?
No. The calculator applies a fixed 0.25 in inside-width deduction and user-entered outside overlap to estimate dimensions. It does not know a product’s bracket, recess or clearance requirements. Check the exact blind’s fitting instructions before ordering or drilling.