Window Blinds Calculator
Calculate blind width and drop for inside and outside mount.
Covers roller, venetian, and roman blinds with deduction guidelines for a perfect fit.
Window blind sizing requires precise measurements of the window opening and a clear decision between inside mount (within the frame) and outside mount (over or around the frame).
Inside Mount Width Formula:
Blind Width = Window Opening Width − 0.25 to 0.5 inches
The deduction (called the deduct) gives clearance so the blind raises and lowers without rubbing the jambs. That figure is the total taken off the width, not per side. Most manufacturers publish their own deduct, commonly a quarter to a half inch, and this calculator uses a quarter inch.
Inside Mount Length Formula:
Blind Length = Window Opening Height − 0.5 inches
Watch this one. Venetian and faux wood blinds want the height deducted as above, but plenty of roller and cellular makers ask for the exact opening and apply their own allowance at the factory. Ordering a deducted height to a maker who expected the raw opening leaves you an inch short of the sill, and custom blinds are not returnable.
Outside Mount Formulas:
Width = Opening Width + (2 × Side Overlap) Coverage only: 1.5–2 inches per side. Blackout: 3 inches or more per side.
Length = Opening Height + Top Overlap + Bottom Overlap Coverage only: 1–2 inches above the frame. Blackout: 3 inches or more.
This calculator uses the blackout figures, 3 inches per side and 3 above, because outside mount is usually chosen for light control in the first place.
Worked example, inside mount: Window opening: 36.25 inches wide × 48.5 inches tall, using a 0.25 inch deduct. Blind width: 36.25 − 0.25 = 36 inches Blind length: 48.5 − 0.5 = 48 inches
Worked example, outside mount for light control: Same window opening: 36.25 × 48.5 Add 3 inches per side: 36.25 + 6 = 42.25 inches Add 3 above and 1 below: 48.5 + 4 = 52.5 inches
Measuring tips:
- Measure width in 3 places (top, middle, bottom) and use the smallest measurement for inside mounts
- Measure height in 2 places (left and right) and use the longest for inside mounts
- Always measure to the nearest 1/8 inch: blind manufacturers work in 1/8-inch increments
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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