Blind makers want the raw window measurements for an inside mount and take the clearance off at the factory. For an outside mount, you add the overlap yourself.
Blinds and shades size calculator
Measure in three places and choose how the blind will hang.
Inside mount
Measure the inside of the frame at the top, middle and bottom and order the narrowest width, rounded down to the nearest 1/8 inch. Measure the height on the left, middle and right and order the longest. Do not take anything off yourself: the maker makes the small deduction so the blind clears the frame. Most blinds need at least 3/4 inch of frame depth (deeper for some styles; check the product).
Outside mount
Add about 1.5 inches on each side (3 inches in total) so the blind covers the light gaps, and measure the height from where the headrail will go down to the sill or below it. Nothing is deducted at the factory.
Sources
- Bali Blinds measuring guide: inside mount, narrowest width and longest height, minimum 3/4 in depth, factory deductions; outside mount 1.5 in overlap per side
- The Home Depot: measuring for window treatments (inside mount: narrowest width rounded down to 1/8 in; outside mount: about 2 in wider than the opening)
Sources last checked 1 November 2026.
Frequently asked questions
Do I take deductions for inside mount blinds?
No. Give the narrowest inside width and longest height; the manufacturer takes the clearance off.
How much wider should outside mount blinds be?
About 3 inches in total, 1.5 inches past the frame on each side, per Bali; The Home Depot suggests about 2 inches wider than the opening.