How to Size Window Blinds

A blind size calculator takes the guesswork out of ordering. The two mounting styles use opposite logic: inside mounts subtract a small clearance so the blind fits inside the frame and slides freely, while outside mounts add overlap so the blind fully covers the opening and blocks light from the edges.

Inside Mount Sizing

Measure the width at the top, middle, and bottom of the frame, then use the narrowest width. Subtract 1/4" from each side (1/2" total) and about 1/4" from the height for clearance. If your window has trim or handles that protrude, factor those into the clearance as well.

Outside Mount Sizing

Add 2" to each side of the window width (4" total) and 3" to the height — about 2" above the frame and 1" below it. This guarantees the blind overlaps the opening on every edge, which maximizes light control and privacy. Outside mounts are also a good choice when the frame is too shallow for an inside mount.