Mount type
Inside mount fits inside the window frame for a clean built-in look. Deduct 1/4 inch from width for proper fit.
Window measurements
Measure each window individually — even windows in the same room are rarely identical.
Frequently Asked Questions About Measuring and Sizing Blinds
Q1. How do I measure a window for blinds?
First decide between inside mount (blinds fit inside the window frame) or outside mount (blinds mount on the wall or trim outside the frame), since the measurement approach differs for each. For inside mount, measure the exact width and height inside the frame at three points (top, middle, bottom for width; left, middle, right for height) and use the smallest measurement to ensure the blind fits. For outside mount, measure the area you want covered, typically extending 2-3 inches beyond the frame on each side.
Q2. What's the difference between inside mount and outside mount blinds?
Inside mount blinds sit within the window frame, creating a clean, built-in look that shows off attractive trim, but they require enough frame depth to accommodate the blind's headrail. Outside mount blinds attach above and beside the frame, which works well for windows with shallow frames, uneven frames, or when you want the window to appear larger, and they also block more light since they cover the entire frame.
Q3. How much should I deduct from my measurement for inside mount blinds?
Manufacturers typically deduct 1/4 to 1/2 inch from your exact width measurement automatically to ensure the blind doesn't bind against the frame as it operates, so when ordering, provide your exact frame measurement and let the manufacturer or calculator apply that standard deduction rather than subtracting it yourself, which can result in a blind that's too small.
Q4. How much extra width and length should outside mount blinds have?
For width, extend the blind at least 2-3 inches beyond the window frame on each side (4-6 inches total) to minimize light gaps and give the illusion of a larger window. For length, mount the header 2-3 inches above the frame and let the blind extend to the sill or a few inches below it, depending on your preferred look.
Q5. What should I do if my window frame is uneven or out of square?
Take measurements at multiple points (both width and height) rather than assuming the window is a perfect rectangle, since older homes especially often have frames that have shifted over time. For inside mount, always use the smallest width and height measurements found to guarantee the blind fits at its tightest point. If the frame is significantly out of square, outside mount is usually the safer, more forgiving option.