How this instrument works
No real window is a perfect rectangle — frames settle, walls aren't perfectly plumb, and a window that measures 24 inches wide at the top might measure 23 at the bottom. That's exactly why blind manufacturers ask for three separate width measurements (top, middle, bottom) and three separate height measurements (left, center, right) rather than a single width-and-height pair: it lets the order size account for the window's actual, slightly imperfect shape instead of assuming it's a flawless rectangle.
The two mount styles use those six measurements differently, and in opposite directions. Inside-mount blinds sit inside the window frame, so the order width uses the smallest of the three width measurements (to guarantee the blind physically fits at the narrowest point) while the order height uses the largest of the three height measurements (so the blind reaches the frame's full extent even at its tallest point). Outside-mount blinds instead sit on the wall covering the whole window opening, so both width and height use the largest measurement plus extra overlap — 3 inches of width overlap and 1.5 inches of height overlap are standard, ensuring full light-blocking coverage past the window's edges.
This calculator computes both mount types from the same six measurements at once, so you can compare an inside-mount order size against an outside-mount order size for the identical window without re-measuring or re-entering anything.
- Measure width at the top, middle, and bottom of the window frame, in inches.
- Measure height at the left, center, and right of the window frame, in inches.
- Enter all six measurements — Width top/middle/bottom and Height left/center/right.
- Read Inside-mount order width and height — uses the smallest width and largest height.
- Read Outside-mount order width and height — uses the largest width plus 3 in and the largest height plus 1.5 in.
Worked example — a slightly out-of-square window
A window measuring 24, 25, and 23 inches wide (top/middle/bottom) and 30, 31, and 32 inches tall (left/center/right): the smallest width is 23 and the largest height is 32, so the inside-mount order size is 23 in x 32 in — guaranteeing the blind fits at the frame's narrowest point while still reaching its tallest point.
For the same window, outside-mount uses the largest width (25) plus 3 inches of overlap, and the largest height (32) plus 1.5 inches: 28 in x 33.5 in — a noticeably larger order size than the inside-mount figure, since outside-mount blinds are meant to cover the wall around the opening, not fit precisely inside it.
Questions
Why does inside-mount use the smallest width but outside-mount uses the largest?
Inside-mount blinds have to physically fit inside the window opening at every point, so sizing to the narrowest measurement guarantees the blind isn't too wide to install anywhere along the frame. Outside-mount blinds instead sit outside the opening on the wall, so sizing to the largest measurement (plus overlap) guarantees full coverage of the window even at its widest point, with no light gaps.
Why does inside-mount use the largest height instead of the smallest, like it does for width?
Height works the opposite way from width for the same physical-fit reason: sizing height to the tallest of the three measurements ensures the blind reaches the full height of the frame everywhere, rather than falling short at the tallest point. Combined with the smallest width, this gives an inside-mount blind that's guaranteed to fit the frame's actual, imperfect dimensions on both axes.
Why measure width and height in three places instead of just once?
Because real window frames are rarely perfectly square — settling, construction tolerances, and older housing stock all commonly produce a few tenths or even a couple of inches of variation across a single frame. Taking three measurements of each dimension and using the appropriate min or max catches that variation instead of risking a blind that's slightly too big or too small based on one unrepresentative measurement.
Should I round my measurements to the nearest inch or keep fractions?
Keep the fractional measurement (to the nearest 1/8 inch is standard practice) rather than rounding to a whole inch — blinds are typically manufactured to fairly tight tolerances, and rounding away a half-inch or quarter-inch of real measurement can be the difference between a blind that fits cleanly and one that's noticeably off.
Do the same overlap allowances (3 in width, 1.5 in height) apply to every outside-mount blind?
These are standard, widely-used overlap allowances for typical residential blinds, giving reasonable light-blocking coverage past the window's edges without an oversized-looking installation. Some specialty or extra-wide/tall blinds, or installations where you specifically want more or less overlap for light control or aesthetics, may call for adjusting those margins from the standard defaults.