Blinds & Shade Size Calculator
Turn your window opening into the exact blind or shade size to order — inside mount with clearance deductions, or outside mount with overlap.
How it works
Inside width = opening − clearance deduction (height exact) · Outside width = opening + 2 × overlap · Outside height = opening + 2 × overlap
The size you order is almost never the size you measure. For an inside mount you give the exact tight opening and the manufacturer subtracts a small clearance (often 1/4"–1/2") so the blind fits and moves freely — you do not deduct the height yourself. For an outside mount you add overlap, typically about 1.5"–2" on each side and top, so the covering hides the frame and blocks side light.
Worked example
A 36" × 48" window, inside mount, with a 1/4" clearance deduction: order 35.75" wide and give the 48" height as-is. The same window outside-mounted with 1.5" overlap per side: order 39" wide (36 + 3) and 51" tall.
Frequently asked questions
Do I deduct the height for an inside mount?
No — for inside mounts you provide the exact opening width and height, and the factory takes its own clearance off the width. Deducting the height yourself leaves an unwanted gap at the sill.
Inside or outside mount?
Inside mounts look built-in and show off the trim but need enough flat depth inside the frame and a reasonably square opening. Outside mounts suit shallow, out-of-square or undersized windows and block more light because they overlap the frame.
How much overlap for an outside mount?
Around 1.5"–2" on each side and at the top is typical, so the shade fully covers the opening and hides light leaks. More overlap improves light blocking; less keeps the look trim.
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This calculator is for educational purposes. Double-check important results before acting on them.