Window Blinds Calculator

Window blind sizing requires precise measurements of the window opening and a clear decision between inside mount (within the frame) and outside mount (over or around the frame).

Inside Mount Width Formula:

Blind Width = Window Opening Width − 0.25 to 0.5 inches

The deduction (called the deduct) provides clearance so the blind raises and lowers without rubbing. Most manufacturers specify their exact deduct — common range is 0.25 to 0.5 inches per side.

Inside Mount Length Formula:

Blind Length = Window Opening Height − 0.5 inches

Outside Mount Formulas:

Width = Opening Width + (2 × Side Overlap) Recommended side overlap: 1.5–2 inches per side

Length = Distance from Mount Point to Desired Bottom + Top Overlap Typical top overlap above frame: 1–2 inches To block light completely: extend 3+ inches on each side and top

Worked example — inside mount: Window opening: 36.25 inches wide × 48.5 inches tall Manufacturer deduct: 0.375 inches Blind width: 36.25 − 0.375 = 35.875 → order 35 7/8 inches Blind length: 48.5 − 0.5 = 48 inches

Worked example — outside mount (for better light control): Same window opening: 36.25 × 48.5 Add 3 inches per side: 36.25 + 6 = 42.25 → order 42 inches Add 3 inches top, extend to sill: 48.5 + 3 + 1 = 52.5 → order 52 inches

Measuring tips:

• Measure width in 3 places (top, middle, bottom) and use the smallest measurement for inside mounts
• Measure height in 2 places (left and right) and use the longest for inside mounts
• Always measure to the nearest 1/8 inch: blind manufacturers work in 1/8-inch increments