Window Blinds Size Calculator
Calculate the correct blinds width and height for any window.
Covers inside and outside mount types with deduction and add-on measurements for each style.
Window blind sizing combines two separate calculations: the correct dimensions for a perfect fit, and the total material cost at your chosen price-per-square-foot.
Width formula:
- Inside mount: Measure the inside of the window frame at three points (top, middle, bottom). Use the narrowest measurement. Deduct 0.25–0.5 inches so the blind slides freely without binding.
- Outside mount: Add 1.5–3 inches to each side of the frame. A 3-inch total overlap (1.5 per side) blocks light leakage; 6 inches total (3 per side) gives a more dramatic, room-darkening effect.
Height formula:
- Inside mount: Measure from the top of the sill to the top of the frame. Use the tallest measurement.
- Outside mount: Add 3–4 inches above the frame to conceal the headrail, and add the sill overlap you prefer (0–3 inches below the sill).
Cost formula: Total Cost = (Width × Height in sq ft) × Price per sq ft
Where:
- Width and Height are in feet (divide inches by 12)
- Price per sq ft — standard roller blinds run $1–$4/sq ft; cellular shades $3–$8/sq ft; motorized shades $8–$20/sq ft
What each variable means:
- Inside vs outside mount — inside mounts look clean and architectural; outside mounts suit windows with shallow depth or make small windows appear larger
- Deduction allowance — prevents the blind from jamming against the frame when raised or lowered; 0.25 inches per side is standard
- Light gap — outside mounts with wider overlap significantly reduce side-light leakage, critical for blackout applications
Reference: standard blind prices by type
- Roller blinds: $15–$80 per blind
- Venetian blinds (aluminum): $20–$100 per blind
- Wood blinds: $40–$200 per blind
- Cellular/honeycomb shades: $50–$250 per blind
- Motorized shades: $150–$600+ per blind
Worked example: You have a window 36 inches wide × 48 inches tall. You want an inside mount roller blind at $2.50/sq ft.
- Deducted width = 36 − 0.375 (3/8 inch deduction) = 35.625 inches
- Height = 48 inches (no deduction needed)
- Area = (35.625 ÷ 12) × (48 ÷ 12) = 2.97 × 4.0 = 11.88 sq ft
- Cost = 11.88 × $2.50 = $29.70
For a set of five identical windows, total cost = $148.50 before installation.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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