Window Blinds Size Calculator

Window blind sizing gives you the correct order dimensions for a clean fit, whether you mount the blind inside the window frame or outside it.

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).

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: on an inside mount the manufacturer trims about 0.25 to 0.5 inch so the blind clears the frame, which is why you order the exact window size and let them deduct
• Light gap: outside mounts with wider overlap reduce side-light leakage, which matters 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 and want an inside mount roller blind.

• Order width = 36 inches (the exact window width; the manufacturer trims the 1/4 to 1/2 inch clearance)
• Order height = 48 inches
• For an outside mount instead, you would order about 42 inches wide (3 inches past each side) and 52 inches tall to hide the headrail and overlap the sill.

Always give the manufacturer your exact window measurements for an inside mount and let them apply the deduction. Deducting yourself on top of theirs leaves a light gap down each side.