Blind Size Calculator — Inside & Outside Mount
Measure your window and enter the details to get the exact blind width and drop for an inside mount (fitted inside the recess) or outside mount (fixed to the wall above the frame), including standard clearance deductions.
Mount type
cm
cm
cm
Deduct 1–2 cm from the measured recess width for operating clearance
- 1
Measured recess width
120 cm - 2
Blind width (minus operating deduction)
120 − 1.5 = 118.5Deduct 1–2 cm so the blind raises and lowers freely inside the recess.
How does this calculator work?
Inside mount: blind width = recess width minus 1–2 cm (operating clearance), drop = recess height. Outside mount: blind width = frame width plus side overlap on both sides (minimum 5 cm each), drop = frame height plus top mounting height (typically 10 cm). Always check the manufacturer's deduction spec before ordering.
Formula
How this is calculated
An inside-mount blind sits inside the window recess, giving a clean, built-in look. To find the right width, measure the inside width of the recess at three heights (top, middle, bottom) and order the narrowest measurement. Most manufacturers require a deduction of 1–2 cm from your measured width to allow the blind to raise and lower without rubbing the sides — check your supplier's specification sheet, as the exact deduction varies by product. The drop is usually the full inside height from top to sill.
An outside-mount blind is fixed to the wall or frame above the window, allowing more flexibility in blind size and better light blocking. A minimum of 5 cm overlap on each side is recommended to prevent light gaps; 10 cm or more gives better black-out performance. The bracket is typically mounted 10–15 cm above the frame to hide the headrail and allow the blind to clear the top of the window when raised. The total drop ordered is the frame height plus the top mounting height.
Always measure twice before ordering. Blind widths are usually ordered to the nearest half-centimetre or quarter-inch depending on the supplier. If your window is not perfectly square (common in older buildings), use the narrowest width for inside mounts and the widest for outside mounts.
Frequently asked questions
The deduction depends on the blind type and manufacturer — typically 0.5–2 cm total from the measured recess width. Roller and cellular blinds usually require 1–1.5 cm; venetian blinds may need up to 2 cm. Always check your blind supplier's specification table before ordering.
A minimum of 5 cm (2 inches) on each side is the practical minimum; 8–10 cm gives better light blocking. For near black-out performance (e.g., bedroom blinds), extend 10–15 cm on each side and mount the bracket 15 cm above the frame.
If the window is out of square, measure the width at the top, middle and bottom and the height on the left, right and centre. For inside mount, order the smallest width and shortest height. For outside mount, order the largest measurements to ensure full coverage.
Also known as
TG we-Calculate Editorial Team. (2026). Blind Size Calculator — Inside & Outside Mount [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/blind-size-calculator
TG we-Calculate Editorial Team. "Blind Size Calculator — Inside & Outside Mount." TG we-Calculate. 2026. https://we-calculate.com/calculator/blind-size-calculator.
TG we-Calculate Editorial Team, "Blind Size Calculator — Inside & Outside Mount," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/blind-size-calculator
@misc{wecalculate_blind_size_calculator, title = {Blind Size Calculator — Inside & Outside Mount}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/blind-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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