Curtains & Windows

Blind Size Calculator

Convert window measurements into an inside- or outside-mount blind order size.

Window-treatment worksheet

Enter the opening and treatment measurements for Blind Size Calculator

Method for this tool

What the Blind Size Calculator actually calculates

For an inside mount, width is reduced by twice the adjustment but height by only one adjustment. For an outside mount, both dimensions increase by twice the adjustment. Outputs cannot fall below zero. This fixed rule does not reflect every blind system, and some suppliers make their own deductions from your recess measurements.

The calculation path

Inside mounts subtract a fitting deduction; outside mounts add overlap for better light control.

Inputs that can change the answer

Opening width: Enter opening width in m.
Opening height: Enter opening height in m.
Mount type: Select the applicable mount type from the listed choices.
Deduction or overlap each side: Enter deduction or overlap each side in m.

Checks that stay outside the formula

Always follow the blind manufacturer’s exact measurement and deduction policy.

A practical order for entering the data

Measure the opening at multiple points and follow the supplier's rule for choosing the width and height to enter.
Choose inside or outside mounting and enter the intended deduction or overlap in metres.
Compare the calculated dimensions with the supplier's ordering policy before submitting an order.

Project walkthrough: Curtains: from window measurement to fabric order

Project walkthrough: Compare blinds and curtains from the same window measurements

Engine check using the prefilled sample

Window-treatment example: opening width = 1.2 m, opening height = 1.5 m, mount type = Inside recess and deduction or overlap each side = 0.01 m. Replace the sample opening and fabric or hardware values with measurements taken for the mounting method you plan to use.

Results for these inputs

Order width: 1.180 m
Order height: 1.490 m

Project-specific method

Worked example and purchase checks

The tool starts from the measured opening width and height, then applies the entered deduction or overlap according to inside- or outside-mount selection. It is deliberately transparent about the adjustment rather than claiming a manufacturer-specific ordering size.

Illustrative worked example

An inside-mount blind in a 1.42 m × 1.58 m recess using an 8 mm planning deduction per side.

Exact inputs for this example

Opening width	1.42 m
Opening height	1.58 m
Mount type	Inside recess
Deduction or overlap each side	0.008 m

Order width: 1.404 m
Order height: 1.572 m

How to read the result

Treat the calculated size as a check against the supplier’s measuring instructions, not a replacement for them. If the supplier specifies a different ordering convention, follow that convention.

Assumptions this example makes

The opening has been measured at the points required by the chosen blind supplier.
The adjustment value is intentionally chosen for that mounting method.

What this calculator does not prove

Suppliers often have their own deduction rules, headrail dimensions, minimum recess depth, and squareness tolerances.
The tool does not decide whether the recess is square or obstruction-free.

Blind recess measurement sketchSketch is explanatory, not to scale.

Mistakes that change this result

Applying your own deduction when the supplier already asks for recess size and makes the deduction.
Taking only one width measurement in a tapered recess.
Ignoring handles, tiles, vents, or trim that affect the mount.