Blind Size Calculator

How to Measure for Blinds

Getting the correct blind size starts with accurate window measurements. The two primary methods are inside mount (fitted inside the window recess) and outside mount (fitted on the wall or frame above the window). Each requires a slightly different approach.

For an inside mount, measure the width at the top, middle, and bottom of the recess. Use the narrowest width measurement. For the drop, measure the left, center, and right. Use the longest drop measurement. This ensures the blind fits without scraping the sides or leaving gaps at the bottom.

For an outside mount, measure the width of the area you want the blind to cover, typically adding 5–10 cm on each side to prevent light gaps. For the drop, measure from the top of the mounting point to where you want the blind to end.

How the Blind Size Calculator Works

This calculator applies standard fitting allowances to your raw window measurements. It accounts for the difference between inside and outside mounts and adjusts for common blind types such as roller blinds, Roman blinds, and Venetian blinds.

The calculator uses the following logic:

• Inside mount: Takes the narrowest width and longest drop from your three measurements, then subtracts a small clearance (typically 5–10 mm) to allow the blind to move freely within the recess.
• Outside mount: Adds an overlap to your width and drop measurements to ensure full coverage and minimal light bleed around the edges.

No complex formulas are required on your end. You simply enter your measurements and select your mount type.

Understanding Your Results

The calculator outputs two key numbers: the recommended blind width and the recommended blind drop. These are the final dimensions you should use when ordering or making your blinds.

If the result seems unexpected, double-check your measurements. Common issues include measuring from the wrong reference point (e.g., including the window sill in the drop) or mixing up inside and outside mount allowances.

For inside mounts, a small gap (1–2 mm) on each side is normal and necessary for operation. For outside mounts, the overlap is intentional and ensures the blind sits flush against the wall or frame.

Common Measurement Mistakes

• Measuring only once: Windows are rarely perfectly square. Always measure width and drop at multiple points.
• Forgetting clearance: An inside mount blind needs a few millimeters of space to operate without friction.
• Including the sill in the drop: For inside mounts, the drop should stop at the top of the sill, not include it.
• Assuming all windows are identical: Even windows in the same room can vary by several millimeters. Measure each one individually.

Practical Use Cases

This calculator is useful for anyone fitting new blinds, replacing old ones, or ordering custom-made blinds online. It removes the guesswork from sizing and reduces the risk of ordering blinds that are too wide, too narrow, or too short.

Common scenarios include:

• Fitting roller blinds in a kitchen with multiple window sizes.
• Ordering Roman blinds for a bay window where each pane has different dimensions.
• Replacing old Venetian blinds where the original measurements have been lost.