Skip to content
MakeCalcs

Blind Sizing Calculator for Windows

The Blind Sizing Calculator determines the correct blind dimensions for inside and outside mount installations with clearance adjustments.

Table of Contents

Calculator

Quick presets

Measured from glass to front of recess

How far the blind extends beyond the recess on each side

Important

Wall covering calculations estimate material needs based on standard wall dimensions and coverage rates. Actual requirements depend on wall condition, surface preparation, and application method. These estimates are for planning — consult product datasheets for specific coverage rates.

How the Blind Sizing Calculator Works

The calculator adjusts your raw recess measurements based on mount type. Inside mount subtracts 10mm from the width for operating clearance — the roller needs space to turn without rubbing the recess sides. Height stays as the full recess height. Outside mount adds the overlap to both sides of the width and to the top of the height, so the blind covers the recess fully and blocks light gaps at the edges.

The minimum recess depth output (45mm for inside mount) checks whether your recess is deep enough for the bracket. If the depth is less than 45mm, inside mount is not suitable and you should switch to outside mount. If you prefer curtains over blinds, the curtain fabric yardage tool works from the same window measurements.

Reading Your Results

Three outputs appear — recommended blind width, recommended drop, and minimum recess depth. Order your blind to these dimensions. Most made-to-measure companies ask for the finished blind size, not the recess.

If buying a standard off-the-shelf size, choose the next size down from the recommended width for inside mount (it must fit inside) and the next size up for outside mount (it must cover the recess). You may also want to estimate paint for the window surrounds while you have measurements to hand.

Common Mistakes to Avoid

Measure the recess width at three points — top, middle, and bottom — and use the narrowest for inside mount. Recesses are rarely perfectly square. Measure depth from the glass to the front edge of the recess. Venetian and pleated blinds need more depth (50–60mm) than rollers (45mm).

For bay windows, measure each panel independently — each gets its own blind. Outside mount is the better choice when the recess is shallow (under 45mm) or when you need blackout coverage — the overlap blocks the light gap at the sides. The same measure-at-multiple-points principle applies to sizing a pond liner — always use the largest dimension to avoid coming up short.

Real-World Adjustments

Increase outside mount overlap to 75–100mm per side for blackout blinds. Standard 50mm leaves visible light gaps. Reduce inside mount clearance to 5mm only if the recess is perfectly square and plumb (check with a spirit level).

If fitting a blind behind existing curtains, make sure there is enough depth for both the blind bracket and the curtain track — they compete for the same space. Our wallpaper pattern coordination guide covers how to match window treatments to your wall covering for a cohesive look.

Worked Examples

Example 1: Inside mount roller blind for a bedroom

Scenario: Sophie is fitting an inside-mount roller blind in a bedroom window with a 1,200 × 1,200mm recess and 70mm depth.

Mount type: inside. Blind width: 1,200 − 10 = 1,190 mm. Blind drop: 1,200 mm (full recess height). Minimum recess depth: 45 mm. Sophie’s recess is 70 mm deep, which exceeds the 45 mm minimum.

Sophie should order a roller blind 1,190mm wide and 1,200mm long. The 10mm clearance (5mm each side) lets the roller turn freely. Her 70mm recess comfortably fits the mechanism.

Key takeaway: Always order inside-mount blinds to the calculator’s recommended width, not the recess width. Ordering the full recess width means the blind will jam against the sides.

Example 2: Outside mount blackout blind for a wide window

Scenario: James wants an outside-mount blackout blind for a wide living room window. The recess measures 1,800 × 1,200mm with a shallow 50mm depth. He wants 75mm overlap on each side.

Mount type: outside. Blind width: 1,800 + (75 × 2) = 1,950 mm. Blind drop: 1,200 + 75 = 1,275 mm. Minimum recess depth: 0 mm (outside mount does not sit in the recess).

James should order a blind 1,950mm wide and 1,275mm long. The 75mm overlap on each side blocks light gaps. Because the blind mounts on the wall above the recess, his shallow 50mm recess is not a problem.

Key takeaway: Outside mount solves the shallow-recess problem and provides better blackout coverage. The 75mm overlap is worth the extra blind width for a bedroom or media room.

Frequently Asked Questions

What is the difference between inside and outside mount blinds?
Inside mount blinds fit within the window recess, giving a clean, built-in look. Outside mount blinds attach to the wall above the recess and overlap the edges. Inside mount works when the recess is at least 45mm deep and square. Outside mount suits shallow recesses, blackout needs, or making a window appear larger.
How much clearance do I need for inside mount roller blinds?
Allow 10mm total clearance between the blind and the recess walls. This gives the roller room to operate without friction. Measure at three heights and use the narrowest width minus the 10mm. Accurate recess measurement follows the same principle as measuring for tiles — use the smallest dimension.
Should I measure the recess or the window for blinds?
Measure the recess, not the glass. For inside mount, measure the inside of the recess — width, height, and depth. For outside mount, measure the recess opening and add your overlap. The blind supplier needs the finished blind size, which this calculator provides.
What size blind do I need for a bay window?
Treat each panel as a separate window. A three-panel bay needs three individual blinds. Measure each recess independently because bay panels are often slightly different widths. Inside mount is the most common choice for bays because it keeps the blinds within the recess line.

Glossary

Inside mount

Installation where the blind fits within the window recess. Brackets attach to the inside top or sides. Gives a clean, flush appearance but requires minimum 45mm recess depth for most roller blinds.

Outside mount

Installation where brackets attach to the wall above the recess. The blind hangs in front and overlaps the edges. Provides better light blocking and works with any recess depth.

Recess depth

The distance from the front edge of the window recess to the glass surface. Determines whether a blind mechanism fits inside. Roller blinds need 45mm minimum; venetian blinds need 50–60mm.

Roller blind

A flat fabric that wraps around a roller tube. Pulling the chain rolls the fabric up. The slimmest blind type, needing the least recess depth for inside mount.

Venetian blind

Horizontal slats (aluminium, wood, or faux wood) that tilt to control light and stack at the top. Needs more recess depth than rollers because stacked slats take up space.

Related pages

Explore Garden Calculators

From paving slabs to raised beds, get accurate material lists for your outdoor projects.

Browse Garden Calculators

More Interior calculators

Browse all interior calculators — Tiles, wallpaper, paint, flooring, and trim calculators.

Danijel "Dan" Dadovic

Commercial Director at Ezoic · MSc Informatics · MSc Economics · PhD candidate (Information Sciences)

Builder of MakeCalcs and 5 other calculator sites. Each applies the same accuracy-first methodology — sourced formulas, known-value testing, multi-material output. Read more about Dan

Independently reviewed by Glen Todd, Construction Professional.

Last updated: