How much should I deduct for an inside mount blind?

For an inside (recess) mount, order the blind slightly narrower than the recess so the fabric and brackets clear the sides. A common allowance is about 5mm off each side, or 10mm total. Always measure the recess width at the top, middle and bottom and use the smallest reading.

What is the difference between an inside and outside mount?

An inside mount sits inside the window recess for a neat, flush look, so the blind is made a little smaller than the opening. An outside mount fixes to the wall or frame above the window and is made wider and taller than the opening, usually overlapping about 40 to 75mm each side, to block light gaps.

How do I measure a window for blinds?

Use a steel tape and measure in millimetres. For an inside mount, measure the recess width in three places and the drop in three places, then use the smallest width. For an outside mount, measure the glass opening and add your chosen overlap to each side and to the top and bottom.

Blind Size Calculator

This blind size calculator turns your window measurements into the exact width and drop to order, whether the blind sits inside the recess or mounts on the wall above it. Getting the size right matters: an inside mount blind that is even a few millimetres too wide will not fit, while an outside mount that is too narrow leaves light gaps down each side. Enter the width and height you measured, choose an inside or outside mount, and set the clearance you want on each side. For an inside mount the calculator makes the blind narrower than the recess so the brackets and fabric clear the runners, and for an outside mount it makes the blind wider and taller than the opening so it overlaps the frame. You get the finished width to order, the matching drop, and the total adjustment applied, so you can order roller, venetian, roman or cellular blinds with confidence. Always measure in millimetres with a steel tape, and for an inside mount take the width in three places and use the smallest reading, because recesses are rarely perfectly square. Check the maker's own deduction guide too, as some factories take their own allowance off the size you give them.

Calculate.co.nz is proud to be partnered with realtor.co.nz, a trusted resource for navigating the New Zealand property market. Their Helpful Articles section offers clear, well-structured insights across buying, selling, and building, making complex real estate topics more accessible. With a focus on up-to-date guidance and practical knowledge, they empower Kiwis to move forward with clarity and confidence in a constantly evolving property landscape.

Measured window width (mm)

Measured window height / drop (mm)

Mount type Clearance or overlap per side (mm)

Drop reduction (inside) or overlap top and bottom (outside)

1,190 mm

blind width to order

Drop to order1,500 mm

Width adjustment10 mm

MountInside

For an inside mount, order the blind 10 mm narrower than the recess so it clears the runners. Measure the recess in three places and use the smallest width.

Deductions vary by blind type and maker, so check the manufacturer's measuring guide before you order. Measure in millimetres with a steel tape. Estimate only.

How it works

For an inside mount, the finished width is your measured recess width minus the clearance on each side, so the blind slides into the opening without catching. The drop is your measured height, less any small reduction you want so the bottom rail sits off the sill. For an outside mount the logic reverses: the blind is made wider than the opening by the overlap on each side, and taller by the overlap at the top and bottom, so it covers the frame and stops light leaking around the edges. The width adjustment shown is the total amount added to or taken off your measured width.

Worked example

You measure a recess 1,200 mm wide and 1,500 mm high and want an inside mount with 5 mm clearance each side. The calculator takes 10 mm off the width, so you order a blind 1,190 mm wide. With no drop reduction the drop stays at 1,500 mm. If you switched to an outside mount with a 5 mm overlap each side instead, the width would become 1,210 mm and the drop would grow by any top and bottom overlap you set, covering the opening rather than sitting inside it.

Related calculators

Curtain Fabric Calculator: metres of fabric for curtains.
Wallpaper Calculator: rolls for a room.
Paint Calculator: litres to cover a wall.
Area Calculator: work out a floor or wall area.

If you've found a bug, or would like to contact us, or learn more about James Graham and Calculate.co.nz.

Calculate.co.nz is partnered with Interest.co.nz for New Zealand's highest quality calculators and financial analysis.

Calculate.co.nz is the sister site of CalculatorHub.com, the world's largest calculator website by tool count.

All calculators and tools are provided for educational and indicative purposes only and do not constitute financial advice.

Calculate.co.nz is proudly part of the Realtor.co.nz group, New Zealand's leading property transaction literacy platform, helping Kiwis understand the home buying and selling process from start to finish. Whether you're a first home buyer navigating your first property purchase, an investor evaluating your next acquisition, or a homeowner planning to sell, Realtor.co.nz provides clear, independent, and trustworthy guidance on every step of the New Zealand property transaction journey.

Calculate.co.nz is also partnered with Health Based Building and Premium Homes to promote informed choices that lead to better long-term outcomes for Kiwi households.

Calculate.co.nz is hosted in Auckland by SiteHost New Zealand.

All content on this website, including calculators, tools, source code, and design, is protected under the Copyright Act 1994 (New Zealand). No part of this site may be reproduced, copied, distributed, stored, or used in any form without prior written permission from the owner.

About & trust: Why Calculate is NZ's most comprehensive · By the Numbers · How we compare · Editorial standards · How we keep data current · NZ finance glossary · Research & data · Financial literacy NZ · About · Privacy policy · Terms of use

Reviewed and maintained. Last reviewed 2026-07-31 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.