How to Measure Windows for Blinds: Step-by-Step Guide

Custom window blinds are cut to your exact window dimensions. Get the measurements wrong and you risk returns, delays, or a fit that doesn't look or work the way you want.

Manufacturers and retailers often cite measurement errors as one of the top reasons for blind returns. The good news: measuring is straightforward once you know whether you're going inside or outside the frame and you have a few basic tools.

This guide covers how to choose inside vs outside mount, what tools you need, general rules (width by height, measure to 1/8"), and bullet-summary steps for four common window blind types: vertical, mini, faux wood, and horizontal. Each of those sections links to our detailed measurement page for that product so you can get full instructions and any PDFs.

Inside Mount vs Outside Mount: Which to Choose

You need to decide this first. It changes where and how you measure.

Inside mount means the blind sits inside the window frame. You get a clean, built-in look, but you need enough depth in the frame for the headrail and hardware.

Outside mount means the blind is mounted on the wall or trim above the window. It works well when the frame is shallow or when you want more coverage and light blockage. The window can look a bit larger too.

Tools You Need to Measure for Blinds

Gather these before you start:

Steel measuring tape that reads to the nearest 1/8 inch. A flimsy or stretchy tape can throw off numbers.
Pencil and notepad (or your phone) so you can record each window clearly. Label which room and which opening if you have more than one.
Step ladder if you have high or hard-to-reach windows so you can measure safely and accurately.

A second person can help hold the tape for wide or tall windows, but it's not required.

General Measurement Rules

A few rules apply no matter which blind type you're ordering:

Always record width first, then height (W × H). Width is the left-to-right dimension; height is top to bottom. Saying "height by width" or mixing them up is the most common measuring mistake.
Measure to the nearest 1/8 inch. Experts and manufacturers recommend this for custom blinds so the factory can cut precisely.
Do not make your own deductions for inside mount. Send the exact measurements; the factory will apply the right deductions. For outside mount, no factory deductions are taken.
Measure every window separately. Two windows that look the same can differ by a quarter inch or more.

How to Measure for Vertical Blinds

Vertical blinds work well on sliding doors and tall or wide windows. The overlap for an outside mount is a bit larger so the track and vanes have room to operate.

Inside mount:

Measure the width in three places: top, middle, and bottom of the opening. Use the narrowest of the three so the blind won't bind if the frame isn't perfectly square.
Measure the length on the left, center, and right. Use the shortest measurement.
Write down the exact numbers to the nearest 1/8". Don't subtract anything; the factory will apply the right deductions.

Outside mount:

Decide how much of the wall or trim you want the blind to cover.
Plan to overlap the window opening by at least 3" on each side (6" total in width).
Mount the headrail at least 3" above the top of the opening so the brackets and hardware fit.
Give the exact size you want covered; no deductions are taken for outside mount.

Double-check that the first number is width (left to right) and the second is length (top to bottom).

For full instructions, stacking options, and diagrams, see our How to Measure for Vertical Blinds page.

How to Measure for Mini Blinds

Mini blinds use narrow slats and are a common choice for standard windows. Once you know inside vs outside mount, follow these steps.

Inside mount:

Measure the width in three places: top, middle, and bottom of the window opening. Use the narrowest measurement so the blind won't rub if the frame isn't perfectly even.
Measure the length on the left, center, and right. Use the shortest of the three.
Record everything to the nearest 1/8". The factory will apply the deductions (typically 1/2" for inside-mount mini blinds); don't subtract anything yourself.

Outside mount:

Measure the area you want the blind to cover.
Overlap the opening by at least ¾" on each side (1½" total width).
For height, allow at least 3" total overlap: about 1½" above the opening and 1½" below so the blind covers the window and the brackets have room.
If you have a windowsill, measure from where the top of the blind will sit down to the top of the sill. No factory deductions; the size you give is the size that gets made.

Always confirm that width is left to right and length is top to bottom before you submit.

For full details and a PDF guide, see our How to Measure for Mini Blinds page.

How to Measure for Faux Wood Blinds

Faux wood blinds are measured the same way as other horizontal slat blinds. Take width and length in multiple spots and use the smallest width and shortest length so the blind fits and operates smoothly.

Inside mount:

Measure the width at the top, middle, and bottom of the window opening. Use the narrowest of the three.
Measure the length on the left, center, and right. Use the shortest measurement.
Write down the exact numbers to the nearest 1/8". The factory will apply the standard deduction (typically 1/2" on the width for inside mount); send the raw measurements and don't deduct yourself.

Outside mount:

Measure the area you want the blind to cover.
Overlap the window opening by at least ¾" on each side (1½" total).
For height, allow at least 3" total overlap (e.g. 1½" above and 1½" below the opening) so the blind covers the window and the headrail has room.
If there's a sill, measure from where the top of the blind will be down to the top of the sill. No factory deductions; you're ordering the exact coverage size.

Before you order, check that width is left to right and length is top to bottom.

For full instructions and a PDF, see our How to Measure for Faux Wood Blinds page.

How to Measure for Horizontal Blinds

Horizontal blinds (wood, aluminum, and vinyl slat styles) all follow the same logic: take multiple width and length readings, use the smallest width and shortest length, and don't deduct for inside mount yourself.

Inside mount:

Measure the width at the top, middle, and bottom of the opening. Use the narrowest measurement so the blind doesn't stick in an uneven frame.
Measure the length on the left, center, and right. Use the shortest of the three.
Record to the nearest 1/8". The factory will apply the right deduction (usually 1/2" on the width for inside mount); send the numbers as measured.

Outside mount:

Measure the area you want covered.
Overlap the opening by at least ¾" on each side (1½" total width).
For height, allow at least 3" total (e.g. 1½" above and 1½" below the opening) so the blind fully covers the window and the mounting hardware fits.
If you have a sill, measure from where the top of the blind will sit down to the top of the sill. No deductions are taken; the size you give is what gets made.

Double-check that your first number is width (left to right) and your second is length (top to bottom).

For full instructions and a PDF, see our How to Measure for Horizontal Blinds page.

Double-Check Before You Order

Before you submit your order, double-check every window:

Width is left to right and height is top to bottom.
You've noted which windows are inside mount and which are outside mount.
You have a copy of your measurements (photo or written) in case you need them later.

When you're ready to install, our Window Blind Installation Guide walks you through the process step by step.