How to Measure Windows for Blinds Without Getting It Wrong (2026 Step-by-Step)

Installing blinds by measuring your windows might seem like a simple process – until you order a beautiful blind to fill the space and they are either too small to stretch across the glass or too large to fit inside! If you’re a homeowner, it is undoubtedly one of the most frequent (and most preventable) errors made by people in the act of purchasing window treatments online.

The good news? Actually, it is very easy to get it right – if you just know where, how and what numbers to record. Whether you want to go for an inside mount, an outside mount or covering a tricky arched or bay window, this guide will take you step-by-step along the way.

At the end of that process, you’ll know how to measure blinds like a pro – and you’ll be able to fit your blinds from BuyHomeBlinds.com the first time.

Why Getting Your Window Measurements Right Matters More Than You Think

An incorrect measurement does not just equate to a blind that looks wrong. It means:

• Gaps to the side allowing light to enter and privacy to be lost
• Blinds that do not fit properly due to the blind’s width
• Again and again things which waste time and sometimes money
• A month of woes when it’s just a simple upgrade

There are three main reasons why most measuring is incorrect – they’re measuring in the wrong spot, they only measure one thing, and they mix up inside mount vs outside mount requirements. This guide resolves all three of these issues.

Step 1: Decide on Inside Mount or Outside Mount First

Before you put your fingers to a tape measure, you are going to have to make one decision – as it will affect how you measure everything.

Inside Mount (Blinds Fit Inside the Window Frame)

Inside mount is a blind installed in the recessed window frame or flush with it. It provides a simple and clean appearance that a lot of interior designers prefer.

To be eligible for inside mount, your window needs:

• Window must be at least ¾” thick (most windows are, but check with the parent/guardian)
• A relatively square frame – windows that are out-of-square can cause fit problems.
• Sufficient blind width based on the type of blind you want (roller shades are narrower than cellular shades, for instance)

Outside Mount (Blinds Mount on the Wall or Trim Above the Window)

Outside mount refers to a blind’s installation around the window’s trim or wall. This is an appropriate option to use when:

• Your window frame is too shallow for an inside mount.
• You want the window to look bigger than it is really.
• You’re covering radiators, handles, or trim you are not wishing to see
• Maximum side gaps are desired 

Record the type of mount you will use prior to taking measurements. It establishes the measurements you’re taking and how you will share them with your retailer.

Step 2: Gather the Right Tools

You don’t need anything fancy:

• Metal tape measure only (fibre or plastic tape measures give inaccurate readings, always use a metal tape measure)
• Pencil and a piece of paper (or your phone’s notepads)
• A helper to manage large or tall windows (optional but helpful)

Always measure in inches. When your tape measure has both inches and centimeters, be certain you are reading the reading on the right side of the tape. Record each measurement as they’re taken, rather than memorizing it.

Step 3: How to Measure for Inside Mount Blinds

When measuring for the inside mount, measure inside the window opening – where blind will fit inside the window’s frame.

Measure the Width

Using a metal tape measure, measure the width of the window opening at three points:

• Across the top of the opening window,
• Across the middle
• Across the bottom

Windows are not always square, particularly older houses. Record all three measurements and use the smallest measurement. This will ensure that your blind will fit comfortably inside the frame without tangling.

Pro Tip: Make sure to not subtract from your inside mount width measurement. We are BuyHomeBlinds.com and do the basic deduction for you to assure proper clearance and smooth operation.

Measure the Height

Determine the window opening’s height at three different locations:

• Down the left side
• Down the center
• Down the right side

Repeat as above with the smallest measurement. Record the measurement, there is no need to add or subtract any numbers.

Record Your Dimensions

Write them as: Width x Height (the industry standard is always width first).

Step 4: How to Measure for Outside Mount Blinds

The outside mount allows you to control the degree to which the window (and adjacent wall) is covered by your blind.

Measure the Width

Determine the distance the blind should travel beyond the opening of the window. The general recommendation is 1.5 to 3 inches on each side of the window frame – so add 3 to 6 inches to your actual window opening width. This overlap removes side gaps and provides the most light blockage.

When measuring for complete window trim, measure between the outside edge of the trim on one side of the window and the outside edge of the trim on the other side and add the overlap you want.

Measure the Height

To see how tall the blind will be, start at the point where you would like the top of the blind to hang (typically 2 to 4 inches above the window frame or trim) and work down to the point where you want the bottom of the blind to land. Per usual, most people install outside mount blinds at the windowsill or 2 to 4 inches below.

Design Tip: Place your blind at least 4-6 inches higher than your window frame, and you’ll make windows feel taller and ceilings will look higher. It’s a simple technique interior designers use routinely.

Record Your Dimensions

Write them as: Width x Height, and indicate these as your actual desired finished dimensions, which is the case with length or width x height (not like inside mount where you’re specifying the dimensions of the frame, but the actual dimensions you want).

Step 5: Measure Twice, Order Once

This is common sense stuff – which it is – and which works. Prior to order, please:

• Re-measure each window separately. Windows may vary by ¼ or more, even if they seem the same make and model.
• Please make sure you are interpreting the width vs. the height (width is always first).
• Verify the mount type that you recorded for each window.
• If you have to go inside mount, measure the depth of the window recess with a small ruler.

Two minute double check equals a 2 week return process.

Step 6: Special Window Types – How to Handle Them

Measuring for Bay Windows

Bay windows feature a prism-shaped glass pane with 3 or more angles. Take measurements for each section separately, width and height for each panel. Do not use one large blind for a bay window as single blinds per pane are more attractive and easier to use.

Measuring for Arched Windows

For arched windows, measure the total rectangular space enclosed by the arch (the width from the widest point to the widest point, height from the sill to the peak of the arc). Specialty arch shades are designed to fit this shape and your special retailer requires the rectangle bound box dimensions for a proper fit.

Measuring for French Doors

French doors must have blinds installed on the door and not the frame. If an inside mount look is desired, measure only the glass panel – width and height of the glass. If mounting on an outside door, carefully add overlap so that the blind does not block the door handle or door swing.

Measuring for Skylights

For skylight blinds, you must have a snug inside mount fit. When ordering, measure the inside of the skylight opening carefully and always specify the skylight angle (flat vs. pitched) because it will impact the mounting accessories included.