How to measure windows

We’ve got the step-by-step instructions you need to measure your windows — a great first step in the window shopping process.
A contractor measures a window while talking with a homeowner.

If you’re thinking about replacing your windows but aren't quite ready to call a contractor, measuring your windows can be a first step to more informed shopping. Let's dig in!

Getting started

Before you start, you’ll want to gather a tape measure, pen, and paper. It can be useful to have a helper too, especially if you have large windows.

Tip: To help you track dimensions and make other necessary assessments, download one of our measurement guides, like this one for the 400 Series.

You also need to understand a few different parts of a window before taking measurements. These include:

  • Jamb: The pieces forming the top and vertical sides of a window or door frame.
  • Head (or head jamb): The horizontal piece forming the top of a window or door frame.
  • Sill: The horizontal piece forming the bottom of the window or door frame. Here’s the equipment you’ll need:

1. Measure to see if your windows are square

The first thing you’ll want to do is check and make sure your windows are in square alignment. This means all four corners meet at 90°. To do this, you’ll take two diagonal measurements from inside your home.

  • Start in the upper left-hand corner where the head meets the side jamb and measure to the lower right-hand corner where the side jamb meets the sill. Measure to the nearest 1/8 inch.
  • Next, measure from the upper right-hand corner where the head meets the side jamb and measure to the lower left-hand corner where the side jamb meets the sill. Again, measure to the nearest 1/8 inch.

If the measurements are within 1/8 inch of each other, your windows are square.

Why measure windows for square alignment?

It’s useful to understand whether or not your windows are in square alignment for several reasons.

  • If your windows are square, you might be able to use an insert replacement window, which fits into an existing window frame. This type of replacement window can help you save money, preserve your existing trim, and offer other benefits.
  • If your windows are out of square, you’ll need to consider sizing adjustments when ordering new windows. Repairs to the rough opening might be needed as well. For this job, you’ll want an experienced window contractor. Find one in your area through our Certified Contractor program.

2. Measure window width

Take this measurement from inside your home. Measure from jamb to jamb or drywall to drywall if you don’t have trim around your windows. Round down to the nearest 1/8 inch, and measure from three different spots: near the top, at the center, and near the bottom.

Use the smallest measurement as your width dimension.

Tip: As you're measuring, you might want to note:

  • The types of windows in your home (double-hung, casement, picture, etc.).
  • The number of windows you have in your home.
  • Any damage to or issues with your windows.

This information will help you communicate your needs when window shopping and get a good sense of the cost right off the bat.

A man measures the width of a window from jamb to jamb.

3. Measure window height

Again, measuring from inside your home, you’ll measure your window’s height from head to sill. Measure to the nearest 1/8 inch in three different spots: left, center, and right

Use the smallest measurement as your height dimension.

A man measures the height of a window inside a home.

With the measurements you’ve taken, you’ll have the basics needed to begin comparison shopping. If you're considering replacing windows yourself, check out the FAQ section below.

FAQs about DIY window replacement

Ready to take the next steps in your window selection process? We’ve got the tools you need to start envisioning your perfect windows. We can also help you find the right professional.

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