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How To Measure Windows Correctly

Written by Joe Roberts

Published on December 8, 2022


How To Measure Windows Correctly

Properly measuring a window can be tricky business. Read our guide to measuring the width, depth, and height of a window frame before buying a new window.

To provide you with the most accurate and up-to-date information, we consult a number of sources when producing each article, including licensed contractors and industry experts.

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Measuring a window frame might seem simple enough, especially for home improvement veterans, but it’s trickier than you might think. And getting incorrect measurements can result in unfit replacements, drafty seams, and inferior energy efficiency.

If you’re replacing a window yourself, taking careful height, width, and depth measurements of the old window is the only way to avoid these subpar outcomes. And even if you hire professional window contractors instead of going the DIY route, you should still measure the window beforehand to start getting estimates of how much you’ll pay for the job. 

Luckily, a tape measure, a pencil, and a pad of paper are all you need to take precise measurements of a window frame. Keep reading once you’ve got all these items on-hand and let’s measure that window!

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How to measure a window

Step 1: Measure the window’s width

Measure a window’s width by spreading your measuring tape between its left and right side jambs. 

Width is the first measurement you should take of your window, and you should measure the width in three different places: across the top of the window, across the middle of the window, and across the bottom of the window. 

You should measure in these three spots because wood and vinyl window frames can expand and shrink unevenly over time, so their width might differ slightly depending on where they’re measured. Steel, aluminum, and fiberglass window frames don’t expand as much, but you should still measure the width of these windows three times to be safe.

Start taking your first measurement by pressing the tab of your measuring tape against the right side of the window jamb, then spread the tape horizontally across the glass to reach the left side. Keep the tape as straight and level as possible to ensure the measurement is exact. When your tape is spread tightly between the two jambs, check the width, write it down, and move down to the next spot.

Be sure you don’t include the window trim in your measurements or they’ll be too large. This might require you to carefully pry off the trim before you get started, though this isn’t necessary if you can see the far edges of the jambs without doing so.

Once you’ve taken the width measurements of your window opening, circle the shortest one. That’s the one you should look for in a replacement window to guarantee it will fit nicely. You can then proceed to the next step, though it wouldn’t hurt to double-check your work. Windows are expensive, so being overly cautious can help you avoid costly mistakes. 

Step 2: Measure the window’s height

Measure a window’s height by spreading your measuring tape between the head jamb and the window sill. 

After measuring your window’s width, you’ll also need to measure its height in three places: along its left side, along its right side, and along its center.

To measure the height of the window, spread your measuring tape all the way from the window sill (the bottom of the window’s frame) to the header (the top of the window’s frame, also known as the head jamb). This top to bottom length is your window’s height. 

Like you did while measuring the width of the window, keep your measuring tape taut at all times and exclude your window’s trim from your measurements or they’ll be a little off.

Once you’ve got three height measurements for your window, circle the smallest one. That’s the height you want the new window to be. With this done, you’re ready to move on. 

Step 3: Measure the window’s depth

Measure a window’s depth by spreading your measuring tape from the window’s front to its back across the sill and sides. 

Depth is the final dimension to measure, though it’s often the least important. Generally, if your window opening is at least 3 ¼ inches deep, it can hold most standard window sizes, so you don’t have to get a replacement window that’s the exact same depth as your old one. If your window opening is more narrow than this, though, you’ll have to look for its specific depth. 

Like the other two dimensions, you should measure your window’s depth in several different spots. To start, open your window and press the tab of your measuring tape against the bottom right corner of your window frame, and then spread the tape forward into the space on the other side. This front-to-back distance is the window’s depth.

Take depth measurements in several places along the window’s sill and one of the sides, then write down the smallest number. That’s the depth the replacement window should be if you want one that’s exactly like the old one.

What to do once you have measured your window

What if you’re replacing multiple windows?

You should carefully measure every window you need to replace, even if they’re supposedly the exact same size. Don’t rely on the measurements from a single window when buying replacements for all of them. This is also true if the windows are directly next to each other in the same wall and their edges perfectly line up. You can never be too careful when getting replacement windows. 

Shop for your replacement windows

Once you’ve got the measurements for your window’s three dimensions, it’s time to shop around for its replacement. This could be as simple as going to the hardware store to pick out an option you like, but if the store isn’t carrying any windows that fit your taste or your old window is an unusual shape or ratio, you might need to get the replacement custom-made.

Additionally, you can use this opportunity to change the type of window in your opening. Say you have a double-hung window but you want to get a casement window instead. That’s an option, though it’s a little more difficult to change window types. 

Window openings aren’t one-size-fits-all fixtures, so getting a different style of window might require a structural change to the opening. If you want to get a different type of window for the replacement, we highly recommend working with a pro instead of doing it yourself. 

If you’re determined to handle your installation yourself, you may just want to get the exact same type of window as your old one for ease. That way, you’ll know that the window opening can properly accommodate the replacement. 

Find a professional installer

If you want to ensure your window is replaced properly, hire a professional to install it for you. This will cost you a bit more than doing the work yourself, but it will safeguard your home from the various risks associated with amateur window installation like drafts, shattered glass, and unopenable sashes. 

How much does window replacement cost?

It generally costs between $550 and $750 to get one window professionally replaced, though this home improvement project can cost several thousand dollars on the high end. There are several factors that make this range so wide. These include size, window type, what kind of glass you get, and what material the window’s frame is made from. 

To get a complete breakdown of how these factors determine your final price, read our window replacement cost guide

Window measurements made simple

Measuring a window is easy to do once you know how, but it’s a little more complicated than many homeowners assume when they do it for the first time. But now that you know how to find a window’s height, width, and depth, you’re ready to go about replacing any window in your home.

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Written by

Joe Roberts Content Specialist

Joe is a home improvement expert and content specialist for Fixr.com. He’s been writing home services content for over eight years, leveraging his research and composition skills to produce consumer-minded articles that demystify everything from moving to remodeling. His work has been sourced by various news sources and business journals, including Nasdaq.com and USA Today. When he isn’t writing about home improvement or climate issues, Joe can be found in bookstores and record shops.