How to Measure Replacement Windows

Professionals measuring for replacement windows

There are a lot of benefits to replacing your windows. From upgrading the aesthetics of your home to improving insulation and decreasing energy costs, new windows can elevate your home and provide a better living experience. But you have to choose the right replacement windows, including the correct size. 

If you plan to order custom replacement windows, it is important to properly measure your existing windows. Thankfully, you do not have to be a contractor or window expert to learn how to measure a window’s size for replacement. Replacement window measurements are fairly simple to determine once you know what to look for. 

We’ve compiled a complete guide on how to measure windows to make the custom ordering process as easy as possible. Keep reading for everything you need to know to measure your windows for replacement. 

Tools to Measure Replacement Windows

Before you can measure your windows, you will need a few supplies. You probably already have these items around the house, making the process even easier. 

  • Quality tape measure: You will need a quality tape measure that is longer than your current windows. 
  • Writing Utensil: Have something handy to write down your measurements, make marks on your tape measure, or mark on the window frame as you measure. 
  • Notepad/Paper: Keep a notepad or piece of paper nearby to write down your measurements. Be sure to store this paper in a safe place to reference when ordering your custom replacement windows. 

Step 1: Check Your Window Frames

If you want to replace your current windows, you have to first check your window frame. Replacement windows are intended to fit within the existing frame, but your window frame must be in good condition and have square corners. Over time, window frames can warp, and corners are no longer 90-degree angles. If this occurs, you may have to find another solution. 

First, check the condition of your window frames. Make sure the wood has not suffered water damage or rot. If these issues occur, your existing window frame may not support the new window. A structurally sound window frame is crucial to replacing your window instead of installing a new-construction window

Next, check for square alignment by measuring the window trim inside your house. “Checking for square” is the process of checking that all four corners of your existing window frame are 90-degree angles.

  • Use your tape measure to measure from the top right corner of the window frame to the bottom left corner. This diagonal should extend from where the trim meets in both places. Write this measurement down.
  • Next, measure from the upper left corner of your window frame to the bottom right corner, where the vertical and horizontal window trim meet. Write this measurement down.
  • The measurements you collected should be within ¼-inch of each other. If this is the case, your window frame is square, and the replacement window should fit within the existing frame securely. 
  • If your window frame is not in square alignment, you may have to replace the frame before purchasing new windows or purchase new-construction windows to resolve the issue. 

Step 2: Measure Your Current Window Width

Once you know your window frame can securely hold a new window, it is time to measure your existing window’s width. The width of your existing window is measured in three places, including the top, middle, and bottom of the window.

Do not include the trim when measuring for width. Remember to keep your tape measure inside of the window jamb on both sides when measuring for width. 

  • Raise the window sash to expose the jamb. 
  • Measure the top of the window inside the jamb from left to right. Write this measurement down. 
  • Measure the bottom of the window inside the jamb from left to right. Write this measurement down. 
  • Close the window and measure the middle of the window inside the jamb from left to right. Write this measurement down. 
  • Select the shortest measurement from the three you collected and record it as your window opening width.

Step 3: Measure Your Current Window Height

pro window installers measuring for replacement windows in kitchen
Young professional worker in uniform using tape measure, measuring window for installing blinds, while his aged colleague making notes. Construction and maintenance concept. Horizontal shot

The process for measuring the height of your current window is almost identical to measuring for width. Once again, you will take three measurements to determine your window height. For height, you will measure the left, right, and middle of the window. 

Do not include the trim of the window when taking measurements for window height. Check that you are measuring from the sill of the window instead of the stool. (For more information on the parts of a window, review the section below on important terms to know.)

  • Open the window to access the windowsill. 
  • Measure from the windowsill to the top of the window opening on the far-left side. Write this measurement down. 
  • Take the same measurement on the far-right side of the window and write it down. 
  • Take the same measurement again in the middle of the window and write it down. 
  • Select the shortest measurement from the three you collected and record it as your window opening height.

Step 4: Measure Your Current Window Depth

In most cases, you will not have to provide the window depth when ordering custom replacement windows. Window depth is only relevant if the opening for your window is too narrow, which may be the case in some mobile homes or modular homes. 

When measuring window depth, ensure your measurement is at least 3 ¼-inches.

  • Open your window. 
  • Measure from the front of the trim to the back of the trim across the sill space. 
  • This measurement is your window depth. 

Step 5: Round Your Measurements Down

While precision is key when measuring for replacement windows, you should still round your measurements down. Using a measurement that is slightly smaller than your current window can help during the installation process. 

It is much easier to fit a new window that is slightly smaller into the existing space, and adjust with shims and insulation, than it is to install a window that is too large. If your window is too big for your current space, you or your installer may try to force it into place and could damage the window or frame. 

  • Round all of your measurements down to the nearest 1/8-inch. 
  • Do not round down if your measurement is already in an eighth-inch increment. 
  • Record these numbers as your final window measurements. 

Step 6: Order Your Custom Replacement Windows

Once you are confident with your window measurements, it is time to order your replacement windows. At ClearMax®, you can order custom vinyl replacement windows by selecting your configuration and entering your window measurements. 

With custom replacement windows, manufacturers use the measurements you provide to deliver a window with the best fit. 

Ordering custom windows from ClearMax® is easy and fast. Keep the following information in mind when entering your window measurements to order custom replacement windows. 

  • Window width must be at least 10 inches and no more than 72 inches. 
  • Window height must be at least 10 inches and no more than 72 inches. 
  • You do not need to provide window depth. Just make sure your window depth is at least 3 ¼-inches. 

Tips for Measuring Replacement Windows

The six steps listed above will help you properly determine your replacement window measurement. However, if you have never measured for replacement windows before the process can seem daunting. 

We’ve learned some tips and tricks along the way that can help make the process simple and ensure you have accurate measurements. Keep reading for valuable information on replacement window measurements. 

Read through every step before getting started. 

It’s tempting to jump in and start measuring step-by-step. For the best results, though, you should read through the entire guide on how to measure windows before getting started. Having a solid foundation before measuring can help make the process smoother and provide more accurate measurements. 

Measure each window separately. 

If you are ordering multiple custom replacement windows, you must measure each window individually. It is important to collect exact measurements for each window you are replacing, even if you think they are the same size. Looks can be deceiving and windows can shift or warp over time. By measuring each window separately, you will have accurate measurements for your new replacement windows. 

Measure your windows multiple times. 

It never hurts to measure again or have someone else verify your measurements. When purchasing custom windows, you are investing in your home and you want to be sure your windows are the right fit. It’s always best to measure twice (or three times) and order once! 

Write down all of your measurements. 

Even if you have the best memory in the world, it is crucial to write down each measurement after you take it. Just a fraction of an inch can make all the difference when ordering custom windows. Keep a notebook and writing utensil handy as you measure for replacement windows to help stay as accurate as possible. If you are measuring multiple windows for replacement, be sure to clearly label each window’s measurements. This will make it much easier to install the right windows in the proper places once your custom windows are delivered. 

When in doubt, ask for help.

If you are not confident in the accuracy of your window measurements, it may be best to enlist the help of a professional. While you can certainly measure for and install replacement windows without a professional, hiring a window installer is the best way to make sure everything is done properly. 

Important Terms to Know When Measuring for Windows

You will come across some very specific terminology when learning how to measure windows and ordering your custom replacement windows. Understanding these terms is key to ensuring you measure your current windows in the right places and record accurate measurements.

Pocket Replacement

This term refers to switching out your current window for a new window of the same size. This is the easiest method for window replacement because it does not require any adjustments to the window frame or alterations on your home. You can use your existing window trim as well. 


The framework that surrounds your window is called the frame. The window frame works as a support for the entire window, including a head, jamb, and sill. 


The horizontal part that makes up the top of the window frame is known as the head. 


Window jambs are the vertical parts that make up with sides of the window frame. 

Rough Opening

A rough opening and a window frame are two different terms. While a window frame has a header, sill, and trimmers, a rough opening is simply the framed opening for your window. 


The window sash holds the panes of glass together and moves up and down or left to right. The window sash is fitted within the window frame. 


Wooden shims can help your replacement window fit within the rough opening. These flat pieces of wood have a taper and provide a tight fit for your window to ensure it does not shift when it is in operation. 


The sill makes up the bottom of the window frame. When the window is closed, the sash rests along the sill instead of the trim board. The sill helps keep water out of the bottom of the window, preventing it from leaking into your house. When measuring from the sill, always take your measurements from the uppermost point of the slope of the sill. 


Window stools and windowsills are often mixed up. The stool is located inside your house and is not actually part of the window. The flat surface extends inward from the bottom rail of the sash. 


Window trim is used to cover gaps around the frame and add decorative elements to a window. 

Order Custom Replacement Windows from ClearMax®

As window and door experts, ClearMax® can help ensure you order the right custom windows for your home. If you have any questions during the window measuring or ordering process, their qualified support team is ready to help. Simply send an email or call to ask your question. If you need a special window, you can always place your order by phone. 

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