The best replacement windows improve your home’s comfort and energy efficiency, allowing you to save money and maintain a more stable temperature year-round.

Whether you use an installer or tackle the project yourself, precise window measurements are essential for an airtight fit, and influence the cost of a replacement window. Take notes on our replacement windows measurement guide to help prepare you for your next upgrade.

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Why Should You Measure Your Windows?

Accurate window measurements ensure your new replacement window fits in the opening. With your window sizes on hand, it’s easier (and more efficient) to shop with a window company in-store or over the phone. Measurements are also necessary for virtual installation consultations. After the window company receives the measurements, it will create a quote for your home improvement project with detailed window replacement cost estimates.

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How To Measure Your Windows

Measuring windows doesn’t require any special know-how. Here are some basic supplies needed for the job:

• Calculator
• Measuring tape
• Paper or notebook (or your phone’s notes app)
• Pen or pencil

Depending on the project size, you may need to record window measurements in multiple rooms. Use a new page per room and label each window based on its position, type, and location. (For example, the label could read: “Kitchen, left double-hung window.”) You also need to measure each existing window separately. Even if they share the same design, their measurements could be slightly different. 

Here’s a step-by-step breakdown of how to measure your old windows for a replacement project. 

1. Check Windows for Squares

First, determine if your windows are square, meaning each corner meets at a 90-degree angle. Standard windows vary in size, so knowing if they need to be square will make the selection process much more manageable. 

Begin measuring the windows inside your home and follow these instructions:

• Position the measuring tape in the top right corner where the horizontal and vertical trim boards meet. 
• Extend the measuring tape diagonally to the bottom left corner where the trim boards meet.
• Record the measurements on your paper or phone. 
• Repeat the same steps for the top left corner to the bottom right corner. 
• Record the measurements on your paper or phone. 

Compare your measurements and see if the numbers are within one-quarter inch of each other. If so, a standard replacement window will work in the opening. If the measurements are off by more than one-quarter inch, you’ll need to find a different replacement option. You can replace the window frame to accommodate a different style or choose new construction windows. With new construction windows, the full frame and window are replaced. 

2. Measure Window Width

Window manufacturers list their products with width-by-height measurements. You need these measurements to ensure a perfect installation. For width, measure in three places: the top, middle, and bottom of the windows. Measure the inside of each window jamb. (The jamb is the main vertical piece that forms the window frame’s outside.) Follow these instructions for a proper width measurement:

• For the top of the window, keep the window closed and measure the distance between the two top jambs. 
• Open the window and measure between the jambs closest to the window’s center.
• For the bottom, keep the window open and take your measurements between the left and right bottom jambs. 

3. Measure Window Height

To find the height, you need left side, middle, and right side window measurements. First, measure the head jamb (or header). The header is the top horizontal board that sits above the windows. Your height measurements will extend from the header to the bottom of the sill (where the sash rests when the window is closed). Don’t confuse the window sill with the window stool, which is the lower trim section that extends from the window into the room. 

Take your three measurements and find the shortest one. This measurement is the window opening height. 

4. Measure Window Depth

You won’t typically find a window depth measurement on new replacement windows because most homes don’t require it. But sometimes, the window depth is too narrow for the replacement, such as in modular or mobile homes. To find the depth, you’ll need to open the window and measure the sill space from the back of the trim to the front. At minimum, this space should be three and one-quarter inches for a replacement window to fit.

5. Measure for Window Screens

Homeowners have two options for window screen measurements: a replacement screen or a new screen. Here’s how to handle either scenario:

If you’re replacing an existing screen, measure that screen’s width and height. These measurements will work for your replacement. 

If you need to add a new window screen and don’t have one installed, take the following steps:

• Measure the space between the horizontal screen channels within the window frame. Subtract one-eighth of an inch from that measurement. This will be the screen’s width. 
• To determine the height, open your window and measure from the lip to the window sash. The window’s lip is the part that holds the screen in place. 
• Add one-eighth of an inch to the measurement for the window screen’s height. 
• Use these two measurements to find the correctly sized window screen.

6. Measure Storm, Double-, and Single-Hung Windows

Measurement instructions for storm windows brands are the same for double- and single-hung windows. Here’s an overview of the process:

• To find the height, measure from inside the trim at the top of the window to the sill (the bottom horizontal component of the window frame) in three areas: the left side, right side, and middle of the window. The shortest measurement represents the height of the window. 
• To determine the width, measure from the right jamb in the inside trim to the left jamb in the inside trim. The smallest measurement is the width. 
• Record your final width and height measurements and use them when shopping for windows. 

7. Measure Bay or Bow Windows

Due to the complex design of bay or bow windows, you must take individual measurements of each window pane. Here’s how:

• Measure the center of each window pane. Start with the middle window first, as it may differ slightly from the others. Measure horizontally from trim to trim or jamb to jamb. 
• To determine the height, measure the center of each panel vertically from trim to trim. 
• Check your notes to ensure you have width and height measurements for each panel. 

8. Finalize Your Window Measurements

Once you complete the window measurements, you need to finalize them. Take the shortest measurements for each window width and height and round down to the nearest eighth of an inch. For example, if your measurements are thirty-six and fifteen-sixteenths of an inch, round down to thirty-six and seven-eighths of an inch. If your measurements are already in eighth-inch increments, you don’t need to adjust them. Use these measurements to order your replacement windows. Make sure to record your measurements using width by height as replacement window measurements follow this order.

9. Order Your Replacement Windows

Place an order with your windows company of choice. If you plan on doing the window replacement yourself, you can pick out windows at a local hardware store or national retailer such as the Home Depot. Although these stores carry a small selection of replacement windows, the window sizes may not work with your installation. Picking windows that don’t match your measurements could lead to drafts, crooked windows, or shaky installation. We recommend ordering custom windows to match your exact dimensions if necessary.

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DIY vs. Professional Measuring

Finding correct window dimensions requires accurate measurements and calculations. While the task doesn’t require advanced do-it-yourself (DIY) knowledge, some homeowners may prefer a professional to handle the task. 

Certain types of windows, such as awning or casement windows, require additional planning for their installation. Since these windows open outwardly, you’ll need to ensure no obstacles hinder their functionality. Other complicated window installations, such as bay or bow windows, could prove too difficult for a DIYer. If you’re not confident in your ability to calculate the right size or hesitating about completing the installation yourself, have a professional do it instead. 

Look for professional window installers in your area that carry the replacement products you prefer. Check their selection of window materials, styles, and customization options. Most professional window installers offer free replacement project quotes. Complete the online form or call to make an appointment with an installer. Some window companies have dedicated showrooms throughout the country for in-person visits. Local window installers may have in-person displays and distribution centers so you can see and feel the replacement windows. 

Typically, the company representative will review your current windows and check the condition of your window frames. The rep will also take window measurements for the quote process. Some installers will send a separate technician to get your final measurements before placing the order. During your in-home visit, the rep will discuss estimated costs, the styles and types of windows available, and your warranty details. We recommend shopping for windows with warranties that are at least 10 years (preferably 20 years). Additionally, look for warranties that will cover replacement parts.

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Our Conclusion

Homeowners should take their time when measuring windows to ensure accurate dimensions. Whether you are going to install replacement windows on your own or hire a professional, accurate dimensions are essential. Record measurements for each window, even if multiple windows have the same style. Windows with the wrong measurements will need to be sent back to the manufacturer. This could result in added time and expenses to your window replacement project. If you’re uncomfortable taking the measurements yourself, a local windows installer can handle the task for you. Installing new windows with accurate measurements will brighten your home and lower your energy bills.

Use the below tool to find reputable window installers in your area, or check out our guide to affordable window replacement companies to keep costs low.

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FAQ About Measuring a Window

What is the difference between a replacement window and a new construction window?

The main difference between a replacement window and a new construction window is that a replacement window uses an existing frame, while a new construction window requires the replacement or removal of the current frame. 

Do replacement windows need to be the exact same size?

Yes, you’ll want your replacement windows to be the same size as your current windows. Matching the size ensures a tight installation with no air leaks or other issues. New windows should have the same shape, size, and functionality as the old ones. 

Can you just replace the windows and not the frame?

Yes, you can replace just the window if your frames are in good condition. This option is known as a pocket window replacement. However, if your window frames are in poor condition, you’ll need a full frame installation instead. This option is known as a pocket window replacement. 

What is the best time of the year to replace windows?

Spring or early summer is the best time of the year to replace windows since the weather is still warm. Since your home would be exposed to the weather during an installation, it’s best to choose a time when the temperatures are comfortable.