How To Measure Windows Step-by-Step

Measuring your windows correctly can help you order replacements that fit properly and perform as intended. With just a tape measure, a notepad, and a few simple techniques, you can accurately measure a window’s width, height, and depth. Our step-by-step guide explains how to measure your windows, avoid common mistakes, and decide when you to confirm your measurements with a professional.

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What To Know Before Measuring Windows

Before you get started with this task, we put together some terms you need to know to measure windows correctly.

Balance Frame Head Jambliner Jamb Lift Rails Sash Sill

These are spring-loaded, mechanical devices commonly found in single- and double-hung windows. They help to counterbalance the weight of the sash during opening and closing.

The window frame surrounds and supports the entire window system. A window frame consists of the head, jamb, and sill.

The window head refers to the central horizontal part that forms the top of the window frame.

This is the strip on each side of a window that helps fit the window sash.

These are the main vertical parts that form the sides of the window frame.

The lift refers to the handle for raising the lower sash.

For double-hung windows, there are three rails. The upper and lower rails create the horizontal piece of the window sash. The middle rail, or the check rail, is where the bottom part of the upper sash meets the upper part of the lower sash.

This is the movable part of a window that holds the glass.

This term refers to the central horizontal part that forms the bottom of the window frame.

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What Tools Do You Need To Measure Windows?

You might already own most of the tools needed to measure your windows. Gathering the right tools before you start helps ensure you can measure your windows accurately—and without making additional trips to your tool shed. Here’s a list of the tools you will need for this task:

• Level: Use a level to check whether the window opening sits properly, especially if you suspect uneven framing. A level can also help identify issues that affect how a replacement window will fit.
• Notepad or measurement worksheet: Write down each measurement as you take it for each window. We suggest labeling every window by room or location so you can match the dimensions to the correct opening.
• Pencil or pen: Keep a pencil or pen handy so you can record measurements immediately. Having notes can help prevent errors and eliminate the need to measure the same window more times than necessary.
• Step ladder: Use a stable step ladder if you need extra height to reach the top of a window opening safely. Always place the ladder on a level surface before climbing.
• Tape measure: Use a sturdy tape measure with clear inch markings to record accurate window dimensions. A tape measure that extends at least 25 feet works well for most window types.

While not required, having an extra set of hands can improve measurement accuracy and make the process easier. Ask a helper to hold the tape measure when measuring large or hard-to-reach windows. We also recommend having a flashlight handy in case your windows have dark trim or are located in a dimly-lit room, as better visibility will help you measure properly.

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How To Measure Windows

Once you have your tools ready, start the measurement process from the interior side of the window, advises Kevin O’Connell, the host of This Old House’s television series. “First, measure the width of the window from the bottom, middle, and top,” says O’Connell. “If the window is parallel, the measurements should all be the same. Make sure to measure the full width of the jamb and not just from the stop bead.”

Here’s a more detailed breakdown of each measurement step below to accurately measure your home’s windows:

1. Width: First, measure the window’s width. To do so, measure between the jambs at the window opening’s top, middle, and bottom. Measure from the surface of the jambs and not from the trim strips or parting beads. When you order a new window, use the smallest of these three width measurements for the dimensions.
2. Height: Next, measure the window’s height. Measure from the high point of the window sill at the surface closest to the inside of the window to the top of the opening. Take this measurement from the left side, center, and right side of the window. Again, use the smallest of these three height measurements when you order the new window.
3. Depth: Finally, measure the depth of the window opening. There must be at least 3 1/4 inches of opening depth between the inside window trim and the outside blind stop strip. This much area is necessary to accept the depth of the replacement window. If your window features any pulleys or parting strips, ignore them when measuring depth dimensions since the installer will remove them.
4. Squareness: Finally, O’Connell recommends checking the squareness of your window by measuring the diagonals from corner to corner. If the measurements match, the window is square.

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How Do I Find a Local Window Installer?

Once you have the measurements and know what type of windows you want, it’s time to figure out installation. While you can install windows yourself, we found in our ride-along with Renewal by Andersen that many variables can quickly complicate the job, like having to spot-repairing window frame rot. Based on our experience, we recommend hiring a professional window installation company rather than trying this project yourself.

Learn more about our top recommendations for professional window replacement below.

Champion Windows

Champion Windows opened its doors for business in 1953 and now provides vinyl windows and various home exterior products across 25 U.S. states. The company’s ColorBond paint uses nontoxic heat-reflective materials to maintain cool temperatures in your home. Champion also offers various wood grain finishes, like cherrywood, dark oak, and English oak.

In addition, Champion Windows provides door replacement, siding, sunroom, and patio services. The company builds all-season and three-season sunrooms with custom-shaped windows tailored to your home’s style.

• Average price per window: $559
• Warranty: Limited lifetime

Castle Windows

Castle Windows has offered high-quality replacement windows, doors, roofing, and siding services since 1977. The company provides accessible windows with double weatherstripping for durability. Castle creates made-to-order vinyl windows rather than offering specific lines. 

The company includes a transferable lifetime warranty covering all window parts, including glass and screens. Castle’s warranty also covers workmanship with free labor on all services—a standout compared to standard manufacturer warranties.

• Average price per window: $487
• Warranty: Transferable limited lifetime

Renewal by Andersen

Andersen is one of the most recognizable names in the window industry. Its subsidiary, Renewal by Andersen, offers customized replacement windows with white-glove installation services, meaning it handles the entire installation process, from start to finish. In addition, you can schedule a free consultation with a window specialist to determine your window needs and specifications.

Renewal has eight different window styles, including specialty shapes. In addition, you can select from various exterior colors, interior colors, glass types, hardware colors, and grille patterns. If you want standard Andersen windows, you can find the brand at popular retailers such as The Home Depot. Renewal by Andersen backs its products with a 20-year limited lifetime warranty on its glass and window frame material, two years on installation, and 10 years on hardware.

• Average price per window: $584
• Warranty: Limited lifetime warranty

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Our Conclusion

Accurate window measurements can help you choose replacement windows that fit correctly while simplifying installation. By following each step in our guide carefully, you can avoid costly ordering mistakes and approach your window replacement project with confidence. If you encounter an unusual window shape, a damaged frame, or an uneven opening, we recommend consulting with one of the professional brands we listed above before placing your order.

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FAQs About Measuring Windows

How accurately do you need to measure your windows?

According to window installation experts featured in This Old House, you should measure your windows to the nearest 1/16 inch for the best results. Even small measurement errors can lead to major installation issues, like poor fitment or air leaks. If you’re measuring windows yourself rather than a pro, we suggest double-checking every measurement before ordering your replacement windows.

How do you read window sizes?

When looking at window sizes online or in a retail store, you’ll see a four-digit figure. The first two numbers are the width of the window, while the last two numbers are the height.

How often should you replace your windows?

You should replace windows roughly every 20 to 30 years. However, this time frame depends on the materials used for your windows, the weather conditions where you live, and the windows’ functionality.

What mistakes should you try to avoid when measuring windows?

Avoid measuring only one point along the width or height of a window because uneven openings can produce different dimensions. Another mistake to avoid is rounding measurements up or down. Always record the exact numbers. Finally, check every measurement at least twice to reduce the risk of ordering the wrong window size.

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Our Rating Methodology

We back up our window ratings and recommendations with a detailed rating methodology to objectively score each provider. We conduct research by speaking with company representatives, browsing product selections, analyzing window line specifications, and conducting focus groups and consumer surveys. We then score each provider against our review standards for window variety, value, trustworthiness and transparency, window features, and customer service to arrive at a final score on a 5-point rating scale.