The DIYer’s Guide to Measuring Replacement Windows


Replacing old windows with new energy efficient windows is a great way to weatherproof your home and increase resale value. While many replacement window manufacturers offer professional installation services, as a skilled home improvement you can save between $ 150 and $ 600 per window by tackling the project yourself. Measuring replacement windows is not difficult, but it requires precision. The tips here will help you get the exact dimensions you need to install custom-fit windows.

Are your window frames in shape?

Before looking for replacement windows, you need to make sure that you can install them in your home. Replacement windows are more DIY-friendly than new windows, but not suitable in all situations. The condition of the wooden frame around the existing window must be in good condition as the replacement window is attached to the frame. If the wooden frame shows water damage or rot, it is structurally not strong enough to support a replacement window and a newly built window will need to be installed instead. However, if the existing frame is solid and “square” (detailed below), you should be ready.

RELATED: Top 6 Reasons to Install New Windows

1. Record your measurements.

Accuracy is critical when measuring replacement windows. Ordering the right windows the first time will save you time and money. Whether you are replacing a single window or every window in your house, carefully record the measurements as you take them. You can download a measurement worksheet like this one from, or you can simply write the measurements on notebook paper or a computer file. If you order several windows, mark each measurement set with its exact position, e.g. B. “North Kitchen Window” or “Southwest Master Bedroom Window”.

When measuring replacement windows, check that they are square


2. Check for square.

Your first measurement will determine whether the existing window frame is square. This means that all four corners have an angle of 90 °. Installing a replacement window in a frame that is not square (that is, crooked) can prevent a window from opening.

Keeping the tape measure taut, measure diagonally from the inside of the upper left corner of the window (where the horizontal and vertical trim boards meet) to the inside of the lower right corner (where the horizontal and vertical trim boards meet) and record that Measurement. Then measure in the other diagonal direction and record this measurement as well. If the two numbers are no more than 1 cm apart, the window frame is suitable for a replacement window. If the difference is greater than ¼ inch, opt for a newly built window.

3. Measure the window width.

Measure the width of the existing window in three places – bottom, middle and top. When measuring the width, position the tape measure against the window jamb on either side. The jamb is the vertical board along which the outer frame of the window (sash) slides when you open and close the window.

  • For the bottom measurement, slide the window up and measure from post to post at the lowest point.
  • For the middle measurement, slide the window all the way open and measure from post to post as close to the center as possible.
  • To take the top measurement, close the window and measure from post to post at the highest point.

Record each measurement, then circle or otherwise mark the shortest measurement.

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4. Measure the window height.

Measuring to determine the correct height for the replacement window is similar to measuring the width. Again, you need to take three measurements, each stretching from the top horizontal plate above the window (head post) to the lower threshold. Take a measurement on the far left, one on the far right, and one in the middle of the window.

To measure correctly, understand that the threshold is the horizontal plate on which the bottom of the window sash rests when the window is closed. The threshold is not the horizontal cladding that extends into the room. While many refer to this board as the “windowsill”, it is actually the “stool” of the window. You have to open the window to access the actual threshold.

Again, record all three measurements and circle or otherwise mark the shortest.

5. Measure the window depth.

When ordering a replacement window, the depth measurement is not taken into account, and in most traditional homes, the window depth does not matter. The only time there is a problem is when the depth is too narrow to accommodate a replacement window, such as a window. B. some windows in mobile homes and modular houses. To determine the depth, open the window and measure the sill space from the front panel to the rear panel. This gap should be at least 3¼ inches for a replacement window to fit.

6. Round the numbers down.

After you measure each window, take the circled or highlighted measurements (the shortest in each set) and round them down to the nearest 3/4 inch. For example, if one of your measurements is 32-15 / 16 inches, round it off to 32⅞ inches. If your original measurement is already in 8 inch increments, do not change it. You will use these final numbers to order your new replacement windows.

7 tips for measuring for replacement windows


7. Buy custom replacement windows.

You can purchase replacement windows online or at your local hardware store. While home improvement centers sometimes have a small selection of replacement windows on their shelves, it is usually better to have a service technician order a custom order based on the exact measurements. A window on the shelf is a narrow size, but rarely the perfect size.

Custom orders are best as window fabricators subtract ¼ “from both height and width dimensions and then design the new replacement window within 3/4” of those dimensions for the best possible fit. Ordering online is also a good option as the manufacturer uses your measurements in the same way to construct your windows.

Are you considering a window replacement?

Find licensed window installers near you and receive free, no-obligation estimates for your project.

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