How to Measure for Replacement Windows Like a Pro

Learning how to measure replacement windows is crucial to guarantee your new windows fit in your existing opening. Measuring a window is fairly easy, but it requires accuracy to ensure you estimate the correct size before ordering replacement windows. Failing to obtain accurate window measurements can lead to inaccurate sizing of replacement windows. As long as you know how to read a tape measure, you just have to know which parts of the windows to measure.

Below, we’ve explained the different parts of a window and how to accurately measure for replacement windows.

Parts of a Window

To better understand the steps for measuring a window for a replacement window, you must learn the different parts of a window frame.

• Head: The head, sometimes called the header, is the top horizontal portion of the window frame. It spans the distance between the two jambs and provides a solid surface for the upper sash to rest against.
• Jamb: The jamb is the vertical portion of the window frame. A window frame has two jambs, one on the left and one on the right.
• Sill: The sill is the horizontal portion of the frame located at the bottom. Like the head, it spans the distance between the two jambs and provides a solid surface for the lower sash to rest against.

Want more home reno project tips and inspiration? Sign up for our free daily newsletter for the latest how-tos, reno guides, and more!

How to Measure Windows

The main measurements you must be concerned with when measuring a window frame to fit replacement windows are the width and the height. We’ll start by measuring the width, as the standard for listing replacement window dimensions includes the width first, then the height.

After finding the width and height, it’s important to check that the window frame is square. It can be difficult to fit a replacement window in a frame that’s out of square.

Measure the Width

To measure the width of the window, follow these steps:

1. Locate the inside edges of the left and right jambs. Be sure to ignore things like jamb liners and stop bead trim to get an accurate measurement of the distance between the jambs.
2. Measure across the top, middle, and bottom, then use the smallest of the measurements as your final measurement.
3. To ensure an accurate measurement, hold your tape measure level.

Measure the Height

To measure the height of the window frame, follow these steps:

1. Locate the head and the sill. When finding the sill, be sure to measure from the sill and not the stool, which is the slightly raised portion on the interior of the window.
2. Place the tape measure on the sill against the edge of the stool and measure up to the jamb. Measure the left, middle, and right, then use the smallest measurement.
3. To ensure an accurate measurement, hold the tape measure plumb.

Check for Square

After obtaining the width and height, you must check the window frame for square. Replacement windows have some slight tolerance to account for out-of-square window frames, but being too far out of square will result in a non-functional window. Typically, most replacement windows can still be installed if the window frame is less than 1/4 inch out of square.

To check a window frame for a square, measure from the bottom left corner to the upper right corner, then repeat this process on the opposite corners, being sure to measure from the exact same position. If the window frame is square, these numbers will match. If not, they will not match.

Consult the manufacturer’s instructions for your replacement windows to see how much tolerance there is for out-of-square window frames. If your window is beyond these parameters, use a level to check the head and sill for level, then check that the jambs are plumb. Take the necessary steps to square the frame.

Tips for Measuring Windows

• Measure each dimension at least twice. The last thing you want to do is order replacement windows that won’t fit your frames, as they can take a long time to arrive and be difficult to return.
• Write everything down. Don’t rely on your memory to remember the window frame dimensions, especially if you’re replacing multiple windows.
• Keep a record of your order. Avoid communicating your dimensions verbally and keep a record of the replacement window dimensions you ordered to ensure you receive the correct sizes.
• Opt for energy-efficient replacement windows. Replacement windows aren’t cheap, but choosing energy-efficient models can help you save money over the long term.
• Don’t install replacement windows inside compromised frames. If a window frame is far out of the square or shows signs of termite damage, rot, water damage, or other damage, this must be remedied before installing your replacement window.