How to Measure Windows for Replacement

Windows can have a long shelf life, and most last between 15 and 30 years. However, you may need a replacement if you’ve bought an older home or a window was damaged. To replace them correctly, you’ll need to know the exact dimensions of your existing windows. While measuring windows might seem easy, there are numerous pitfalls you can fall into, especially if you’re new to home improvement. This article will make getting replacement windows a breeze by reviewing everything you need to know about finding the exact window sizes. 

Obtaining Window Measurements

Getting accurate measurements for your old window ensures your replacement fits correctly. If even the smallest measurement is too short, the new window will be too small, creating drafts leading to higher electrical bills. Conversely, if you overmeasure, your new window will be too large for the existing window frame. If this happens, you’ll need to remeasure the space, return the ill-fitting window, and then purchase a new one, wasting time and money on your window replacement project. 

Obtaining Width Measurements 

The first and easiest measurement you’ll want to get is the window’s width. To do this, you’ll need to check the window’s width in three places, the top, middle, and bottom.

1. Start at the top right side of the window. 
2. Extend the tape measure horizontally across the window from the inside of the jamb until it reaches the left side. The jamb of a window is the vertical part of the window frame. 
3. Repeat this process for the middle and bottom parts of the window. 
4. To expose the jamb at the bottom of the window, you’ll need to move the sash upward to the top, then measure in the window opening. The sash is the internal, moving part of the window that holds the glazing (glass). For standard vinyl windows, the sash will slide up and down; for awning windows or casement windows, it will swing outward.

Remember the following tips when measuring window widths: 

• If there are different measurements for every section of the window, always use the shortest measurement. 
• If a measurement seems off, double-check it. It’s always better to get exact measurements before purchasing a new window.
• Don’t include the window frame or header in the measurements.

Obtaining Height Measurements

Next, you’ll need to find the height of the window. Like when measuring the width of a window, you’ll want to measure its length three times, on the left, center, and right. To do this, follow these steps:

1. Start in the top right corner, inside the window frame.
2. Extend the tape measure straight down to where the window sash meets the sill.
   • Remember that the sill is not the horizontal board that extends along the bottom of the window; that is the window’s stool. The window sill is the base of the window frame on which the sash rests. You’ll have to open the sash to access the sill at the bottom of the frame. 
3. Repeat this step for the center of the window and the left-hand side. 

Keep the following tips in mind when measuring the height of a window:

• Like with the width of a window, always use the smallest of the three measurements when determining the window’s length. 
• Do not include the window stool or head jamb in the window’s height measurement. The head jamb is the top portion of the window frame. 

Obtaining Depth Measurements

Measuring a double-hung window’s depth is not always required for installing replacements. However, it can be needed if your walls are abnormally thin – trailer homes often run into this problem. To measure a window’s depth, you must: 

1. Open your window sash. 
2. From within the window trim, run the tape measure from one side of the window frame to the other. 
3. Repeat this process thrice each for the frame’s left, right, and base. 

Tip for finding window depth measurements:

• Ignore all internal components of the frame, like parting strips and blind stop strips, as these will all be removed in the window replacement process. 

Measure Square 

The final step in checking your window’s measurements is to ensure that the frame is square. While you could use an equation to check and see if the existing measurements of the window square up, there is a much faster and easier way to do this. 

1. Begin in the top right-hand corner of the window. 
2. Extend the tape measure from the corner, where the jamb meets the top of the frame, diagonally to the bottom left-hand corner. 
3. Repeat this step from the top-left corner to the bottom right.
4. If the distance between both corners is within one-quarter of an inch of one another, the window is square. If not, the window must be squared before a new replacement is installed. 

Tips for Measuring Your Windows

Round Down for Simplicity

When measuring standard windows, if you’re getting complex measurements, round down to the nearest one-eighth of an inch. For example, if you measure a side as 36 and 24/32 of an inch, round it down to 36 and six-eighths of an inch.   

Measure Each Window Individually 

Even though windows in the same room can appear to be a similar size, they’re often different enough to require individual measurements. Always measure each room window individually to ensure all replacement windows fit properly. 

Always Measure From the Inside

There is rarely a need to measure outside a window. It is almost always easier and safer to measure a window from indoors. 

Always Check To Make Sure a New Window Is Square

If you’re Installing a window DIY style, it always pays off to make sure the new window is 

square. Measure it by using the same method described above. An off-square window can lead to structural problems like broken seals and drafts. 

Final Notes on Measuring Windows

Keeping your windows in tip-top shape is essential to being a responsible homeowner. Run-down windows can lead to problems like drafts, reduced energy efficiency, and water damage. A good window installation or repair ensures your home is comfortable and resistant to further damage.