How to Measure for Replacement Windows (5 Steps)

Is it time to replace your home’s windows? Collecting measurements for your new windows is the perfect place to start.

If measuring your windows sounds a little intimidating, don’t worry – the process is relatively easy and can be done on any type of window. In this guide, we’ll give you the exact steps you need to measure for a replacement window and how to familiarize yourself with each part so you can feel confident while shopping for new windows. 

Key takeaways 

• Measuring for replacement windows requires precise measurements. 

• If you are unsure of your ability to record window measurements, a professional can do the job for you. 

• Collected measurements will need to be reduced by ¼ inch to find the correct size of your new windows. 

• Consider upgrading to windows with energy efficiency features to cut down on your energy bills. 

How do you measure windows for replacement?

Measuring your windows for replacement is a fairly straightforward process that doesn’t require too much effort or tools. Regardless of the type of window you have (excluding specialty shapes), you’re going to go about the same steps:

1. Check for square

2. Measure the width

3. Measure the height

4. Reduce your measurements

What are the parts of a window?

Before you start collecting your measurements, it’s important to know the parts of a window since they impact the overall size you need:

• Jamb: The two vertical sides of the window against the sides of the window frame. 

• Head: The horizontal top and bottom part of the window connected to the jamb sides. 

• Sill: The horizontal piece at the bottom of the window that runs straight across the ends of the jamb and lower window sash.

Tools needed to measure replacement windows

Now that you know roughly what you’ll be doing and what parts you’ll be measuring, here are the tools you’ll need to have on hand to complete the process:

• Measuring tape

• Pen or pencil

• Notepad paper

Step 1: check for square 

This step is all about measuring to ensure your frame sits at a 90° angle, which means everything is aligned and fitting properly. To do this, follow the steps below:

• Start at the top of the window in the upper left-hand corner where the head jamb and side jamb meet.

• Take your tape measure from this corner and pull it down to the lower right-hand corner where the jamb and head connect. Record this measurement.

• Repeat the process from the other corners, starting at the upper right-hand corner and measuring down the lower left-hand corner. Record this measurement as well.

• Compare the recorded measurements, making sure they’re within ⅛ of each other, as this means the corners are square. Measurements that are off by more will need a different replacement option and may require the help of a professional to find the right replacement.

Step 2: measure the width

Now that you’ve seen if everything is square, it’s time to collect the width measurement of the windows you’re replacing. To measure the window width, you’ll need to measure horizontally across three different places: the top, middle, and bottom. 

Here’s how to collect the measurements for all three places with your tape measure:

• Top: Start inside the left jamb and stretch your tape measure across the window to the inside of the right jamb. Record this measurement.

• Middle: Repeat the process at the center, going from the inside of the left jamb to the right jamb, and record the measurements. 

• Bottom: Repeat the process at the bottom, going from the inside of the left jamb to the right jamb, and record the measurements

After collecting the measurements, circle the smallest one, as this is the measurement to start shopping with. 

Step 3: measure the height

Similar to measuring width, you’ll also measure three places for window height: the left side, center, and right side. It will work fairly the same way as the previous step, but follow the instructions below for proper measurement collecting:

• Left side: Position your tape measure at the top of your sill (the part right underneath the bottom head), and pull your tape measure upward to stop at the top of the head. Record this measurement.

• Middle: Repeat the process at the center of the window and record the measurement.

• Right side: Repeat the process at the right side of the window and record the measurement.

Similar to the previous step, note the smallest measurement and use this one for shopping. 

Step 4: Measure window depth

Once you begin shopping for replacements, you'll notice that the dimensions won’t include depth, but this measurement is important so you don’t end up with windows that are too narrow to fit in your opening. 

To measure your window’s depth, open the window and use your tape measure to record the dimensions of the window sill from the back of the window’s trim to the front. Record this measurement. You will need at least 3 ¼ inches to fit a replacement. 

Step 5: reduce your measurements

Because window materials expand over time due to heat and temperature shifts, you’ll need to alter your measurements slightly to find a more accurate fit for your window sizes. Refer back to the smallest dimensions you collected when measuring the width and height of all your windows and reduce that dimension by ¼ inches. These will be the new dimensions you have to shop with!

Tips for measuring windows

• Never assume that all windows measure the same. Measure every individual window you plan to replace. 

• When measuring from the sill, always measure from the highest point, which is the part directly underneath the bottom head. 

• Keep the tape measure straight and flat, never bent.

• Always record the measurements you collect to refer back to when shopping. 

• Check for signs of water damage or other necessary repairs when replacing windows.

For more information on window measurements, check out our article on standard window sizes.

When to hire a pro

If you’re not as confident in your measuring skills or if your window is an uncommon shape, know that a professional is just a phone call away to do the work for you. Since precise dimensions are key for correct replacement sizes, it’s never a bad idea to let a window dealer come to your home and measure each window you want to replace just to ensure the measurements are properly collected. 

Measuring replacement windows: the bottom line

Measuring for replacement windows is fairly simple but crucial to making sure the replacements you purchase are accurate in size. Because of the precise measurements you will need, it’s incredibly important to take your time measuring the windows and never skip a step to hurry along the process. Do not record measurements from one side of the window.

When recording width and height dimensions, make sure you record from all three points. If you do not think you can record the dimensions of your windows correctly or do not have the time to measure all the windows you’re replacing, consider hiring a professional to measure for you. 

Frequently asked questions