How to measure a window to replace it

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Whether due to a rogue baseball or regular wear and tear, eventually it will come time to replace your windows. If you’re a handy homeowner, you can learn how to measure for replacement windows in a few simple steps. The process is simple, but precise measurements are essential for accurate quotes and ordering the right size window replacement. Use this guide to learn how to measure windows without the help of a professional.

Why Do I Have to Replace My Windows?

If you’re measuring for replacement windows, it may be because you’re doing an optional upgrade for weatherproofing or energy efficiency. In other cases your windows may urgently need to be replaced. Address damaged windows as soon as possible, especially if you’re seeing or feeling signs of that damage.

Increased humidity levels or visible water damage can lead to wood rot that can cause serious structural damage to your home. Moisture can also cause mold or mildew, which can lead to health issues for you and your family. The longer you leave window damage unattended, the more difficult and costly it can be to fix.

How Much Does It Cost to Install a Replacement Window?

If you hire a pro to replace your windows, they’ll include measurement as part of the job. Window replacement installers typically charge $40 to $65 per hour. The average cost of window replacement is between $100 and $300 per window for simple installations or about $650 per window for more complex installations.

How to Identify Damaged Window Frames 

Replacement windows are more DIY-friendly than new construction windows. However, your window frames need to be in good condition or they won’t be structurally sound enough for replacement windows. Before measuring, inspect the condition of your window frames, sills, sashes, and trim for the following signs of water damage or rot:

• Wood that is visibly rotten

• Wood that is soft or crumbles to the touch

• Discoloration

• Peeling or bubbling paint

• Gaps in the corners of the interior casing trim

• Fog or condensation that you can’t remove

• Light shining through cracks around the frame

• Difficulty opening or closing

If your windows are damaged or are not square, you may need to contact a window frame repair service to remedy the issue before installing new windows.

How to Measure Windows

There are several ways to measure for replacement windows depending on exactly which measurements you need. Regardless of how you measure, you’ll need the following:

• Measuring Tape (non-retractable strips work best)

• Pencil

• Notepad

Check If Your Windows Are Square

Before measuring for a replacement window, window screen, or storm window, it’s important to ensure that your current window frames are square. To be square means that all four corners are at 90-degree angles. Any new windows you purchase will be square. If the current glass is lopsided, this could prevent the windows from opening properly. Here’s how to check for square:

1. From inside your home, start at the top right corner of the glass where the vertical and horizontal trims meet.

2. Hold the tape measure taut and measure diagonally to the bottom left corner where the two trims meet.

3. Record the measurement.

4. Take a second measurement diagonally down from the top left corner, just as you did for the other side.

5. Record and compare the two measurements.

If both measurements match within 1/4 of an inch, you can safely order replacement windows for a correct fit. If not, you will need to replace the window frame beforehand or order a new-construction window instead of a replacement window.

Measure Window Width

Photo: Gerald Carter/Creative Flame/ Adobe Stock

To measure the true width of a window opening, you’ll need to take three measurements from the top, middle, and bottom of the window.

1. Raise the window sash to expose the jambs, which are the vertical parts forming the sides of the frame where the sash slides up and down.

2. Ignoring the width of the trim, extend your tape measure straight across the bottom of the window from just inside the left jamb to just inside the right jamb.

3. Close the window and repeat this measurement from the middle and top of the window.

4. Compare your width measurements and note the shortest of the three. This is your true window opening width.

Measure Window Height

To measure the true height of a window opening, you’ll need to take three measurements from the windowsill, which is where the sash rests when closed and which is only accessed by opening the window. Note that the sill is not the horizontal piece of trim board that extends out from the wall. While often mistakenly called the sill, this is actually the stool.

1. Starting on the far left side of the window, extend your tape measure straight up from the windowsill, which is where the sash rests when closed, vertically to the head jamb.

2. Ignoring the height of the frame and trim, record your measurement.

3. Repeat steps 1 and 2 from the center and right side of the window.

4. Compare your height measurements and note the shortest of the three. This is your true window opening height.

Measure for Storm Windows

The steps to measure for new storm windows are the same as the steps to measure window width and height for single-hung or double-hung windows.

1. Take measurements from the bottom, middle, and top of the window. Extend your tape measure from inside the trim on the left jamb to the trim on the right side. Your shortest measurement is your true width.

2. Measure from the sill to the inside of the trim at the top of the window on the left side, center, and right side. Your shortest measurement is your true height.

Using the shortest measurements for the window width and height will ensure that the flanges of the storm window you purchase will fit inside the window opening’s exterior trim.

Measure for Screen Windows

How you measure for window screens will depend on whether or not your window currently has a screen.

Measuring to Replace an Existing Damaged Screen

If your window already has a screen that you need to replace because it’s damaged, simply measure the width and height of the existing screen. These are the dimensions you’ll need when purchasing a replacement.

Measuring to Add a Screen to a Window That Does Not Currently Have One

1. Pull your tape measure straight across the window horizontally to measure the distance between the screen channels on the window frame.

2. Subtract 1/8 inch from your measurement. This is your width dimension.

3. Raise the window sash and measure up from the lip, which is the ridge that holds a screen in place, along the screen channel to the sash.

4. Add 1/8 inch to the measurement. This is your height dimension.

How to Order Replacement Windows

Photo: yuriygolub / Adobe Stock

After you’ve recorded the smallest width and height measurements (width x height), you’re all set to get your replacement windows ordered. Replacements should be within1/8 inch of your measurements, rounded down. You should also decide which type of replacement windows will work best for your home and budget, whether it’s vinyl windows, composite, or another style.

You may find windows close to the size you need at your local home improvement store, but it’s typically best to order custom windows based on your exact measurements for the perfect fit. Order online, talk to a customer service technician at your local home improvement store, or contact a window replacement service near you to finish the project.

Tips for Properly Recording Window Measurements

When you measure a window for replacement, accuracy is vital. Carefully record each measurement as you take it, and always record them as width x height. For example, if your windows are 60 inches tall and 34 inches wide, your measurement should read 34 inches x 60 inches. Round down each measurement to the nearest 1/8 inch.

If measuring multiple windows, label each set of measurements with the exact window location. For example, write down “east side bedroom window” with your measurement. Even if all the windows in a room look the same size, they may have slightly different measurements. To stay on the safe side, measure each window separately before you order new windows.

DIY vs. Hire a Pro

Measuring for replacement windows is a simple DIY project, but not every homeowner will be comfortable replacing windows on their own. Especially if you plan on replacing multiple windows or installing a custom or complex window design, hiring a local window installer can help you avoid problems like mold, code violations, cosmetic damage, and structural issues.

Frequently Asked Questions

When measuring for replacement windows, measure from inside your home. Take horizontal measurements inside the two window jambs, and take vertical measurements inside the sill and head jamb.

The difference between a replacement window and a new construction window is that replacement windows are designed to fit right into the existing window frame in your home. In contrast, new construction windows cannot be installed unless the existing frame is removed and replaced.

To measure a rough opening for a replacement window, take three horizontal measurements from the bottom, middle, and top of the window. Take three vertical measurements from the left, center, and right sides. The smallest of each set of three measurements are your rough opening dimensions.

Are replacement windows measured from inside or outside?

How much smaller should a replacement window be than the opening?

Can I replace just the window and not the frame?