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Measuring For Replacement Windows

Last updated on 08/04/2022

Energy loss through windows is responsible for up to 30% of your home heating and cooling bill. When you consider the cost of utilities, the savings from energy-efficient windows is plain to see.

Ill-fitting, worn-out windows can cost you. If you're seeing signs of wear, like beads of water between the panes, or your windows have become hard to open or close, then it may be time for replacements.

Measuring for a replacement window isn't difficult, but it does require some know-how and precision. Here's what you need to know to ensure you measure correctly.

How to Measure Windows - the Basics

Window dimensions are indicated as width by height (WxH). You'll round each replacement window measurement down to the nearest 1/8 inch. So, if you measure the width of a window as 44 7/16 inches, use 44 3/8 inches.

The two types of replacement windows available include replacement window inserts and full frame replacement windows. The type you get dictates the measurement methods you'll use.

Measuring the height and width of a window

Replacement window inserts are installed inside your existing window frame. They're less expensive than full frame replacements, but there are some drawbacks. They're generally only available for wooden frames, and they may reduce the amount of light let in.

A full frame replacement window, as the name suggests, is a replacement window that includes a new casement as well as new windows. For this type of replacement window, the old window, frame, sill, and trim are removed. What's left is the rough opening in the wall.

Full frame replacement windows are more expensive and take longer to install, but there are some significant advantages. Removing everything exposes any water damage or rot to be repaired. And full frame windows allow you to change the look of your home.

Replacement Window Inserts

Understanding how to measure windows for inserts is critical to ensure the replacement windows will fit. There's no need to replace the casing, trim, or siding with inserts. But you'll need to take precise measurements.

Make Sure the Frame Is Square

Measuring a window to make sure it is square

All four corners of the existing window frame must be 90° angles. If your frame isn't square, replacement window inserts won't fit. You'll need a full frame replacement window instead.

To check for square, measure your window diagonally. Record the distance from the inside upper left corner to the inside lower right corner. Then, take the same measurement from the upper right to the lower left.

Compare the numbers. If the difference between each diagonal window measurement is less than one-quarter inch, your window frame is square. More than that, and replacement window inserts may not work for you.

Is the Opening Deep Enough

In most homes, the depth of the window casement isn't an issue when installing replacement window inserts. But, sometimes it is. Measure the sill depth to ensure you have enough room to accommodate replacement windows.

A cut away diagram of window parts to consider when installing a window

The sill is the bottom of the window casement where the window comes to rest when it's closed. It is not the flat piece that juts into your room where your cat likes to sunbathe. (That's the stool.)

You'll need to open the window to measure the sill. Check the distance between the trim on either side of the sill. It needs to be at least 3 1/4 inches for a replacement window insert to fit.

Measure the Width

When measuring for replacement windows, you don't measure the window itself. Instead, you'll measure the window opening from jamb to jamb. The jamb is the vertical strip inside the window casement. When you open a window, the sides of the window slide along the jambs.

To ensure a good fit, you'll want to take three measurements. Take one at the top, one in the middle, and one at the bottom of the window.

First, measure the distance between the jambs as close to the top of the frame as possible. For the bottom measurement, slide the window open and measure the distance across at the lowest point. For the middle, open the window all the way and measure between the jambs as close to the middle as you can.

The shortest of the three measurements is the width for your replacement window insert.

How to measure the height and width of a window

Measure the Height

Just like measuring for width, for height, you'll measure the opening itself, not the window. And again, you'll take three measurements—one on the left, one on the right, and one in the middle.

You'll need the length between the head jamb and the sill. The head jamb is the horizontal part of the window casement at the top and the top of the window butts against it.

Measure the distance between the head jamb and the sill on the left, in the center, and on the right. Again, use the shortest of the three measurements.

Full Frame Replacement Window

A diagram a full frame replacement window, parts, and assembly diagram

Sometimes a full frame replacement is the only way to go, like when the window frame isn't square. It's usually necessary when replacing vinyl or aluminum windows too. Full frame window replacements are also the best option if you want to change the style or size of your window.

When planning a full frame replacement, measuring for replacement windows starts with the window trim. The most accurate way to measure for a full frame replacement window is to remove the trim and measure the rough opening.

If you don't want to remove the trim, there are other ways to get an accurate measurement.

The Drill Method

Start by drilling small holes through your jambs. Use a wire to measure the depth of the holes to the rough opening (studs) on each side.

Measure the width of the window from jamb to jamb. Then add both depth measurements to the window measurement. That's your total width.

For the height, drill holes in the sill and header jamb. Again, use a wire to gauge the depth of each hole to the rough structure (header and sill plate). Measure the height of the window from sill to top jamb. Add the window measurement to the top and bottom depth measurements to get the height.

The No-Drill Method

If you don't want to remove the trim and or drill holes, you can still get an accurate enough measurement for a full frame replacement window.

For the width of the opening, measure the distance from the middle of the trim on the right side of the window to the center of the trim on the left side. That's your width.

Measuring the height works the same way. Measure from the middle of the top trim to the center of the bottom trim piece (below the stool). That's your height.

Replacement Window Installation

Now you know how to measure for window replacement inserts and full frame replacements. Both options are great home improvements. But you only get the benefits if they're installed correctly.

It pays to hire professional replacement window installers. They can get the job done quickly and affordably. Where do you find the installer you need?

At Window Joe, we're here to help! We'll match you with the right window replacement contractor for your needs. Fill out the form at the top of this page to get free, no-obligation estimates from local contractors.

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