Replacement Window Sizes

One of the first steps in replacing your windows is measuring your current ones. While this may seem arbitrary and straightforward, getting the right replacement window sizes is the first step toward a successful project. Fun fact: mismeasured windows are the most common cause of window leaks, excess trim, callbacks, and project delays.

Our article covers a few best practices your local window company should follow and answers common questions. This article answers the following questions:

  • How to properly measure replacement windows?
  • Why did your sales rep measure windows from the inside?
  • What sizes do windows come in?
  • Does rough opening window size impace that type of window can be chosen?
  • What is a rough opening window measurement?
  • How can you tell if the wrong sizes were ordered?

How do you properly measure replacement windows?

An experienced window contractor will measure the width and height of your original windows from the outside at three points—top, middle, and bottom—to get an accurate reading. The smallest of the three will be chosen and then reduced by 1/4″ for ordering purposes. This ensures your new windows fit in the opening once the old window has been removed.

Measurements for most windows are typically taken from the outside because the outside opening is larger than the inside. Interior trim and casing can reduce the interior size of windows by 1/4″ to 3/4″. This is not always the case. Wood windows are often measured from the inside and outside to account for trim.


Why did your sales rep measure windows from the inside?

It’s common for window company reps to measure windows to quote a replacement window project. If their quote is accepted, a technician or installer will come out a 2nd time to measure for ordering and installation purposes. Reps that take inside measurements for quoting may add 1″ to the width and height.

At Window Depot of Dallas, our window consultants will approximate the size of your windows for pricing purposes. Once you agree to move forward, we schedule an experienced craftsman to return to your home and measure. These measurements are used to order your new windows.

What sizes do windows come in?

Most custom-size replacement windows are available in 1/8″ increments. However, the standard new construction sizes are in 6″ increments. For example, the Andersen 100 Series Casement Window Sizes come in 18″, 24″, 30″, 36″ widths and 23″, 30″, 36″ etc. but can be built in 1/8″ as needed.

A builder will always default to the standard sizes unless otherwise specified by the architect because the standard sizes usually cost less.

Does rough opening window size impact what type of window can be chosen?

Yes. You won’t be able to select some window types based on your rough opening window size. For example, most casement windows can’t be built wider than 36″. So, if your rough opening is 42″, a single casement window type isn’t possible. Casement, awning, and shaped windows have the most size limitations.

Single hung, double hung, and picture windows are the most flexible and can be built in most standard dimensions.


What is a rough opening window measurement?

The rough opening for a new window is the actual height and width measurement of the “hole” the window will go in. Rough opening is used primarily in new construction, so framers know what size a window needs.

Rough opening sizes are typically 1/2″ taller and wider than the actual size of the new window. The 1/2″ gap around the perimeter allows room for shims to level the opening and secure it to the frame. Any surrounding gaps will be filled with insulation or caulk. Rough openings are not used to order windows. We order the actual frame size needed for each window, whether in brick, stucco, wood, or siding.


How can you tell if the wrong replacement window sizes were ordered?

Excess trim and caulk around your windows indicate that your new window is too small. Ideally, windows are installed with minimal or no trim. When measured tightly, a single caulk line and insulation are needed to close the gap between the window and your exterior wall cladding. You want to avoid excess exterior trim as much as possible.


In closing

Ordering and installing the correct size replacement window can make all the difference in how efficiently it operates and looks once installed in your home – not to mention its ability to keep out moisture and air infiltration. Taking accurate measurements of existing window frames is critical.

At Window Depot of Dallas, we pride ourselves on our craftsmanship, attention to detail, and expertise when installing replacement windows. If you’re in the market for new windows and live near Dallas, Fort Worth, Texas, call us at 214-399-9592 or request a free estimate to discuss your replacement window project.