How to Measure for a Replacement Window?

There are several methods for installing a window. And, depending on the method you choose, the points from which you calculate a window can differ.

Did you take measurements from the outside of the brick opening or the inside of the stud opening? Does your opening measurement include the reveals?

Your response could mean the difference between your brand new windows fitting correctly the first time or not fitting at all.

If you’re unfamiliar with any of these terms, the reveal is a surrounding timber frame that is fitted to the nailing fin of your window or door product and is used for installing onto wall studs. As a result, the reveal serves as the internal frame to which the architrave is attached to cover the distance between the product and the plasterboard (or internal lining).

The dimensions of a reveal can depend on whether you are using brick, double brick, timber, or steel.

How to Measure Home Windows?


1. Take 3 Horizontal Measurements of the Window Frame for the Rough Opening Width

  • Measure the gap horizontally around your frame from each side jamb at the top, middle, and bottom.
  • Make a circle around the smallest of the three figures.
  • The smallest measurement is the rough opening width, which is the primary measurement to use while shopping for a new window.

Always list window dimensions with width first, followed by height.

2. Take 3 Vertical Measurements of the Window Frame for the Rough Opening Height

  • From the head jamb to the window sill, measure the gap vertically through the frame from top to bottom on the left, middle, and right sides of the window.
  • Circle the smallest measurement of the three figures, this is your rough opening height.
  • If your window sill is sloped, take the measurement from the highest point of the sill, which is usually right against your window.

3. Measure the Depth of the Window’s Opening

It is important to determine the right depth because some windows may be too deep to fit into a replacement window opening.

  • Run a tape measure from the outside to the inside of the window frame and take the measurement from edge to edge.
  • Do this in several places around the window frame, and then use the smallest measurement as the final dimension.
  • If you can’t open the window, measure the depth on each side of the glass and sum it up. Then add the thickness of your window glass pane. A single window glass pane is usually 1/8′′ or 3mm thick.

How to Measure Rough Openings?

It is important that you get your window measurements right. Otherwise, drafts will enter your home, increasing your energy costs, exposing you to the elements, and reducing your comfort.

First, take the horizontal length of the replacement window frame. To do so, measure the gap around the frame from top to bottom, at the middle, and at the bottom. Take the smallest of the three measurements and write it down. When buying a window, the smallest measurement should be used.

When measuring height, follow the same procedure. Take measures in the middle of the window as well as on either side to decide which part is the smallest. Keep the two smallest numbers in mind, one for width and one for height, and base the buying choices on those.

The width of the window opening must also be measured. Any windows would be too deep to fit into an opening if the width is too great. Measure from edge to edge using a tape measure from the outside to the inside of the frame. Do this several times around the window, focusing on the smallest figure.

Now that you have the dimensions, you can use them to buy windows that would fit into the available space. The windows should be two inches narrower and two inches shorter than the space measured. This leaves an inch on either side of the window for adjusting and changing it to match precisely where you want it.

Use the depth measurement to guarantee that the windows do not protrude from the frame opening—the windows do not need to be any smaller than the depth. It could be a good idea to look for a local window replacement contractor.

What to Know Before Purchasing a Replacement Window


Bring your rough opening window dimensions with you and use them when buying new windows to ensure you are paying a reasonable price and are aware of the average replacement window installation costs.

You will almost definitely be provided the measurements of any windows you wish to buy. You should, however, measure them yourself to guarantee that they are the correct size for your window openings.

Measure the height of the window from top to bottom in three different positions using a tape measure. Make a list of the largest of the three measurements in case the window’s height isn’t uniform. Then, from outside jamb to outside jamb, measure through the window to determine its width. Take three measurements and choose the largest of the three.

Finally, measure the depth of the window unit in several areas to ensure that it will match in the opening. To see if the windows are compatible, compare the window measurements to the space available in your rough opening.

Most of the time, you won’t have to do this while buying windows because the measurements would be readily accessible. It’s always a smart idea to take window measurements if you have windows physically available to you, even if it’s just to double-check someone else’s job. There’s nothing worse than bringing windows home that doesn’t fit.