How to measure windows?

Before you can go shopping for a new window, it is crucial to find the correct size for replacement. It is not a complex task, but only if you know the right way to make measurements. But in actual measurement, many homeowners are confused about measuring windows from inside or outside, and make mistakes or inaccurate measurements when doing it.

So, while you're in the middle of a renovation venture, you can't afford to order the wrong sizes. Otherwise, you're ultimately responsible for the added time and expense of making the windows fit. That's why it's good to take your time, take multiple measurements and avoid guesses. It will just take a few minutes, and make sure your record the measures so you can take those numbers to the supplier to get a quote or take a new piece.
Taking measurements for windows is crucial before you begin your project. The first step is to determine if your window frame is square.

Check window frame for square alignment

Standard replacement windows may vary in size but have 90-degree corners. So, it is essential to check the window for square alignment. That means to determine the corners of the existing window frame are square. Take a measuring tape and measure the width from the top at the center and the bottom. A square window will have the exact measurement at each point you measure.
The other way to check for squares is to measure the window diagonally. So, hold the meter tape in the top right corner at the junction of the horizontal and vertical window trim. Extend the tape diagonally towards the lower-left corner and record the measurement. The next is to measure the other diagonal, i.e., from the top left corner to the lower right corner. Now compare the two readings. If the difference between the two measurements is less than 1/4th of an inch, you can purchase the standard window sizes because they will easily fit in the opening. If your window is not square, you might have to square up the frame or find some alternate replacement as a solution.
Generally, the windows in older homes – let say a home built before the 90s or something might have a wrecked window, but for a newer home, you might not have such trouble. But anyhow, it's always good to check for the square.

Measure horizontally

Many homeowners are confused about the term generally used in window measurement that is rough opening. So, what exactly does this mean? A rough opening refers to the opening of the frame of a window. A typical window frame has a header strip at the top, a sill plate at the bottom, and vertical trimmers on either side. So, a rough opening of 3 feet wide and 4 feet tall would be listed as 3/0-4/0, but there would be a difference between the rough opening and actual window size with a margin for paper flashing on each side.
As you can see above, we first write the window's width and then the window's height to designate the size. So, we can write the standard window dimensions as width x height.
To measure the width of the window:
Start from the top. Open the window sash to expose the vertical side of the frame, called the jamb.
Place the tape horizontally between the inside of the frame jamb (excluding the trim) from left to right. Close the window and take a similar measurement from jamb to jamb at the middle of the window.
In the end, measure the width of the window at the bottom.
Record all the readings and highlight the shortest one that would be the width of the window opening.

Measure vertically

The next step is to record the vertical measurement or height of the window.
So, take the tape and measure from the sill to the top of the window opening. Again make sure to not include the trim in the measurements. We refer to the piece where the window sash rests when closed by the sill.
Don't confuse the window sill with the trim board that extends from the wall.
Like you did in width, take at least three measurements, i.e., on the left, center, and right. Just highlight the smallest of the readings that would be the window opening height.

Measure depth

Generally, depth is not a big concern for the replacement window unless you live in a modular or mobile home with thin walls. For reference, you can measure the depth from the exterior blind stop strip of the jamb to the interior trim of the existing window. If the measurement is more than three ¼ inches, you can proceed with ordering without much trouble because this area is enough to accept the depth of the replacement window.
Measure the depth from several locations around the window frame and rely on the smallest measurement once again as your final dimension. When measuring depth, don't forget to ignore all pulleys and parting strips because they'll be removed for replacement window installation.
If you're unable to open the window for depth measurement, you can measure each side of the glass and add the thickness of the glass pane, usually 1/8" or 3 mm.

Measure for window screen

If your existing window has a screen, the measurement process is straightforward. Remove the current screen and measure its width and height to order a new screen. But if the windows don't have a screen, you need to measure the distance between screen channels on the window frame. But be sure to subtract 1/8 inch from the measurement to get the width and add 1/8 inch from the size to get the screen's height.

Key takeaways

Taking measurements for your windows is essential. Accurate sizes of windows are necessary while ordering replacement windows. Measuring each window is necessary even if they look similar and are in the same room. Even a difference of an inch would be enough to make things annoying.
After you have the width and height of all the existing windows, you can then order and select the type of window you want to install.