How to Measure a Window for Replacement ( With Steps )

Wondering how to measure a window for replacement? Find the complete detailed guide in this article.

The window is the part of your room that takes weather abuse, so they wear out with time. If you feel like your window needs replacement, be sure to measure it correctly to avoid ending up with drafts and condensation.

Why Measure a Window for Replacement?

Getting window measurements exactly right is vital. If the windows don’t fit, you might have drafts in your home, which can increase energy bills, reduce your comfort at home, and expose you to harmful elements from outside. When you purchase replacement windows, bring your rough window opening measurements with you to ensure you are paying a fair price.

window measurements

In most cases, you will have the measurements of all available windows or the most popular ones. Most of the time, they will be readily available, but the measuring is still a good practice. This way, you will double-check someone else’s work because otherwise, the windows just won’t fit.

3 Steps to Measure Window Correctly

Step 1: Measure Width

To measure the width, you should take three horizontal measurements of the frame. This way, you will get a rough opening width. The right way to measure is to consider the top, middle, and bottom of your window. You should measure at three points between the jambs: top, bottom, and middle of the window opening; measure from the surface of the jambs.

Avoid measuring from the parting beads or trim strips of your old window opening. The right way to measure is to use the smallest of three measurements. Once you’ve got your measurements, circle the smaller numbers that you’ve got out of the three figures. The smallest one is important to you because it’s your rough opening width. When you purchase the window, you need to name the smallest measurement.

Step 2: Measure Height

To take vertical measurements, you also need to measure three times. You should measure across your frame from top to bottom. Measure from the top of the window opening to the high point of the sill. That is the surface closest to the inside of your window.

You should take three measurements: one on the right side, one at the center, and one on the left side of the window. To get correct measurements, use the smallest of them to order your window. After you’ve finished, circle out the smallest one to get your rough opening height.

Step 3: Measure Depths

Measuring correct depths is vital because some of the windows are too deep to fit into a replacement opening. You should run a tape measure from the inside to the outside of your frame. Do it three times on each side, from edge to edge of your window frame. Measuring in several locations is vital because you will need a smaller measurement. When you give the results for the replacement, the smallest one is your final dimension.

Apart from height and width, you also need to measure the depth of your window. Ignore all parting strips and pulleys for depth measurement, as the specialist will remove them for the installation of a new window. There should be an opening at least 3 ¼ inches deep between the outside blind stop strip and the inside window trim. That will provide enough space to correctly assess the depth of the window you’re going to replace.

When you have the measurements, you can use them anytime to purchase windows. They will fit into the available spaces. The window itself should be two inches shorter and two inches narrower. This way, there is an extra inch of space around each of the sides.

The specialists then can adjust and position your windows and fit them exactly where you want it. Do not forget about the depth measurements. This way, you ensure that the windows won’t stick out of the frame opening. They should be neither bigger nor smaller than the depth.

Useful Tips

Window Replacing Useful Tips

  • When you hand out measurements, always list width before height.
  • If your window sill is sloped, you can measure from the highest point of your window sill. Usually, it is right against your window.
  • If you can’t open the window, first measure the depth of each side of the glass. Add these together. After this, add the thickness of the window glass pane. A single one is 3mm or ⅛” in thickness.
  • Check your exterior space. If you want to install a swinging window, like an awning or casement window, think about how much space you have outside. The windows might not fit if you have them over the roof or next to the garage.
  • Watch the space between the glass and the frame. Usually, an inch of space around windows is perfect, as it keeps windows from pressing against the frame and ensures secure anchoring.
  • Do not try installing windows on your own. That will save you time and money if something goes wrong or the window won’t fit.

How to Use a Tape Measure for Correct Window Measurements

To get the exact measurements of the window opening, use a tape measure. It is the best tool because it is rigid and can fix the blade to ensure that you measured it correctly.

Tape Measure for Correct Window Measurements

First of all, you will need to correctly locate the housing, tang, lock, and blade.

The housing holds the tape, and you can also use it as a measurement tool. As for the tang, this metal clip at the end indicates zero. The lock is the button on the front of your housing that locks the tape in place when you need to measure something. The blade is the tape itself, and it has numbers and symbols on its face.

To measure correctly, find the measurement system on the tape which meets your needs. After this, pull the tang out from its housing and hook it on the edge of the window. If you cannot do it, hold the tang at one edge of the surface and extend the tape with the other hand. Hold firmly. If possible, locate the strip next to the lines of your window so that the measurements are more exact.

After you stretched the blade across the object, you should press the lock and check if it meets the end. The nearest blade line to the end of the window indicates the final measurement. After this, note the result, unhook the tang. For safety measures, hold the blade with your hand while you recoil. Otherwise, it might cause injury. Press the lock to return it to the housing.

5 Signs Your Window Needs Replacing

Signs Your Window Needs Replacing

From time to time, windows wear out, and you need to replace them. But how to determine that the window is old enough to replace? Pay attention to the signs below.

  • Drasfts

If your window gives you drafts, this might lead to discomfort and even cold or other health problems.

  • Increase in Energy Costs

You will pay much more to heat or cool your room because the windows will let in the air.

  • Condensation

If you see it on your windows, that means they fit poorly. Consider changing them as soon as possible.

  • Outside Noise

Noise pollution is bad for you, so do not hesitate about changing windows.

  • Moisture Build-Up

If you see moisture building up between the panes, that means your windows let it in from the outside.


Do you measure replacement windows from the inside or outside?

Measure your windows “inside” the frame. Do measurements of the rough window opening, so measuring windows “from inside” is the best way to do this.

Are replacement windows smaller than the original?

The process of installation for replacement windows is different for full-frame ones. For replacement, specialists will pocket a new window unit into a frame that already exists. That means the original glass surface of the window will be a bit smaller than before.

Can you replace your own windows?

If you are handy and have experience in home-improvement projects similar to this, you might try. Also, if you know how exactly to reinstall your own windows, give it a try. Still, mind that it will take a lot of time, especially for multiple windows.

Can you replace windows without replacing a frame?

Yes. Insert windows will change only the window itself, not the frame. All exterior trim, siding, and the interior casing will stay the same.


Windows need replacement from time to time. If your frame is okay, but the window causes drafts and condensation, think about changing them. It’s better to take correct measurements because otherwise, you may end up with a poor-fitting window. Measure from the “inside” of the frame and consider only the window opening.

Measure the width at the top, bottom, and middle of the window. Height also needs three measurements at the left, right, and bottom. Measure the depth of the window when it’s open by doing multiple measurements in different places. Later, use only the smallest figures to have your rough window figures. That will help you when you order new windows for replacement.

How useful was this post?

Click on a star to rate it!

Average rating 5 / 5. Vote count: 3

No votes so far! Be the first to rate this post.