How To Get The Right Measurement For Your Replacement Windows & Doors

When you are selecting windows and doors for your home, it is very important to get the correct size. If you order the wrong size, it could cost you significantly more money. In this blog we will go over how to measure for replacement windows and doors properly. This is the best way to avoid costly mistakes.

Taking accurate measurements is the most important part of ordering replacement windows and doors. In order to accurately measure, you will need a steel tape (a wooden yardstick or cloth tape measure may not be accurate), a pencil, and a pad of paper.

When you are ready to order, you will need three measurements.

·      The Width

The first step in measuring for replacement windows and doors is to measure the width of the opening at the top, middle, and bottom. It is important to measure all three areas because even though the width may be uniform, it could be out of square. If this is the case, you will need to order your window or door with custom measurements. If there is a difference in width among these three areas, then measure at 1/3rd points as well and use those measurements when you order your window or door. The height should also be measured at three different points (left, center, right) for both windows and doors. Again, if there are differences among these measurements then you will need to order a custom size window or door.

·      Take Three Measurements

When measuring for a replacement window or door you want to take three measurements along each side (top/bottom & left/right). Each measurement should be no more than 1/8” apart. Then record the smallest width and height measurements you get. This will ensure your replacement window or door will fit properly.

·      Depth

Second, measure the depth of your windowsill in three places: from front to back at the far left, middle and far right. Use the smallest number as your measurement. Be sure to measure deep enough so that your new window can rest on something sturdy when it is closed, but not so much that it might hit anything inside your home when it’s open.

·      Height

Third, measure for height from top to bottom in three places: far left, center and far right side of the window opening. Again, use the shortest measurement. This measurement is especially important for windows that sit above a radiator or sink because if it’s too short you could have problems with heat or water damage.

Measuring Tips

  • Take a measurement in three places: the top, middle and bottom, as well as at the left, center and right. Record each measurement in millimeters (mm) to the nearest 5mm.
  • Take measurements of both the width and height for each window opening and door opening.
  • For any opening that is out of square by more than 10mm (the width is not equal to the height), measure the offset from one corner of the window or door opening to its diagonal corner using a long steel tape measure or straight piece of timber.
  • Take all measurements from the inside, except for a roof window which is taken from the outside. The width and height of a window frame are always measured at the inside of the window. Measurement should be taken in millimeters.
  • Measurement should be taken in three places – on the left side, in the middle and on the right side. Add these three figures together and divide by three to get an average measurement. Then do this with all other measurements (height, width, depth).
  • The height is measured from the top of the lower window frame to bottom of upper window frame. This can be either double or single glazed units. If it is a double glazed unit, measure between the glass panes rather than including their frames.


One of the most important things to consider when installing replacement windows and doors is that they fit correctly. You can take it a step further and say that they fit correctly, look good and operate properly. The only way to achieve this is by getting the right measurements.

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