Replacement windows can bring a lot of value to your home. There are so many benefits to replacing your windows, especially if you do not have to change the size. These benefits can include better sound reduction, energy efficiency, creating a warm and solid feel, reducing drafts, allowing for ventilation through operable windows, adding a modern look, and best of all, increasing the value of your home.

How to Measure for Replacement Windows

For starters, you have to evaluate how important it is for your preference, to have custom sized windows. Chances are, many of the windows in your home are off-the-shelf (in-stock at home improvement stores). This gives the best value, as these standard/stock windows are lowest price. . . they don’t have to be custom made. If you can install a store-bought in-stock window, you can potentially save 20-50% off of the cost of the windows. In addition, if you can stick with the same exact size, or close to it, you will not have to make major adjustments to the jambs or sills, saving you valuable labor time and costs.

How to Measure Windows

Start by measuring inside the window box, both horizontally and vertically. From jamb to jamb (horizontal, or “width”). Then measure the top sill to the bottom sill, or “height”. It is best to measure in three spaces each; near the sides, and the middle, and pick a close average of each. While some DIY advocates encourage you to measure diagonally (from the top of one corner, to the bottom opposite corner), we find that it is not necessary, and has more potential to vary, due to slack in the measuring tape. Simply measure horizontal and vertical.

Now that you have these measurements, in inches, you’ll want to first check your local home improvement stores to see if they have any in-stock windows of at or near that size. Keep in mind, most new vinyl replacement windows have thicker frames than older builder-quality windows that were originally installed in tract home developments. This is a good thing, as these frames have better insulation, and greater quality for energy efficiency and sound reduction. However, the new window measurements will likely be slightly larger than the previous window.

Important window size issues to be aware of

When replacing windows with nearly identical size replacements, it is helpful to beware of these possible scenarios:

  • Too small – If the replacement window is too small, typically by 1 inch or greater on either width or height, you may be stuck with having to frame in additional space to make up for the gap. This will also require the labor intensive work of texture and painting, as well as removing and trimming the old sills and casing. This can be a lot of extra time and work, possibly a half-day or more for the average do-it-yourselfer.
  • Too large – If your replacement window is too large, typically any small amount larger than the current width or height measurements, trimming of the boards, sheetrock wallboard, and possibly the siding will be needed. This can be easily done with a handheld oscillating saw. But it can also be dusty and dirty, especially if you have to cut through chalky wallboard.




In addition to these, keep in mind that your exterior and interior trim may need to be cut and repainted if there are major size changes made to the window box. So, our rule-of-thumb, find a window that has less than a 1-inch difference from the original, on any of the dimensions. Ideally, if you can find a off-the-shelf window that is slightly smaller than the current, by about ½ inch, you will likely not have to do any cutting, or any additional framing You can simply pop out the old one, and put in a new window, with little to no adjustments.

How to Measure for Replacement Windows - measuring

What if I cannot find an identical window in stores?

In this scenario, you will want to compare the pricing of custom-order windows to your exact current window box size, with the pricing of different sized windows with added labor. Chances are, major added labor can lead to unknown cost overruns. So we recommend custom ordering, and simply shopping around for the right price. You’ll have to wait; . . . sometimes a month or more, but it is worth the wait, to have a window that easily and seamlessly pops into your current existing window box space. If you are going to go the route of different sized windows, consider larger windows, which will add more light and value to your home.

What if all of this measurement stuff stresses me out?

Door and replacement windows installation companies can handle everything, from design, to measuring, permits, the sale and delivery of the window, removal and disposal of the old one, any changes needed to the current window box, installation and follow up service. While their services may cost more than doing it yourself, it may actually save you money in the long-run by avoiding costly mistakes. Window installers do this work every day, and can get it done quickly and done right the first time. This is an important option to consider. Replacement windows can be a valuable addition to your home, making it feel warm, modern and inviting. Measuring windows yourself is a doable task that can save you money, and help you plan for the right replacement window.

This article is provided by Mike Cutler is a commentator on modern home design and DIY home improvement projects for energy efficiency. He is especially passionate about ways to turn a house into a home.