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How to Measure for Replacement Windows

Are you thinking of installing new windows? Then it might be time to dust off your measuring tape and put your skills to the test! You’ll need to know how to measure for replacement windows if you’re planning to purchase and install them yourself, or if you’re comparing DIY pricing to those from window and door companies.

However, if you plan on outsourcing the job to professionals, measurements are unnecessary. Don’t waste time doing it, unless it’s a virtual appointment and they specifically request measurements. Allowing them to handle measurements ensures they’re responsible for any mistakes, not you, and they’ll take measurements multiple times throughout the estimate and installation process.

DunRite Windows & Doors will happily measure and order the correct windows for a flawless install every time, if you’re interested in a price quote. However, here’s a quick tutorial if it’s something you prefer to do yourself. 

1.Use a spreadsheet. If your home has a lot of windows, then some find it helpful to print a form or spreadsheet and specifically label each window with the name and measurements. For example, “North-Facing Office Window – 36” x 60.” Just be sure to list width, then height, then depth for each measurement, in exactly that order. To make the job easier, here’s a simple measurement sheet you can download.

2. Take all measurements from both interior and exterior the home. This will allow you to understand the wall and window size better.  Plus it will let you know if there is something different that you need to accommodate.

Angi 2021 Super Service Award3. Consider using a helper. Those large windows can be tough to measure, and having one person write down the measurements a second person is calling out can really speed up the process. Plus when the window is too wide, you’ll need someone at the other end of the tape measure

4. Check that the opening is square and true. Take a level, and check both vertically and horizontally to see if the opening is level and plumb. Then, check all four corners to make sure each right angle is the correct 90 degrees. If the window isn’t square or true, you’ll need to reduce the size of the new window to make it work in the opening.

5. Measure width in three places. Measure the distance horizontally across the window opening – not the frame – in three places, and write down the smallest one.

(Professional tip: if it’s a true and square window, professional installers will then subtract ½” – 3/8” from the width and height measurement to allow room for adjustment. It ensures the replacement window isn’t too tight, but with gaps that are small enough to easily fill with sealant or foam.) 

 After all, if you write down the largest, then the new window will be too tight in that smallest width. It’s better to fill small gaps where the opening is slightly larger than the frame, instead of trying to cram something in a too-small space.

6. Do the same thing for height. Now, take the vertical measurements by following the same process: capture the distance vertically across your frame on the left, middle and right side of the window. Jot down the smallest number.

7. Capture the depth. Last, open the window and measure depth. This is important, because different styles and materials require specific depths of windows. For example, triple-pane and wood windows are very thick, requiring a substantial depth of opening to accommodate that much material. And a vinyl window will be thicker than an aluminum window. So take your measuring tape and run it from the inside of the opening to the inside, and again write down the smallest number that captures the distance from edge to edge.

BBB Torch Award Finalist 2021(Professional tip: if the new window is not as deep, you’ll need to understand how to make that adjustment during installation. And if it’s thicker, you’ll want to make sure it isn’t thicker than the wall opening itself. You also might want to ensure you’re comfortable how much sill on the interior will be left after the thicker window is installed, and that it doesn’t interfere with window coverings, shutters or blinds. If the new window is thicker than the total depth of the opening, it won’t fit. It must be flush to the exterior wall, and would look weird extruding past the interior.)

8. Take a break, then go back and do it all over again. The old adage “measure twice, cut once” applies to windows, just as much as it applies to woodworking or fabric. If a DIY homeowner is off by a small amount, a product that’s ordered to size is not returnable, so the error can be quite costly. It can also delay installation. It’s better to be cautious, so just like the professionals, you’ll want to measure the window openings two or three times. You’ll also want to measure the product yourself once it arrives, and thoroughly inspect the quality before installation. If it’s incorrect from the factory, you can’t return it once you’ve begun installation. You have to catch the mistake beforehand.

Here are a few ways the replacement installation might look, depending on your frame style.

9. Order windows that are 3/8 of an inch SMALLER than the opening. DunRite’s Owner, Sal Sucato, has this tip to share: professional installers subtract three-eighths of an inch from the height and width before placing a product order. It’s just a little bit of insurance that allows adjusting the window once it’s set in the opening, and allows for a nice bead of flexible sealant or caulking all the way around the window. All houses shift slightly over the years, plus windows typically expand and contract with the weather, so the extra space helps accommodate those shifts.

Pella Limited Lifetime Warranty details
Did you know we’re one of very few installers in Arizona to offer a lifetime installation warranty? It’s valid for as long as you own the home.

Doesn’t sound too difficult, right? We do have a few words of caution, though, since we answer phone calls on a regular basis from homeowners who’ve begun the DIY process, only to need help. Like any handy homeowner knows, there’s always a hiccup or two.

Don’t underestimate your DIY skills. Window and door companies won’t install windows that aren’t purchased from them, or finish a partially-completed job if a homeowner runs into problems.  If the stucco crumbles, there’s water damage or you simply don’t know how to get around an issue, you’ll be stuck hiring a handyman or contractor.

Avoid standard off-the-shelf sizes. Unfortunately, there aren’t guidelines on window sizes that homebuilders follow, they simply create windows based on the architectural plans. And even windows that are supposed to be the exact same size can be off by a few inches here and there. It’s even more likely on older homes, since it settles and shifts over the years. If you’re following the DIY path, order each window to exactly fit the opening, never settle for the “closest” off-the-shelf size that might work. You never want more than a quarter inch gap between window frame and opening, and product ordered to fit ensures a clean, snug fit. It looks better, prevents leaks and drafts, and ensures you get the best possible energy efficiency.

To learn more, here are some tips on how to decide if it’s safe to DIY, or if you’re better off choosing a professional.

And if you’re going with a vinyl window product, we recommend ensuring it’s a higher quality brand that can withstand decades of Arizona heat, and a solid warranty in case something goes wrong.

If you’d like a quote from a windows and doors professional with decades of experience installing windows in Phoenix homes, and a five-star reputation, look no further! Schedule an in-home appointment today. No pressure, and no obligation. We’re an Angi Super Service Award winner, a BBB Torch Award finalist, and a family-owned company that promises a smooth, hassle-free experience. We’d be honored to come out to your home.

Published Aug. 4, 2022

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Please note that we install replacement windows and doors, but we don’t repair them or replace broken glass. Three-item minimum purchase, some doors excluded. Refer to our FAQ and warranties page for details.


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