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The Definitive Guide to Measuring Your Windows Before a Replacement

Windows are essential elements of any building, affecting both its aesthetic and functional aspects. Whether you're undertaking a remodeling project or simply looking to upgrade, accurate measurements are critical. In this comprehensive guide, we'll outline the step-by-step process of measuring your windows before a replacement, ensuring you get the right fit for a seamless installation and a beautiful transformation of your home.

Breakdown of Necessary Tools and Materials

To start on the right foot, it's crucial to gather all the tools and materials you will need. This includes:

  • A reliable tape measure: Ensure it's long enough to measure both the width and height of your windows accurately.

  • Pen and paper: Keep track of measurements and note which window they correspond to.

  • Ladder or step stool: For hard-to-reach or upper-floor windows.

  • Assistant: A second pair of eyes can help ensure precision, especially for larger windows or those in odd-shape enclosures.

Taking Accurate Measurements

Measuring a window might seem straightforward, but to get it right, you'll need to follow precise steps:

Step 1: Understand the Window Type

Understanding if you have a single-hung, double-hung, casement, or any other type of window is your first step. Each type can have specific requirements for measurement due to the way they operate and fit within the wall.

  • For single-hung or double-hung windows, the sashes slide vertically and can be measured in sections.

  • For casement or sliding windows, you'll need to measure the frame size, ensuring you account for any obstructions that could affect the new window’s fit.

Step 2: Measure the Width

When measuring the width, take three measurements: at the top, middle, and bottom. Record the smallest measurement. Here's how you do it:

  • For single-hung or double-hung windows, measure from the jamb on one side to the jamb on the other.

  • For casement or sliding windows, measure from the inside of the brickmould or casing.

Step 3: Measure the Height

Like with the width, three measurements at different points of the height should be taken, and the smallest recorded:

  • For sash windows, measure from the sill to the head jamb at three locations.

  • For casement windows, measure from the head jamb to the windowsill.

Step 4: Measure the Depth

The depth will determine how far into the wall your new window fits. Measure from the outside part of the window frame to the inside part to get the depth.

Accounting for Trims and Obstructions

Trims, such as brickmould or wooden casings, can make a significant difference in the measurements. Be sure to take this into account:

  • Measure the width, height, and depth of the trim around your window and note these in your measurements.

  • Identify any obstructions, like protruding pipes or electrical fixtures, that may require a custom window fitting.

Re-Measuring and Verification

With all measurements in hand, double-check to ensure accuracy. Run through the steps once more and have your assistant do the same. Here's a tip:

  • Utilize the 3-4-5 triangle method. If you measure three feet across and four feet up, the diagonal measurement should be five feet for a perfectly square window.

What to Do With Your Window Measurements

Armed with precise figures, you’re ready to either purchase a standard-sized window that fits your measurements or to order a custom-size if your window is an odd shape or size.

  • If you're ordering custom windows, the manufacturer might have a specific ordering form to fill out. Ensure your measurements are correctly converted according to their requirements.

  • If purchasing standard windows, you need to compare your measurements with typical window sizes available and select the closest match.

Common Pitfalls to Avoid

Even when you're meticulous, mistakes can happen. Here are some common pitfalls to steer clear of:

  • Neglecting to measure all dimensions can result in an ill-fitting window.

  • Ignoring the trim measurements can lead to a window that doesn't fit within the wall opening.

  • Allowing tape to sag or bend can decrease the accuracy of your measurements. Keep it taut and level.


Proper window measurement is a critical step that ensures a successful window replacement. With this guide, you can confidently take on the task, knowing every step to get the job done right. Remember, the more precise your measurements, the better the fit, and the more efficient and attractive your new windows will be. Whether you're a homeowner or a professional, investing time in accurate measuring will save you time and money in the long run, and ensure your new windows are a perfect match for your space.

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