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How to Measure Windows

Getting the correct window measurements is critical for a successful installation. Follow this guide to learn how to measure for new windows or replacement windows.

Tips:

  • Measure each window individually, even if they appear to be the same size.
  • Window sizes are presented as Width x Height.

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Three Types of Window Installation

There are three types of window installation: new window construction, window replacements and insert window installation. How to measure your windows will vary depending on your installation type.

How to Measure for New Window Construction

How to measure for new windows construction

Measure the window opening at three points on the width (top, middle and bottom) and height (center and opposite sides). If the sizes differ slightly, use the smallest measurement for each category. Then, simply remove ¾-inch from the width and height and it will give you the window size you need.

Window Width Size = Smallest Window Opening Measurement - ¾ inch

Window Height Size = Smallest Window Height Opening Measurement - ¾ inch

Window Depth = Jamb Extension

How to Measure for Full Window Frame Replacement

Man removing existing window for replacement

This type of window installation is needed if:

  • Your existing window frame is unusable.
  • You would like to maximize the glass area.
  • You would like to maximize your energy efficiency.

For this type of window installation, you will require a jamb extension. You may also require a brickmould depending on the existing window.

How to measure windows for brickmould

Step 1: Take exterior brick-to-brick or siding-to-siding measurements at three points on the width and height (close to top, close to bottom and close to center). If the sizes differ slightly, use the smallest measurement for each category. Then, simply remove ¼-inch from the width and height. The exterior dimension is critical to ensure minimal exterior finishing.

Exterior Brickmould Measurements

Window Width = Smallest Exterior Window Width Measurement - ¼ inch

Window Height = Smallest Exterior Window Height  Measurement - ¼ inch

How to measure windows for brickmould

Step 2: You’ll need to take interior window measurements to confirm if brickmould is required. To take these measurements, remove all interior trim (casing) to expose the window frame. Measure the window opening at three points on the width, height, and depth

(close to top, close to bottom and close to center). If the sizes differ slightly, use the smallest measurement for each category.

Interior Window Measurements

A. Window Width Size = Smallest Interior Window Width Measurement

B. Window Height Size = Smallest Interior Window Height Measurement

Window Depth = The overall depth of the window from the very exterior frame to interior frame where the casing trim attaches.

window without brickmould

Window without brickmould

window with brickmould

Window with brickmould

Step 3: If the exterior and interior measurements are equal (within ½-inch) a brickmould is not required. If exterior measurement is larger than interior by 2 – 2-1/2 inches a brickmould will work perfectly. The window should be ordered using the brickmould size measurements determined in step 1.

Step 4: For the jamb extension you will need to measure the overall depth of the window opening from the existing caulking line on the exterior wall to the interior casing.

Jamb Extension = Overall depth of window cavity - window frame size to be used

How to Measure for Insert Window Replacement

Man replacing window glass

This type of window installation is needed if:

  • Your window frame is in good condition.
  • You wish to maintain your home’s original window trim - for example, the elaborate mouldings often found in older homes.
  • You’re looking for a more economical option.

For this type of installation, you will not require a jamb extension or brickmould

Step 1: Measure Window Width

Measure between the jambs at three points: top, middle and bottom of the window opening. Measure from the surface of the jambs, not from the trip strips or parting beads in your old window opening. Use the smallest measurement.

Step 2: Measure Window Height

Measure from the high point of the sill of your window (the surface closest to the inside of the window where the sash rests when closed) to the top of the window opening. Make three measurements: left, center and right side of the window. Use the smallest measurement.

Window Width = Smallest Window Width Measurement

Window Height = Smallest Window Height Measurement

What You Need for This Project

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