If you’re thinking about replacing your windows but aren't quite ready to call a contractor, measuring your windows can be a first step to more informed shopping. Let's dig in!

Before you start, you’ll want to gather a tape measure, pen, and paper. It can be useful to have a helper too, especially if you have large windows.

Tip: To help you track dimensions and make other necessary assessments, download one of our measurement guides, like this one for the 400 Series.

You also need to understand a few different parts of a window before taking measurements. These include:

• Jamb: The pieces forming the top and vertical sides of a window or door frame.
• Head (or head jamb): The horizontal piece forming the top of a window or door frame.
• Sill: The horizontal piece forming the bottom of the window or door frame. Here’s the equipment you’ll need:

The first thing you’ll want to do is check and make sure your windows are in square alignment. This means all four corners meet at 90°. To do this, you’ll take two diagonal measurements from inside your home.

• Start in the upper left-hand corner where the head meets the side jamb and measure to the lower right-hand corner where the side jamb meets the sill. Measure to the nearest 1/8 inch.
• Next, measure from the upper right-hand corner where the head meets the side jamb and measure to the lower left-hand corner where the side jamb meets the sill. Again, measure to the nearest 1/8 inch.

If the measurements are within 1/8 inch of each other, your windows are square.