How to Measure for Replacement Windows

Replacing windows can be a tough task, but it’s something with which just about every homeowner must contend. Newer window designs not only help to deliver an appealing and modern facelift to the property but can also provide added security and reduced energy bills. It’s also a project that many may choose to tackle themselves to save on costs. 

While replacing your windows may not be the most difficult project for homeowners to undertake themselves, it is critical you know how to record accurate measurements. Otherwise, you may find yourself wasting both time and money. Want to know how to measure windows? This article will guide you through the process of taking correct measurements and determining whether your window project can feasibly be completed on your own. 

Assess the Condition of Your Current Window Frames

Before taking window measurements, you’ll need to figure out if you can even take on the project yourself. One of the biggest determining factors will be the condition of your window frames. Replacement windows are often easy to install yourself because they are designed to simply fit into built-in frames. However, if the frames themselves are in poor condition due to water damage or rot, they may not be structurally sound enough to support your replacement windows. Additionally, water damage may cause the wood to warp and not provide the tight angles needed for a proper window seal. 

Inforgraphic describing a process for replacing windows.

Once you’ve determined the window frames are still in good enough condition for a DIY project, you can begin taking measurements. 

Before you get started, make sure you have the following items on hand: 

  • Tape Measure 
  • Pencil 
  • Note Pad 
  • Ladder (you may or may not need one depending on your style of home. Open-concept homes can have harder-to-reach windows and will require some ladder assistance.) 

Tools Needed to Measure Windows Include a Tape Measure, Pencil, Notepad, and Ladder.

Measurement 1: Make sure the Frame is sQuare

The first measurement you’ll want to take will determine whether the corners of your window frames are square. To do this, follow the steps outlined below: 

  • Hold the end of your measuring tape in the top left corner where the vertical and horizontal trim come together. 
  • Extend the tape measurer on a diagonal line to the bottom right corner and record the measurement. 
  • Repeat the measurement, this time beginning at the top right corner and measuring diagonally to the bottom left corner. 
  • If both measurements are within ¼ inch of each other, you should be able to easily install most standard replacement windows. If the difference in measurement is greater than ¼ inch, you’ll need to square up the frames before new windows can be installed. This may require professional assistance. 

An inforgarphic describing the proper way to measure a window.

Measurement 2: Width

Measuring the width of your window frames will ensure the frames are parallel and of equal distance. To determine this, you will need to take three different measurements: At the top of the frame, the middle, and again along the bottom. Then, record the shortest of the three measurements. This is the one you will want to use to determine your window opening width. 

Important: You will want to make sure you are taking your measurements from the window jam and NOT from the trim. 

A diagram showing how to measure a window.

Measurement 3: Height

To measure the height of your window you will want to repeat the same process as before, but this time you’ll measure vertically three different ways: left, middle, and right. For each measurement, extend the tape from the jamb at the top of the window to the bottom sill. The sill is closest surface to the inside of the window and where the window sash rests when closed. Make sure you take measurements from this point and not from the window stool (horizontal board of trim attached to the wall).  

Again, you’ll take the shortest of the three measurements as your measurement of record. 

Diagram showing how to measure a window vertically.

Measurement 4: Depth

For most traditional homes, depth is usually not an issue and typically is not a measurement included when ordering replacement windows. However, if you live in a mobile or modular home, you will want to make sure you have enough jamb depth required for most replacement windows, which is at least 3 1/4-inches. With the window open, measure the sill space from the trim in the front to the trim in the back. 

Round Down Your Measurements

Once you are finished taking all your measurements, round each one down to the nearest 1/8-inch. Remember to use the shortest of the three measurements from each direction.  

Example: If the shortest of your three height measurements is 55-9/16 inches, round it down to 55-1/2 inches. 

Note: Do not round down if your original measurement is already an eighth-inch increment (ex. 42-5/8 inches). 


When measuring for replacement windows, it is vital you remember these key guidelines: 

  • Make sure each frame is squared up, and that the two diagonal measurements are within 1/4-inch of one another. 
  • Remember to take three horizontal and vertical measurements. Use the shortest measurements when ordering new windows. 
  • Round your height and width measurements down to the nearest 1/8-inch. 
  • Ensure that your window depth is at least 3 1/4-inches. 

You may also be interested in replacing your current windows with a different style. There are many different options to choose from, and each one can help provide a personal touch to your home. Not sure about which style to choose? Visit our home window style guide to learn more. 

Unsure if replacing your windows is a job worth tackling alone? West Shore Home is here to provide you with a free in-home consultation for your window replacement needs. We’ll do all the measuring, ordering, and replacing for you. Schedule your free consultation today!