How to Measure for Replacement Windows

14th Jun 2023

When it’s time to replace the old windows in your home you will have to measure to know what size replacement windows you will need to purchase/order.

To start let’s go over some terms:

Jambs: This is the frame or box the window’s panes are in.

Sashes: Another word for the windowpanes.

Sill: The bottom of the jamb, often angled to the external facing side is lower than the internal one. Commonly the sill is at a 12° angle.

Header: The top of the jamb.

Rough Opening: The actually physical opening the replacement window is going into, window sizes are rounded up to make ordering easier.

You will need:

  • Tape Measure
  • Pencil
  • Paper (for noting measurements)
  • Click here for a form you can use.

Remember, all window dimensions are Width first and then Height.

Measure Width:

Measure the space between the jambs at three points: the top, middle and bottom of the window opening. Measure to the interior space of the jambs, not from the casing or the sash stop of your old window. Use the smallest of these three measurements. If you are measuring multiple windows to replace make sure you make it clear in your notes what measurements are for which window.

Measure Height:

From the highest point of the sill, which is normally the surface closest to the inside of the window, measure to the top of the window’s opening, not the top of the header or casing. Make three measurements: one at the left side, one at the center and one at the right side of the window. Use the smallest of these three heights.

Measure Diagonal:

Measure from the upper right corner of the window opening to the lower left corner. Do the same for the upper left corner to the lower right. The two measurements should be the same or within a ¼” of each other. If they are not the same, the window is not square. You will need to take extra care when installing the replacement window that it is level and plumb or it will not open and close easily or operate properly.

Subtract an 1/8” from the height and width dimensions you measured (1/4” for brick or stone sided houses) from the old jamb so you have room to shim and square your replacement window when you install it.

Measure Depth:

Most replacement windows are constructed to fit with a 2”x4” framed wall and come with a 4 9/16” jamb. If you have an older home, it may use thicker framing, like 2”x6”s, if you have stone or brick faced siding you will want to talk with one of the partners at our store to find your best options.

Home Outlet has an assortment of in stock vinyl windows that are ready to be in your home today. If you need special sizes or styles, we have over a 1000 available through our Spend Less Express program delivered for you at our stores within 2 to 5 weeks of placing the order. You can increase your home’s energy efficiency and at the Guaranteed Lowest Price.

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