Replacement Window Sizes

What replacement window sizes do you need when replacing your windows ? Installers have differing opinions on how much smaller a replacement window should be than the opening it will fit in. Some installers make their windows 1/4" smaller than the opening while others prefer 1/2". Mostly it comes down to how accurately you can measure the opening that the window will fit in.

A great deal of window installation are replacements of sashless sliders. These are the old builder windows which are basically sheets of glass sliding in grooves made in the sills, or vinyl framed glass sliding in vinyl tracks stapled to the sill. These windows are the easiest to replace because so long as the sills and jambs are in good condition, you can use these as the opening for your new window. Most of the time these old sills will be plumb and true. If you are going to do this style of window replacement, measurement is easy.

Just take three vertical measurements inside the sill to the header, one on the left, one on the right and then another in the middle. Make note of which vertical dimension is smallest. Then make three horizontal measurements from inside the jambs. One at the top, one at the bottom and another in the middle. Once again note down the smallest measurement. You now have your vertical and horizontal opening dimensions. Subtract 1/4" from each of these. This will be your replacement window sizes.

Before ordering your window, double check your dimensions and place a small level on the bottom sill and check to make sure its not angled up toward the inside of the room. Sometimes water gets in underneath the sill and the wood expands, warping the bottom sill. If you have a badly warped sill, the whole sill and jambs will have to be removed and the window installed in the original rough opening. A new window size will have to be calculated for this.

If you are replacing a window right to the rough opening measurement gets a little more difficult. This is where installers prefer to make the new window smaller because it covers them in case they cant see something during measurement. To do a proper job of measuring you need to take the casing off the inside of the window so you can actually see the original wood framing that the original window was nailed into. Sometimes taking the casing off, does not actually reveal the framing. You might have to make some small holes on the plaster or drywall to be sure.

If you can see the framing, you can measure inside the framing and apply the same three horizontal and three vertical measurements as described before. Take the smallest horizontal dimension and the smallest vertical dimension. Subtract 1/4" from each number, this will be your new window size provided there is no warped framing that you cant see. If in doubt go for 1/2" off the smallest vertical and horizontal dimensions.

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