When you plan to replace window, careful measurement of the window opening is critical. To expand the size of a window requires significant reconstruction of the surrounding walls. Replacing a window with a new window of the same size is much easier. A same-size replacement is called a “pocket replacement.” This allows a homeowner to keep the trim intact around the windows. In this case, the replacement window should measure the same as the old window (or slightly smaller) but certainly no larger than the wall’s rough opening.
Depending on the window’s construction, it may help to remove the old interior trim for a more correct measurement of the rough opening. If a window manufacturer offers specific measuring instructions, defer to those guidelines.
Otherwise, follow these basic steps for measuring a window for replacement:
When measuring the existing window for width, place the tape against the jamb and measure to the other jamb—not the stop molding or the parting bead.
The parting beads are the wooden strips attached to the jamb on double-hung windows that separate the sashes. The parting bead is removed from the jamb when the new windows are installed. Replacements for double-hung windows are sold in a standard thickness to fit the 3-1/4-in. pocket between the outside and inside stop molding after the parting bead has been removed.
Measure the window’s width in three places: the top, middle, and bottom. Use the smallest of these measurements to determine the final width around the window.
Next, measure the height of the window, placing the tape on the very top of the sill and measuring up to the top jamb. The sill should not be confused with the stool, which is a decorative piece of interior molding at the bottom of the window. On a double-hung window, the sill is the board the sash closes onto. The sill is usually sloped to divert water away from the house, so it is important to take measurement from the uppermost point of the sloped sill to the top jamb (not a stop or parting bead).
Measure the window’s height in three places—right, center, and left—and use the smallest of these measurement to determine the height of your replacement. Once you have taken all of the existing window measurements, consider deducting slightly from them so the replacement window is easier to install. Some professionals suggest deducting 1/4 inch from the width and 1/2 inch from the height, to make sure that the installer won’t try to force the new window into place, potentially damaging it. The replacement window can be adjusted using shims, and the extra space can be filled with insulation.
Remember that the replacement window must fit into the opening plumb, level and square, even if the opening may not be any of these.
Most manufacturers will offer standard sized windows. For a pocket replacement, order a unit that most closely matches your measurements. (Some manufacturers customize their windows to the exact measurements that are requested, so the standard sizing issues don’t apply.)
Remember: If the window is slightly smaller than the window frame, it can be shimmed to properly fit, but if the window is too large, the wall will need substantial reconstruction to fit it.
Measuring Windows for Replacement in Brick
Metal windows are generally fastened with a flange inside the wall behind the brick. To measure the window’s height and width for replacement, follow the same three-step measuring guidelines above, but strike the tape from brick to brick to measure the window’s width, and measure from the brick at the bottom to the metal lintel at the top of the opening for the height. The brick opening may be larger than the window opening; always use the smaller measurement to determine the size of the replacement.