How do I measure for replacement windows?

Measuring for replacement windows takes a tape measure, a pen and about ten minutes per window. You check whether each window is square, then take three width readings and three height readings and keep the smallest of each. Andersen's guide calls for two diagonal measurements taken from inside the house, each to the nearest 1/8 inch,^[1] and the same 1/8-inch precision for every other reading.

These numbers are for comparing prices and choosing a window type. The installer should take the final measurements before anything is ordered, for reasons explained below.

Know the parts you are measuring

Three parts of the frame matter:

• Jambs are the vertical sides of the frame.
• The head (or head jamb) is the top piece.
• The sill is the bottom piece.

Measure to the frame itself, not to the edge of the glass or the sash, which is the part that moves.

Check whether the window is square

Square means all four corners meet at 90 degrees, and it decides which kind of replacement window can go in. Measure from the top left corner, where the head meets the side jamb, to the bottom right corner at the sill. Then measure the other diagonal, top right to bottom left. If the two diagonals are within 1/8 inch of each other, the window is square.^[1]

Houses settle, and older windows are often slightly out. That is common and not a reason for alarm, but it changes the order.

Measure the width and height

Width. From inside, measure from jamb to jamb, or drywall to drywall if there is no trim, in three spots: near the top, at the centre and near the bottom, rounding down to the nearest 1/8 inch.^[1] Keep the smallest number.

Height. In the same way, measure from the head down to the sill at the left, the centre and the right, again to the nearest 1/8 inch, and keep the smallest.^[1] Taking three readings and keeping the smallest accounts for the chance that the window is out of square.^[4]

Write the result as width by height, for example 30 by 58, and label it with the room. A short list with every window's size, type and any damage makes quotes much easier to compare.

Insert or full-frame?

Your measurements also help you see which of two kinds of replacement window will work.

Insert windows. An insert window fits inside the existing frame, which can save money and keep your interior trim in place.^[1][4] It only works on a sound frame: the opening must be plumb, level and square, and Andersen's inserts need a jamb depth, the space between the inside and outside stops, of at least 3 1/4 inches.^[1] Measure that depth while you are there.

Full-frame windows. Full-frame replacement windows include both the window and a new frame, either with a nailing flange secured on the outside, much like new construction, or without a flange, fastened through the jambs and sealed inside and out.^[4] This is the usual route when the old frame is rotted or badly out of square. If the window is out of square, Andersen says you will need sizing adjustments when ordering and the rough opening might need repairs, which is work for an experienced window contractor.^[1]

Masonry walls. On brick, stucco and concrete houses, flanged windows are usually not an option, and the new window is set back in the opening with exterior trim covering the gaps.^[4] The size of the brick opening often sets the size of the window.

Why your numbers are for shopping, not ordering

Replacement windows are made to order, and a window that is 1/4 inch too big does not go in. Andersen recommends using your own measurements for comparison shopping, not for ordering, unless you have experience with home projects.^[2] Its support page explains why: old windows are often out of square, and the sizing differs depending on whether you choose a flanged, full-frame or insert window.^[3] To get it exact, an installer may need to remove the trim around a window to get the most precise measurement.^[2]

Ask in writing who is responsible for the measurements. If the installer measures and a window arrives the wrong size, that should be the installer's problem, not yours. On installation day, check that the windows delivered are the ones you ordered and carry both ENERGY STAR and NFRC labels before any old window comes out.^[7]

Code changes to keep in mind

A new window can be held to rules the old one was not. Andersen notes that sometimes tempered glass is required in bathrooms, in a low-placed window, or in a transom above a door, and a bedroom might need a larger window to meet egress requirements.^[1] Mention bathroom, stairway and bedroom windows when you ask for quotes.

Size also affects permits. Under Virginia's building code, replacing windows with windows of similar operation and opening dimensions that do not require changes to the existing framed opening needs no permit,^[5] but altering a required means of egress or any structural component does need a permit.^[5] If you want a bigger window, the measurement becomes a permit question; see whether you need a permit to replace windows in Virginia.

Stop and call a pro if

• The diagonals differ by more than 1/8 inch, or the frame looks twisted, so the opening may need repair.
• The wood is soft, stained or crumbling at the sill or the bottom of the jambs.
• You would need to pull off trim or interior casing to measure, especially in an older house. If your home was built before 1978, it may have lead-based paint, and the EPA says to assume it does if you cannot find out the age.^[6]
• The window is above the first floor and you would need a ladder to measure from outside.
• You want a larger opening, a different window type, or a bedroom window that meets egress rules.

Next steps

Once you have your list, read what window replacement costs in Virginia and how to tell whether your windows really need replacing. You can browse window replacement businesses, find exterior businesses around Blacksburg, or see the other exterior answers.

When you are ready for a measured quote, describe your windows once and local businesses will reply.