How to Measure Windows for Replacement

You can measure your own windows, and it is worth doing. A list of rough sizes lets you get budget numbers by phone or email, compare companies before anyone visits, and catch a quote that has miscounted your openings.

What you should not do is treat those numbers as an order. This page explains both kinds of measuring and why they are different jobs. It is part of our California window guide.

Measuring for a budget quote

You need a steel tape measure, a notepad, and a way to label each window. A helper makes large windows easier.

Step 1: make a window list

Walk the house and number every opening. For each one, write down:

• The room and the wall it is on (for example, "primary bedroom, west").
• How it opens: slider, single-hung, double-hung, casement, awning, fixed. If you are unsure of the names, see how to choose a window style or the glossary.
• Which side moves, as seen from inside.
• Whether the glass is obscure (bathroom glass) or has grids.
• Whether it is upstairs.

Never assume two windows are the same size because they look it. Milgard's measuring guide makes the same point: measure each window individually.

Step 2: measure width

From inside, measure between the side jambs at three points: near the top, at the middle, and near the bottom. Andersen's guide says to measure jamb to jamb, or drywall to drywall if there is no trim around the window. Write down all three and circle the smallest.

Step 3: measure height

Measure from the sill to the head at three points: left, center, and right. Milgard describes the sill here as where the sash rests when closed. Again, record all three and circle the smallest.

Step 4: check for square

Measure both diagonals, corner to corner. If the two numbers match, the opening is square. Andersen treats a difference within 1/8 inch as square. Milgard's guide says that if the diagonals are within 1/4 inch of each other, a standard replacement window will fit securely, and that beyond that you may need to replace the frame or use a new-construction window.

Write the difference next to that window on your list. An installer will want to know.

Step 5: note depth and surroundings

For houses with wood windows where an insert might be used, Andersen and Milgard both give a minimum jamb depth of 3 1/4 inches between the inside and outside stops. For any house, note what is around the window: tile, a countertop, shutters, built-in cabinets, an alarm sensor, security bars, a deck or roof below it outside.

What to hand a contractor

A simple table is enough:

Those example rows are illustrations of the format, not typical sizes. With a list like this, a contractor can give you a realistic budget range before visiting, and you can use the same list with every company so the quotes cover the same openings. See how to compare window quotes.

How installers measure for the order

The order measurement is done on site, by the company that will install the windows, usually after you have signed and before anything is manufactured. It differs from yours in three ways: what gets measured, what gets inspected, and who carries the risk.

Three widths, three heights, smallest wins

The method is the same in principle, and it is written into the installation standard. AAMA 2410, the industry practice for flush-fin retrofit windows, calls for three horizontal measurements between the existing jambs, at the corners and the center, recording the smallest, and three vertical measurements from head to sill, recording the smallest. It then calls for checking squareness by cross-measuring the diagonals.

The smallest dimension governs because a window is a rigid rectangle going into a hole that is not one. Order to the largest dimension and the frame will not go in. The standard says sizing must allow a proper fit "without having to rack or force" the window into the opening.

The deduction

The window is ordered smaller than the tightest measurement, to leave room for shims, sealant, and adjustment. AAMA 2410 sets the retrofit window at no less than 1/4 inch smaller in width and height than the smallest measurements, and notes that out-of-square conditions may require additional allowance. Individual manufacturers set their own figures; JELD-WEN's flush-fin instructions call for the opening to be 1/2 to 5/8 inch larger than the window.

This is where experience shows. Too small a deduction and the window binds. Too large and the installer is filling a wide gap with shims and sealant, and on a retrofit the exterior flange may not fully cover the old frame.

Level, plumb, and plane

Installers put a level on the sill, the head, and both jambs. Andersen recommends the longest level possible for judging plumb, level, and square. They also check whether the wall is in one plane. JELD-WEN's instructions call for the exterior face of the opening to be in a single plane with less than 1/8 inch of twist corner to corner. A sill that sags in the middle or a wall that twists changes how the window is sized and set.

Retrofit versus full-frame: different things get measured

Andersen's guidance for professionals adds that horizontal measurements may be taken at different reference points inside and out, jamb to jamb on the interior and brick to brick on the exterior, and that an in-person assessment of both sides is what makes accurate measurement possible. The differences between the methods are covered in retrofit versus new-construction windows and nail fin, flush fin, and block frame.

What else the measure visit checks

A good measure tech is inspecting as much as measuring. AAMA 2410 requires a pre-installation inspection of the walls next to the window and of the existing frame for damage, deterioration, or decay. Beyond that, the visit should settle the code items that depend on size and position:

• Bedroom escape openings. A new frame inside an old one makes the clear opening smaller. The California Residential Code's allowance for replacement windows, Section R319.5, depends on the replacement being the manufacturer's largest standard size that fits the existing frame or rough opening, in the same operating style or one that gives an equal or greater opening area.
• Safety glazing. Whether tempered glass is required depends on measurements: the size of the pane, the height of its bottom edge above the floor, and its distance from doors, tubs, and stairs. See California tempered glass requirements.
• Low sills on upper floors. CRC Section R321.2.1 addresses operable windows whose opening is less than 24 inches above the floor and more than 72 inches above the ground outside. See window fall prevention.
• Total area for Title 24. The Energy Code defines fenestration area as the rough opening area. The total being replaced decides whether a job falls within the 75-square-foot small-job exception. See Title 24 window requirements.

Why the installer's measurement is the one that gets ordered

Three reasons, in order of importance.

Responsibility. Replacement windows are built to order. If a window arrives the wrong size, someone pays for a new one and the job waits while it is built. When the installing company measured, that is their problem. When the homeowner supplied the sizes, it is the homeowner's. No reputable installer will order from a customer's numbers for that reason, and you should be wary of one who offers to.

The measurement depends on the method. Your inside jamb-to-jamb number is not the number a flush-fin retrofit is ordered from, and neither is it the rough opening a full-frame window needs. Until the frame type is chosen for each opening, there is no single correct size.

The details beyond size. An order includes the frame type, the operating direction, the glass package, tempering, obscure glass, grid pattern, color, hardware, and sometimes the jamb depth. Milgard notes that each window needs the right combination of size, style, frame material, glass package, color, and options. Each of those is checked against the opening at the measure visit.

Common measuring mistakes

• Measuring the glass or the sash instead of the opening. Glass size is only useful when ordering glass.
• Measuring trim to trim. Interior casing is wider than the opening.
• One measurement per dimension. Openings are rarely the same top and bottom.
• Swapping width and height.
• Rounding up. Andersen says to round down to the nearest 1/8 inch.
• Noting the wrong handing. State which side moves and whether you are describing it from inside or outside.
• Forgetting what the window sits next to. Tile, counters, and shutters all affect how the new frame can be set.

A note on patio doors and odd shapes

Sliding patio doors are measured the same way, with one addition: the sill. The installer checks whether the threshold is level across its full width and what it sits on, because a door that is not level will not roll or lock properly. Arched, round, and angled windows are measured with templates or a set of reference dimensions the manufacturer specifies, and are not something to estimate from a single width and height.

After the measure

Ask for a copy of the order schedule before it is placed: every opening, with size, type, handing, glass, and frame type. Check it against your own list. This is the last cheap moment to catch an error. What happens next is on our window replacement timeline page.

Sources

• FGIA, AAMA 2410-13, Sections 5 and 6.1 to 6.2 (licensed copy published by Anlin)
• Milgard, How to Measure for Replacement Windows
• Milgard, Window Replacement Process: What to Expect
• Andersen, How to measure windows
• Andersen, How to assess windows for replacement
• JELD-WEN, Installation Instructions for Vinyl Windows with Integral Flush Fin (JII-90024), retailer-hosted copy
• 2025 California Residential Code, Sections R319.5, R321.2 and R324
• California Energy Commission, 2025 Building Energy Efficiency Standards, Section 100.1 (fenestration area) and Section 150.2(b)1B

Working with Heritage Exteriors

Send us your own list and we will give you a budget range from it. If you go ahead, we measure every opening ourselves, inside and out, and the sizes we order are ours to stand behind. Licensed in California, CSLB #1055919. Call (916) 546-2991.