United inches explained: how windows are sized and priced
United inches — simply width + height — is how the trade sizes and prices windows. Learn what it means, why it beats area for comparing quotes, and how to measure for it.
Walk into any window quote and you will hear a number that has no obvious meaning to a homeowner: "that's about a hundred and ten united inches." United inches is the fenestration industry's shorthand for window size, and once you understand it, comparing a tall casement to a wide picture window — and reading a price list — suddenly makes sense.
What united inches actually is
United inches (UI) is simply the width plus the height of a window, in inches. A 36 × 60 inch window is 36 + 60 = 96 united inches. That is the entire formula; there is nothing hidden in it. What makes it useful is that it collapses two dimensions into one number that tracks how much a window costs to build. Glass, frame, hardware and shipping all scale roughly with the perimeter and the size of a unit, and width + height is a fast proxy for both. So suppliers band their pricing by united inches: a window up to 101 UI falls in one price tier, up to 120 UI in the next, and so on. The united-inches calculator returns the number for any width and height, and the united-inches sizing convention table lists UI for common stock sizes.
Why not just use area?
Area — width × height ÷ 144 — tells you how much glass and daylight you get, and it is the right number for energy and cost-per-square-foot thinking. But area alone can mislead a price. A 24 × 72 window and a 48 × 36 window both come out to 12 square feet, yet the tall narrow one has more perimeter frame, more sash structure and a different glass aspect, so it does not cost the same to make. United inches (24 + 72 = 96 versus 48 + 36 = 84) captures that difference; the tall unit reads larger, which matches its higher build cost. Use united inches to compare and price and square footage to think about glass and energy — the window area calculator handles the second number.
A worked comparison
Suppose you are comparing three quotes for the same opening, each priced by UI. Your opening is 36 × 60, so 96 united inches. Quote A prices to 101 UI (its next tier up), quote B prices to exactly 96 UI, and quote C rounds down to a 90-UI tier because it is substituting a slightly smaller stock unit. Now the numbers are comparable: A is padding to the tier ceiling, B is honest to your size, and C is quietly shrinking your window. Without united inches, three different dollar figures look arbitrary; with it, you can see what each installer is actually selling.
Measuring for united inches
Because UI drives price, measure it carefully. Use the smallest-of-three method: three width readings, three height readings, take the smallest of each, and add them. Measure to the frame for an insert or to the rough opening for a full-frame job — and be consistent, because mixing an opening width with a unit height gives a UI number that describes no real window. For rough sizing, the frame outside dimension is what you want; the daylight opening (the visible glass) is a little smaller and only matters where a spec explicitly calls for it.
Standard versus custom
United inches also hints at whether you are ordering a stock size or a custom one. Manufacturers stock windows on a grid of common sizes — think even-inch widths and heights like 24, 30, 36 wide and 36, 48, 60, 72 tall. A 36 × 60 lands on that grid; a 33.25 × 57.5, measured to an existing opening, does not, and becomes a custom order at a custom price and lead time. The standard vs custom size reference checks your dimensions against the labeled stock grid so you know which conversation you are having with the supplier.
Where united inches stops
United inches is a sizing and pricing convention, not a code or performance figure. It does not tell you whether a bedroom window meets egress — that depends on the clear opening when the sash is open, which is a different measurement covered in the egress guide. It does not tell you the window's energy rating — that is U-factor and SHGC on the NFRC label. And the price tiers themselves are set by each supplier, so treat any band as a planning guide and confirm it against your own written quotes. What UI gives you is a common language: one number that lets you line up quotes, read a price list, and talk about window size the way the trade does.