There is no building code that defines a standard window size. What the industry has is a set of common dimensions that manufacturers hold in stock or run frequently, and those cost less and arrive sooner than anything else.
The dimensions that recur
Common widths cluster around 24, 28, 30, 32, 36, 48 and 60 inches. Common heights around 36, 48, 54, 60 and 72 inches.
Some manufacturers still quote sizes in the old four-digit shorthand, where 3050 means 3 feet 0 inches wide by 5 feet 0 inches high — feet and inches, not inches. It appears on older plans and in some order systems, and misreading it as inches produces a very wrong window.
Why it matters less than it sounds
Replacement windows are made to size. Nearly every manufacturer builds to the eighth of an inch at little or no premium, because a replacement job is custom by definition — no two openings in a fifty-year-old house are identical.
So "will a standard size fit my opening?" is the wrong question. The right ones are:
Is it in stock? Stock sizes ship in days. Custom runs commonly take four to twelve weeks, and that gap matters far more than the price difference.
Does it cross a price break? Manufacturers set maximum sizes per product line and per operating type. Going one inch over a line's maximum for a casement moves the order to a different, more expensive line — and that step is far larger than the cost of being custom.
Is it a stocked replacement part later? A common size is more likely to have a glass unit available in fifteen years when the seal fails.
Where new construction differs
In new work the size is chosen, so stock sizes are worth using: the framing is laid out to suit, and the rough opening is simply the unit size plus about 1/2 to 3/4 inch all round for shimming.
In replacement, the opening already exists and the window follows it.
Operating type constrains size more than the catalogue does
Casements are limited by hardware. Beyond roughly 3 feet wide the sash weight exceeds what standard operators handle, and the window either needs heavy-duty hardware or has to become a fixed unit with a smaller vent beside it.
Double hungs get heavy as they get tall, and the balances have to be specified for the weight. Triple glazing pushes this further than most people expect.
Sliders span wide comfortably but seal least well of any type, and a very wide slider gets harder to operate as the roller load rises.
Fixed units span the furthest and cost the least per square foot, which is why large windows are often a fixed centre with operating flankers rather than one enormous opening sash.
The dimension nobody checks
Frame depth. A replacement unit needs a minimum wall depth to sit in, and a 2×4 wall with thick exterior insulation or an interior furring layer can be tighter or deeper than the standard assumption.
Measure it before ordering. It is the dimension that most often produces a window that fits the opening in two directions and not the third.
Practical approach
Measure the openings properly, give the supplier the real numbers, and ask two questions: whether any of them happen to land on a stock size, and whether any of them are close to a line's maximum. The first can save weeks; the second can save a great deal of money for the sake of half an inch.
Mixing sizes across one elevation
A house rarely has one window size, and the eye is good at spotting near-misses. Two windows on the same wall that differ by two inches in height read as a mistake; two that differ by a foot read as a design decision.
This matters when only some windows are being replaced. Matching the sight line — the height of the head, and where the meeting rail of a double hung falls — does more for how the elevation looks than matching the exact overall size. Where the old and new cannot match, keeping the heads aligned and letting the sills differ is almost always the better-looking compromise.
What to send the supplier
A list, one line per opening, with the room, the wall, the three width readings, the three height readings, the diagonals and the frame depth. Then let them tell you which are stock and which are custom.
That order of operations matters: measure first and ask about stock sizes second. Choosing a window because it is a stock size and then trying to make the opening suit it is how a replacement job turns into carpentry.