Measure the framed opening in the wall, or the old window. You get the window size to order in the makers’ call-out (a 3050 is 3′0″ × 5′0″), its rough opening, what it opens to, and whether it passes as a bedroom’s escape window.
Window to order
A 3050 double-hung window (3′0″ × 5′0″)rough opening 36″ × 60″ · frame 35½″ × 59½″ · opens 30.5 × 24.3″
Quick answer: windows are ordered by the rough opening they fill, in feet and inches: a 3050 is 3′0″ × 5′0″, 36 × 60″, and its frame is ½″ smaller each way (Pella). Standard double-hung sizes run 24 to 54″ wide and 36 to 78″ tall. A bedroom’s escape window must open 5.7 sq ft, at least 24″ high and 20″ wide (IRC R310).
How to use the window size finder
- Pick the window: double-hung, sliding, casement or picture.
- Measure the rough opening, stud to stud and sill to header, or the old window’s frame, outside edge to outside edge (add ½″ and you have its rough opening).
- Say if it is in a bedroom. The finder checks it as the room’s escape window, and the sill’s height if you give it.
- Read the size to order, the frame, what it opens to, and what to do with any gap. Copy link keeps the result.
The rules behind the numbers
The sizes: Pella’s tables
The finder uses Pella’s 250 Series vinyl windows, from its architectural design manual. Each size is named by the opening it fills: 3-0/5-0 in Pella’s tables, a 3050 on the building site. Double-hung windows come 2′0″ to 4′6″ wide and 3′0″ to 6′6″ tall; sliding windows 2′0″ to 6′6″ wide and 2′0″ to 5′0″ tall; casements 1′3″ to 3′0″ wide, never wider than tall; picture windows 1′2″ to 8′0″ wide and up to 6′0″ tall. Other makers’ call-outs follow the same pattern: check their size table before you order.
The rough opening
Pella, on every table: “Rough Opening = Frame Dimension + 1/2″”. The frame is ½″ smaller each way than the opening, which leaves ¼″ around it to shim and seal. When your opening is more than 1″ bigger than a standard size, frame it in, or order a special size built to it: Pella builds them in ⅛″ steps.
What it opens to, and the bedroom escape rule
The finder works out the clear opening with Pella’s own formulas for each window: a double-hung opens only its lower half, a slider one pane, a casement nearly all of it, a picture window not at all. The International Residential Code asks every bedroom for one escape window with a net clear opening of at least 5.7 sq ft (5 sq ft at the ground floor), 24″ high and 20″ wide, its bottom no more than 44″ above the floor (R310.2.1–R310.2.3). The formulas give the same verdicts as the egress marks in Pella’s own tables.
Common double-hung window sizes
| Call-out | Rough opening | Frame | Opens | Bedroom escape |
|---|---|---|---|---|
| 2030 | 24 × 36″ | 23½ × 35½″ | 18.5 × 12.3″ | no |
| 2440 | 28 × 48″ | 27½ × 47½″ | 22.5 × 18.3″ | no |
| 3040 | 36 × 48″ | 35½ × 47½″ | 30.5 × 18.3″ | no |
| 3046 | 36 × 54″ | 35½ × 53½″ | 30.5 × 21.3″ | no |
| 3050 | 36 × 60″ | 35½ × 59½″ | 30.5 × 24.3″ | ground floor only |
| 3252 | 38 × 62″ | 37½ × 61½″ | 32.5 × 25.3″ | yes |
| 3056 | 36 × 66″ | 35½ × 65½″ | 30.5 × 27.3″ | yes |
| 3060 | 36 × 72″ | 35½ × 71½″ | 30.5 × 30.3″ | yes |
| 3650 | 42 × 60″ | 41½ × 59½″ | 36.5 × 24.3″ | yes |
| 4050 | 48 × 60″ | 47½ × 59½″ | 42.5 × 24.3″ | yes |
Pella 250 Series, equal sash. The smallest double-hung that passes as a bedroom’s escape window is a 3252; a casement passes from a 3036, because it opens nearly its whole frame.
Three worked examples
A rough opening 37 × 61″. A 3050 double-hung: its frame is 35½ × 59½″, leaving 1″ each way to shim.
An old bedroom window, frame 35½ × 47½″. A 3040, the same size. But it opens only 18.3″ high, and a bedroom’s escape window needs 24″: a double-hung passes from a 3252 up, so the opening has to grow, or a casement goes in.
A casement for a rough opening 40 × 50″. The standard 3040 leaves 4″ across and 2″ up: frame it in, or order a special 39½ × 49½″ frame. Either way it opens 7 sq ft, enough for a bedroom.
Common window sizing mistakes
- Ordering the rough opening as the window size. The frame is ½″ smaller; a 36″ frame will not go into a 36″ opening.
- Measuring the glass, or inside the trim. Measure the rough opening, or the old frame’s outside edges.
- Trusting a big window to be an escape window. A double-hung opens half its height: a 3040 opens only 18″ high.
- A sill too high. The code allows the bottom of a bedroom’s escape opening 44″ above the floor at most.
Dressing the window next? The free curtain size calculator gives the panels and the rod for it, the blinds size calculator the blind to order inside or over the frame, and the door size finder does the same job for doors. Plan the room around it in the free 2D room planner.
Size the curtainsRead next
Window size questions
What are standard window sizes?
Pella’s double-hung windows come 24 to 54″ wide and 36 to 78″ tall, named by the opening in feet and inches: 2030, 2440, 3040, 3050, 3060, 3650 and so on. Sliders run 24 to 78″ wide, casements 15 to 36″ wide, picture windows up to 96″ wide.
What is the rough opening for a window?
The framed hole in the wall, stud to stud and sill to header. For Pella’s windows it is the frame plus ½″ each way: a window with a 35½ × 59½″ frame needs a 36 × 60″ rough opening.
What size is a 3040 window?
3′0″ × 4′0″: a 36 × 48″ rough opening, with a 35½ × 47½″ frame. As a double-hung it opens about 30.5 × 18.3″, too low to be a bedroom’s escape window.
What size does an egress window have to be?
It must open at least 5.7 sq ft (5 sq ft at the ground floor), 24″ high and 20″ wide, with the bottom of the opening no more than 44″ above the floor (IRC R310). The smallest double-hung that passes is a 3252; a casement from a 3036.
What is a standard bedroom window size?
Any size that passes the escape rule: a 3056, 3060 or 3650 double-hung, or a 3036 casement and up. A 3050 double-hung passes only on the ground floor.
How do I measure for a replacement window?
Measure the old window’s frame, outside edge to outside edge, across and top to bottom: that plus ½″ is the rough opening, and the finder names the size that fits it. For an insert that goes inside the old frame, the maker’s own measuring guide rules.