Window Size Calculator
Enter the room's length and width, how many windows it will have and whether it is a bedroom. The calculator works out the glass and opening area needed and suggests a standard window size with its rough opening.
- Daylight and air
- Standard sizes
- Bedroom escape
Window sizes are written width by height, in inches.
Building codes set a minimum window size for each room. In the US residential code, glass must be at least 8% of the floor area and the part that opens at least 4%. Bedrooms also need one window big enough to climb out of in a fire.
Window size chart
8% glass rule, one window. Search for a room.
| Room (ft) | Floor (sq ft) | Glass needed (sq ft) | One window (in) |
|---|---|---|---|
| 10 × 10 | 100 | 8 | 30 × 48 |
| 12 × 12 | 144 | 11.5 | 36 × 60 |
| 14 × 12 | 168 | 13.4 | 48 × 60 |
| 16 × 14 | 224 | 17.9 | 60 × 60 |
| 20 × 15 | 300 | 24 | Two 48 × 60 |
Glass area is taken as 75% of the window size.
Floor area and window
One window. Slide the floor area.
The wall shows the suggested window drawn to scale.
How to size a window
Floor area first. Step through it.
Floor area
Length × width.
The rough opening in the wall is usually the window size plus half an inch in each direction.
Casement, sliding or double hung
Window types. Compare them.
Hinged at the side.
One sash slides.
Two sashes slide up and down.
Casements open almost fully, so a smaller casement can meet bedroom escape rules that a sliding window cannot.
Before ordering
Window checks. Tick each item.
Tick each item as you go.
Measure the rough opening in three places for width and height, and use the smallest.
What you get
Window sizing. Each tile explains one result.
Standard.
8% of floor.
4% of floor.
In the wall.
For bedrooms.
More glass brings more light but also more heat loss. Choose good double or triple glazing.
Frequently Asked Questions
No question matches that search.
How big should a window be for a room?
The glass should be at least 8% of the floor area, and the opening part at least 4%.
What size window for a twelve by twelve foot room?
About 11.5 square feet of glass, such as one 36 × 60 inch window or two smaller ones.
What is a bedroom egress window?
A window large enough to escape through: at least 5.7 square feet open, 20 inches wide and 24 inches high.
What is a rough opening?
The hole in the wall framing, usually half an inch bigger than the window each way.
Are window sizes width by height?
Yes. A 36 × 48 window is 36 inches wide and 48 inches high.
Do UK rooms have the same rules?
UK rules focus on ventilation and escape windows rather than a glass percentage, but the 8% guide still gives good daylight.
Can I use several small windows?
Yes. The total glass area is what counts.
Light and air for every room
Work out the glass the floor area needs, split it across your windows and choose standard sizes. For bedrooms, make sure one window is big enough to climb out of.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.