In short
Multiply 3.14159 by the radius squared. The radius is half the diameter, so a circle 12 ft across has a radius of 6 ft: 3.14159 × 6 × 6 = 113.1 sq ft. Measure the diameter through the middle, at the widest point.
The formula
Square feet = π × radius × radius, with π about 3.14159 and the radius in feet. If you measured the diameter, halve it first.
Examples
| Diameter | Radius | Area |
|---|---|---|
| 4 ft | 2 ft | 12.6 sq ft |
| 6 ft | 3 ft | 28.3 sq ft |
| 8 ft | 4 ft | 50.3 sq ft |
| 10 ft | 5 ft | 78.5 sq ft |
| 12 ft | 6 ft | 113.1 sq ft |
| 14 ft | 7 ft | 153.9 sq ft |
| 16 ft | 8 ft | 201.1 sq ft |
| 20 ft | 10 ft | 314.2 sq ft |
| 24 ft | 12 ft | 452.4 sq ft |
Half and quarter circles
A half circle is half the area: a half circle 12 ft across is 56.5 sq ft. For a bay or a rounded end of a room, add the half circle to the rectangle next to it.
How to measure a round room
- Find the widest point and measure straight across through the middle: that is the diameter.
- Measure again at a right angle to the first line. If the two differ, the room is an oval: use the average, or draw it.
- Type the diameter in the calculator, or draw the room in the editor with curved walls.
Draw it instead
Draw this room opens the editor with a round room of exactly this size: two curved walls with the room inside them. The editor measures the same area as the calculator, and you can add doors, windows and furniture.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
About this page · Written by the Squaremetry team. Last checked on October 1, 2026. Found a mistake? Send us a message.