Circle Square Feet Calculator
This calculator finds the square footage of a circle from either its diameter (the distance across) or its radius (center to edge), in feet. Choose a full, half or quarter circle and you get the area in square feet, the radius it used, and the same area in square yards, square meters and acres.
How to calculate the square footage of a circle
Multiply pi (3.14159) by the radius squared. Measure the radius in feet and the answer is already in square feet, as Clemson Cooperative Extension's lawn measuring guide shows.
Square feet = 3.14159 × radius × radius
- Measure across the circle at its widest point. That's the diameter: 16 feet.
- Halve it for the radius: 16 ÷ 2 = 8 feet.
- Square the radius: 8 × 8 = 64.
- Multiply by pi: 64 × 3.14159 = 201.06 square feet.
The big mistake: squaring the diameter instead of the radius gives exactly 4 times the real area. A 16 foot circle is 201 square feet, not 804.
Circle sizes in square feet
A 10 foot circle is 78.5 square feet and a 20 foot circle is 314.2. Area grows with the square of the width, so doubling the width multiplies the area by 4.
Square footage of circles from 4 to 30 feet across
| Diameter | Radius | Square feet |
|---|---|---|
| 4 ft | 2 ft | 12.6 |
| 5 ft | 2.5 ft | 19.6 |
| 6 ft | 3 ft | 28.3 |
| 8 ft | 4 ft | 50.3 |
| 10 ft | 5 ft | 78.5 |
| 12 ft | 6 ft | 113.1 |
| 14 ft | 7 ft | 153.9 |
| 15 ft | 7.5 ft | 176.7 |
| 16 ft | 8 ft | 201.1 |
| 18 ft | 9 ft | 254.5 |
| 20 ft | 10 ft | 314.2 |
| 21 ft | 10.5 ft | 346.4 |
| 24 ft | 12 ft | 452.4 |
| 25 ft | 12.5 ft | 490.9 |
| 30 ft | 15 ft | 706.9 |
Half circles, quarter circles and rings
A half circle is the full circle's area divided by 2, and a quarter circle is divided by 4. Pick either one in the calculator's Portion box.
Radius is the distance out from the straight wall.
Radius is the run along each fence.
Outer circle minus inner circle; never subtract the widths.
Finding the radius from the area
Divide the square footage by 3.14159, then take the square root. A round bed of about 492 square feet has a radius of 12.5 feet, so it's 25 feet across.
Radius = √(square feet ÷ 3.14159)
To check it, enter 25 as the diameter above: the calculator returns 490.87 square feet.
Measuring a round patio, pool or bed
The longest straight line across a circle always passes through the center, so you never need to find the center itself. Stretch a tape edge to edge and pivot one end until the reading peaks: that's the diameter.
Measure the diameter twice at right angles and average the two before you enter it.
Measure around the outside and divide by 3.14159. A planter 37.7 feet around is 12 feet across.
A 24 foot round pool or a 9 foot round rug is measured across, so enter it as the diameter.
For a pool, the pool calculator gives the surface area and the gallons together. A 16 foot round patio (201 square feet) goes into the concrete calculator for cubic yards, and the lawn calculator handles the grass around it. For corners and gables, use the triangle calculator; for a whole project with several shapes, the main square footage calculator adds them up, and how to calculate square footage covers every shape.
Frequently asked questions
How do you calculate the square footage of a circle?
Multiply 3.14159 by the radius squared, with the radius in feet. A circle with an 8 foot radius is 3.14159 × 8 × 8, or about 201 square feet. If you have the diameter, halve it first.
How many square feet is a circle with a 5 foot radius?
78.5 square feet: 3.14159 × 5 × 5. If 5 feet is the diameter instead, the radius is 2.5 feet and the area is 19.6 square feet.
How do I get square feet if I measured in inches?
Work out the area in square inches, then divide by 144. A 36 inch round table has an 18 inch radius: 3.14159 × 18 × 18 = 1,017.9 square inches, or 7.07 square feet. Or divide the inches by 12 first and enter feet.
How many square feet is a 24 foot round pool?
About 452 square feet. The radius is 12 feet, and 3.14159 × 12 × 12 = 452.39. An 18 foot pool is 254.5, so the extra 6 feet of width adds about 198 square feet.
What value of pi should I use?
3.14 is fine for most jobs. A circle 21 feet across comes to 346.2 square feet with 3.14 and 346.4 with the full value of pi, which the calculator uses.
How much cover does a 19 foot round pool need?
About 283.5 square feet: the radius is 9.5 feet, and 3.14159 × 9.5 × 9.5 = 283.5.