Circle Square Footage Calculator
Enter the radius (or half the diameter) of any circle. We'll calculate the area in square feet using the formula π × r².
Last updated:Tip: If you only have the diameter, divide by 2 to get the radius.
L-shape = two rectangles sharing a corner.
Add one row per rectangle and we'll sum them.
Circle Square Footage Formula
The area of a circle is π × radius². Using π ≈ 3.14159:
- Radius = 5 ft → π × 5² = 78.54 sq ft
- Radius = 10 ft → π × 10² = 314.16 sq ft
- Radius = 15 ft → π × 15² = 706.86 sq ft
If you only have the diameter, divide it by 2 first. A 20-ft diameter pool has a 10-ft radius and therefore an area of 314.16 sq ft.
Common Uses for a Circle Square Footage Calculator
Round pools, garden beds, silos, trampolines, and patios are the most common reasons to calculate circular area. For a round above-ground pool, measure the inner diameter at the water line, divide by 2, and enter the radius.
For a circular concrete pad, this gives its footprint, not the concrete volume. For a pool, it gives the flat water-surface area, not the curved liner area or water capacity. Those quantities require more measurements.
Half-Circle and Quarter-Circle Areas
A semicircle (half-circle) is exactly half the area of a full circle: (π × r²) / 2. A quarter-circle is one-quarter: (π × r²) / 4.
Calculate the full circle area in the tool above, then divide.
A Circular Border: Which Area Are You Buying For?
A round patio has an outside diameter of 20 ft and a uniform 1-ft border inside its edge. The inner surface and the border need different material quantities. Subtract the border on both sides, not just once.
- Inner diameter:
20 − 2 × 1 = 18 ft. - Inner surface:
π × 18² ÷ 4 ≈ 254.47 sq ft. - Border only:
π × (20² − 18²) ÷ 4 ≈ 59.69 sq ft.
For outside diameter D and border width w, use border area = π × [D² − (D − 2w)²] ÷ 4. This assumes concentric circles, uniform width, matching units, and 0 ≤ 2w < D. An off-center or uneven border needs separate measurements.
To check with the calculator above, enter radius 10 ft, note Total Area, then enter 9 ft. Subtracting the displayed results gives 314.16 − 254.47 = 59.69 sq ft here; those displays are rounded. For an unrounded calculation, use π × (10² − 9²) and round the final answer. Apply any cutting allowance to the material you are ordering, not automatically to both surfaces. For a concrete volume, also supply thickness in the area-to-volume calculator.
What If Your Diameter Measurement Is Approximate?
Suppose you measure 20 ft across and judge that the actual diameter could be up to 2 inches smaller or larger. Two decimal places on a calculator do not make that tape measurement exact.
| Diameter assumption | Calculation | Area |
|---|---|---|
| 19 ft 10 in | π × (20 − 2/12)² ÷ 4 | 308.95 sq ft |
| 20 ft | π × 20² ÷ 4 | 314.16 sq ft |
| 20 ft 2 in | π × (20 + 2/12)² ÷ 4 | 319.42 sq ft |
With diameter D and your chosen error bound e in the same units, the limits are π(D − e)²/4 and π(D + e)²/4, where 0 ≤ e < D. This is a measurement-bound example, not a statistical confidence interval or a recommended waste percentage. It assumes a true circle; measure across the center in more than one direction to check that assumption.
These worked examples are derived from the circle area formula. Calculations keep full precision until the displayed result is rounded.
Frequently Asked Questions
What is the square footage of a 10-foot radius circle?
π × 10² = 314.16 sq ft (approximately 29.19 sq m).
How do I calculate area from a diameter?
Divide the diameter by 2 to get the radius, then use π × r². For a 24-foot diameter, the radius is 12 ft and the area is 452.39 sq ft.
What's the area of a 20-foot round pool?
If "20 feet" refers to the diameter (most common), radius = 10 ft and area = 314.16 sq ft. If it's the radius, area = 1,256.64 sq ft.
Does this work for circular patios?
Yes. Measure the radius from center to edge for the footprint area. For pavers, choose a cutting allowance appropriate to the layout. Concrete also requires the specified thickness to calculate volume; an area waste factor alone does not give the order volume.