Circle Square Footage Calculator
Calculate the area of a circle using the formula π × radius². Useful for round patios, pools, garden beds, and circular construction projects.
Circle Area Calculator
The formula and quick examples
Circle area = π × radius². π (pi) ≈ 3.14159. Multiply π by the radius squared.
Quick examples: a 10-ft diameter circle has a 5-ft radius and an area of 78.5 sq ft. A 20-ft diameter circle has a 10-ft radius and an area of 314 sq ft. A 30-ft diameter has a 15-ft radius and an area of 707 sq ft.
Doubling the diameter quadruples the area. A 20-ft circle has 4× the area of a 10-ft circle, not 2×.
Measuring a real circle
In the field, measure diameter (across the widest part of the circle through the center) - it's easier than measuring radius. Then divide by 2 for the radius.
For a circular patio: measure across at multiple angles to confirm it's actually a circle (not slightly oval). If diameters vary by more than a few inches, treat it as an ellipse - approximate with two semicircle ends or just average the diameter.
Pro tips
Measure diameter, not radius
The center of a circle isn't marked in the real world. Measure across (diameter) and divide by 2.
Half a circle uses half the formula
A semicircle area = (π × r²) ÷ 2. A quarter circle = (π × r²) ÷ 4. Useful for round-end patios and garden beds.
Annulus = ring shape
For a ring (donut shape), area = π × (R² - r²) where R is the outer radius and r is the inner. Use the Annulus mode on the multi-shape calculator.
Use 3.14159 for >1% accuracy
Using 3.14 introduces ~0.05% error. For most projects that's fine. For large patios or pools, use 3.14159 or the calculator's built-in π.