Circle Area (Square Footage) Calculator

By Vinod Kondeti · Updated October 14, 2026

Round areas such as a fire-pit patio, an above-ground pool pad or a tree ring use π × radius². Most people measure across the circle, so enter the diameter and the calculator halves it for you.

Diameter (ft)

Cost estimate (optional, enter your local prices)

Price per ft² (material or installed) ($)

Square feet	–
Square yards	–
Square meters	–
Acres	–
Estimated cost	–

Working: –

How do I measure for a circle project?

Measure across the widest point, through the centre, for the diameter.
For a semicircle, halve the result.
For a ring (a circle with a hole), work out both circles and subtract the inner one.

How is the circle amount calculated?

Rectangle: length × width
Circle: π × (diameter ÷ 2)²
Triangle: ½ × base × height
Square yards = ft² ÷ 9; m² = ft² × 0.09290304; acres = ft² ÷ 43,560
Cost = ft² × your price per ft² (flooring: boxes × price per box)

Circle calculator questions

What is the area of a 10 ft circle?

π × 5² = 78.54 ft².

Why does doubling the diameter quadruple the area?

Area depends on the radius squared, so a 20 ft circle (314.16 ft²) has four times the area of a 10 ft circle.

Sources

Estimates only. Use the coverage or bag size printed on your product. Read the square footage guide for worked examples.