Circle Area Calculator

The formula

Area = π × radius², where π ≈ 3.14159.

The result is in the square of the unit you use: a radius in centimetres gives an area in cm², a radius in metres gives m².

Worked examples

• A circle with a 5 cm radius: π × 25 = 78.54 cm².
• A round table 1.2 m across (radius 0.6 m): π × 0.36 = 1.13 m².
• A circular garden bed 3 m in diameter (radius 1.5 m): π × 2.25 = 7.07 m² — useful for ordering soil or mulch.

The pizza question

Because the radius is squared, area grows much faster than diameter:

A 30 cm pizza is 50% wider than a 20 cm one but has 2.25 times more to eat; a 40 cm pizza is as big as four 20 cm ones. Comparing price per cm² often shows that the large size is the best deal.

Common mistakes

• Using the diameter instead of the radius: halve the diameter first. Using the diameter by mistake gives an area four times too large.
• Mixing units: convert everything to the same unit before calculating, then use the area converter if needed (1 m² = 10,000 cm²).
• Rounding π too early: 3.14 is fine for a rough estimate, but keep more decimals for precise work.

Related formulas

• Radius from area: r = √(A ÷ π). A 50 m² circular lawn has a radius of √(50 ÷ π) ≈ 3.99 m.
• Area from circumference C: A = C² ÷ (4π).
• Half circle: π r² ÷ 2; quarter circle: π r² ÷ 4.

The circumference calculator gives the length around the circle, and the cylinder and sphere calculators extend these formulas to three dimensions.

Formulas and content last reviewed: October 2026 · About