Circle Circumference Calculator

The formula

Circumference = 2 × π × radius, or equivalently π × diameter.

π is precisely the ratio between a circle's circumference and its diameter, about 3.14159 — the same for every circle.

Worked examples

• A circle with a 35 cm radius: 2 × π × 35 = 219.9 cm.
• A bicycle wheel about 70 cm in diameter (radius 0.35 m) travels 2.2 m per turn — about 455 turns per kilometre.
• The Earth, with a mean radius of 6,371 km, has a circumference of about 40,030 km. The metre was originally defined in the 18th century so that a quarter of a meridian would measure 10,000 km.

Practical uses

• Edging and trim: a round flower bed 3 m in diameter needs π × 3 ≈ 9.4 m of border.
• Round tables: allow about 60 cm of edge per person. A 1.5 m diameter table has a circumference of 4.71 m, enough for 7 to 8 people.
• Measuring a diameter you can't reach: wrap a tape around a tree trunk or a pipe and divide by π. A trunk measuring 1.57 m around is 50 cm in diameter.

Reverse calculations

• Radius from circumference: r = C ÷ (2π).
• Diameter from circumference: d = C ÷ π.

A surprising property

If you added 1 metre to a rope wrapped tightly around the Earth's equator and spread the slack evenly, the rope would rise about 16 cm above the ground everywhere — because the circumference grows by 2π for each unit of radius, whatever the size of the circle (1 ÷ 2π ≈ 0.159 m).

For the surface inside the circle, use the circle area calculator; for an arc, multiply the circumference by the angle ÷ 360°.

Formulas and content last reviewed: October 2026 · About