Circle Calculator

Step 1: Enter any one value — radius, diameter, circumference, or area. Leave the other three blank.

Step 2: The calculator is unit-agnostic — square units for area, linear units for the rest. Inputs in inches give results in inches and square inches.

Step 3: Click Calculate to see the other three properties.

Every circle is completely defined by a single measurement — its radius — the distance from the center to any point on the edge. From the radius, the diameter (twice the radius), circumference (distance around), and area (space enclosed) can all be derived.

Circles appear everywhere: wheels, clock faces, pizza, planetary orbits, pipe cross-sections, and countless design elements.

The constant π (pi) relates the circumference to the diameter of every circle — a ratio that is the same for all circles, regardless of size.

Radius → Diameter: d = 2r
Radius → Circumference: C = 2πr
Radius → Area: A = πr²

Reverse: r = d/2 = C/(2π) = √(A/π)

Example: r = 5
Diameter = 2 × 5 = 10
Circumference = 2π × 5 ≈ 31.416
Area = π × 5² ≈ 78.540

Example 1: A pizza with radius 8 inches has area 201.06 sq in and circumference 50.27 in.

Example 2: A circular pool with circumference 25 feet has radius 3.98 ft and area 49.74 sq ft.

Example 3: A coin with diameter 2 cm has area 3.14 sq cm and circumference 6.28 cm.

π ≈ 3.14159 — this calculator uses full precision.

Units matter. Inputs in inches give results in inches or square inches.

C/d = π for every circle — the original definition of pi.

Area scales with r² — doubling radius quadruples area.