Circle Calculator - Find Radius, Diameter, Circumference & Area

⭕ Enter One Known Value

What do you know?

Value

Radius

Diameter

Circumference

Area

Circle Formulas

From Radius (r)

Diameter = 2r
Circumference = 2πr
Area = πr²

From Diameter (d)

Radius = d ÷ 2
Circumference = πd
Area = π(d/2)²

From Circumference (C)

Radius = C ÷ (2π)
Diameter = C ÷ π
Area = π × [C ÷ (2π)]²

From Area (A)

Radius = √(A ÷ π)
Diameter = 2 × √(A ÷ π)
Circumference = 2π × √(A ÷ π)

Frequently Asked Questions

Area = π × radius². For example, a circle with a radius of 5 has an area of π × 5² = π × 25 ≈ 78.54 square units.

Circumference = 2 × π × radius, or equivalently π × diameter. A circle with a radius of 5 has a circumference of 2 × π × 5 ≈ 31.42 units.

The diameter is always exactly twice the radius (Diameter = 2 × Radius), since the diameter is a straight line passing through the center connecting two points on the circle, while the radius is the distance from the center to the edge.

Rearrange the area formula: Radius = √(Area ÷ π). For example, if the area is 78.54 square units, the radius is √(78.54 ÷ π) = √25 = 5.

Circle Calculator - Radius, Diameter, Circumference & Area

Every measurement of a circle - radius, diameter, circumference, and area - can be derived from just one known value, since they're all mathematically connected through π (pi). This calculator lets you enter whichever single measurement you have and instantly computes the other three, useful for geometry homework, DIY projects (like sizing a round tabletop or garden bed), and engineering estimates.

Key formulas: Area = πr² · Circumference = 2πr · Diameter = 2r

Understanding Pi (π)

Pi is the ratio of a circle's circumference to its diameter - approximately 3.14159, though it's an irrational number with infinite non-repeating decimal digits. This constant is the same for every circle, regardless of size, which is exactly why all four circle measurements can be calculated from just one known value using formulas built around π.

Common Real-World Uses

DIY and crafts: Figuring out how much material is needed for a round tabletop, rug, or garden bed border.
Construction: Calculating pipe cross-sections, circular foundations, or round window openings.
Education: Geometry homework involving circles, sectors, and related shapes.
Everyday estimates: Comparing pizza sizes by area rather than diameter alone (a 16-inch pizza has nearly twice the area of a 12-inch one, not just 33% more).

How this calculator works, and where the numbers come from

The Circle Calculator applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.

Please note: Results are provided for general information and are calculated from the values you enter.

Sources and further reading

Last reviewed: October 6, 2026 by the CalcQube Editorial Team. See our editorial policy for how we build and check calculators, or report an error.

⭕ Circle Calculator