MATH · CIRCLE
Circle Calculator
Find any circle measurement — radius, diameter, circumference, or area — from any single known value. Enter one quantity and get all four instantly.
Radius5
Diameter10
Circumference31.4159
Area78.5398
About This Calculator
A circle is fully defined by a single measurement. This calculator lets you start from any one of the four key quantities — radius, diameter, circumference, or area — and instantly computes all the others.
How It Works
Choose which measurement you know from the dropdown, enter its value, and the calculator derives the remaining three using these exact relationships: d = 2r, C = 2πr, A = πr². For any given input the radius is derived first, then all other quantities follow. Results are rounded to 4 decimal places.
The Formula
d = 2r C = 2πr A = πr²
- r
- radius
- d
- diameter (d = 2r)
- C
- circumference (C = 2πr)
- A
- area (A = πr²)
Frequently Asked Questions
- What is the circumference of a circle with diameter 10?
- C = πd = π × 10 ≈ 31.4159. Equivalently, r = 5, so C = 2π × 5 ≈ 31.4159.
- How do I find the radius from the area?
- Rearrange A = πr² to get r = √(A/π). For example, if A = 78.54, then r = √(78.54/π) ≈ 5.
- What is the relationship between circumference and diameter?
- C = πd, so the ratio of circumference to diameter is always π (≈ 3.14159). This is one definition of π.
- How is this different from the area calculator?
- The area calculator covers six shapes including the circle. This calculator focuses exclusively on circles and lets you start from any of the four circle measurements, not just the radius.