Circle Area Calculator

Enter the radius, diameter, circumference, or area of a circle and get every other measurement instantly, with the formula and worked steps shown.

C = 31.4159 r = 5 d = 10 centre A = 78.5398
Area —
Circumference —
Radius —
Diameter —

How to Find the Area of a Circle

Fill in exactly one field — radius, diameter, circumference, or area — and the rest are derived from it.

1

Enter one measurement

Type a single positive number into radius, diameter, circumference, or area. Clearing a field or typing in another one resets the rest, so only one value drives the calculation at a time.

2

The calculator solves for the radius

Every formula starts from the radius: r = d / 2 from the diameter, r = C / (2π) from the circumference, or r = √(A / π) from the area. Once the radius is known, the other three measurements follow directly.

3

Read the result and steps

Area, circumference, radius, and diameter all appear together, with the exact formula work shown below. Working with a right triangle instead? The Pythagorean Theorem Calculator solves for a missing side the same way.

Frequently Asked Questions

What is the formula for the area of a circle?
The area of a circle is A = πr², where r is the radius and π is approximately 3.14159. So a circle with radius 5 has an area of π × 5² = π × 25 ≈ 78.54 square units. If you only know the diameter, first halve it to get the radius (r = d / 2) before squaring. This calculator accepts any one of radius, diameter, circumference, or area and derives the rest automatically.
How do you find the circumference of a circle?
The circumference is the distance around a circle, found with C = 2πr, or equivalently C = πd using the diameter. A circle with radius 5 has a circumference of 2 × π × 5 ≈ 31.42 units. Circumference and area both come from the same radius, so once you know one of them you can work backward to find the radius and then compute the other.
How do you find the radius from the area or circumference?
From the area, solve A = πr² for r to get r = √(A / π). From the circumference, solve C = 2πr for r to get r = C / (2π). For example, an area of π square units (≈3.14159) gives r = √(π / π) = 1. This calculator runs those inversions for you, then reports the radius, diameter, circumference, and area together.
What is the difference between radius and diameter?
The radius is the distance from the center of a circle to its edge; the diameter is the distance all the way across through the center, which is always exactly twice the radius (d = 2r). Area and circumference formulas are usually written in terms of the radius, so if a problem gives you the diameter, halve it first. Enter either one here and the calculator fills in the other automatically.