🔵

Circle Calculator

Calculate area, circumference, diameter and more from any circle measurement.

Radius

What is this tool?

A circle calculator is a free geometry tool that calculates all measurements of a circle — area, circumference, diameter and radius — from any single input value. Enter any one measurement and instantly get all others, making it perfect for maths problems, design work and construction projects.

How to Use — Step by Step

Enter any one measurementType in the radius, diameter, circumference or area — whichever you know. The tool calculates all others automatically.

Select your unitChoose from millimetres, centimetres, metres, inches or feet depending on your project.

View all circle measurementsSee radius, diameter, circumference and area displayed together, ready to copy or use in calculations.

Common Uses

📐

Geometry & Maths

Solve circle-related problems for school and university maths, including arc length and sector area.

🏗️

Construction

Calculate the circumference of pipes, circular rooms, roundabouts and curved walls.

🎨

Design

Work out the dimensions of circular logos, icons, buttons and design elements.

🍕

Everyday Use

Calculate pizza area to find out which size gives better value, or work out circular garden pond dimensions.

Frequently Asked Questions

What is the formula for the area of a circle?

Area = π × r², where r is the radius. Using diameter: Area = π × (d/2)². Pi (π) ≈ 3.14159.

What is circumference?

Circumference is the perimeter of a circle — the total distance around the outside. Formula: C = 2πr or C = πd.

What is the relationship between radius and diameter?

Diameter = 2 × Radius. The radius is the distance from the centre to any point on the circle. The diameter passes through the centre from one side to the other.

How accurate are the results?

Results are calculated to full floating-point precision and displayed to several decimal places for accuracy.