Home › Calculators › Circle Calculator
⭕

Circle Calculator

Free circle calculator: enter the radius to instantly get the diameter, circumference and area of any circle. Perfect for homework, DIY and design.

About this tool

Every measurement of a circle flows from a single number: the radius. Double it for the diameter, multiply by 2-pi for the circumference, square it times pi for the area.

Enter the radius once and this circle calculator shows you all three results together - no need to run three separate calculations.

How to use

Type the radius in any unit - centimetres, inches, metres - and press Calculate Circle. Diameter and circumference come back in the same unit, area in square units.

If you only know the diameter, just halve it first and enter that as the radius.

The math behind it

Diameter = 2 x radius. Circumference = 2 x pi x radius. Area = pi x radius^2, with pi approximately 3.14159.

These three formulas cover nearly every circle problem in school maths, DIY and design work.

Frequently asked questions

How do I find the circumference if I only know the diameter?
Halve the diameter to get the radius, then enter it here - or just multiply the diameter by pi (3.14159) directly. Both give the same circumference.
What is pi and why 3.14159?
Pi is the constant ratio of any circle\u2019s circumference to its diameter, approximately 3.14159. It never changes, whatever the circle\u2019s size - that is what makes these formulas work universally.
Can I use this for a semicircle?
Partly. Enter the radius to get the full circle\u2019s numbers, then halve the area and circumference - but remember a semicircle\u2019s perimeter also includes the straight diameter edge, so add that back for fencing-type problems.

Related tools

📐

Area Calculator

Calculators
⬛

Perimeter Calculator

Calculators
🔺

Triangle Calculator

Calculators

💬 Comments

Have a question about this page, or an idea to make it better? Share it below — your feedback helps everyone who uses this site.