This circle calculator lets you enter one known measurement and instantly works out the rest.
Whether you provide radius, diameter, circumference, or area, the tool shows all circle properties with formulas and steps.
- Radius: distance from centre to edge of the circle.
- Diameter: full width across the circle (2 × radius).
- Circumference: total distance around the circle (π × diameter).
- Area: space inside the circle (π × radius²).
How to use: Select which value you know, enter it with your preferred unit (cm, m, in, ft), and hit Calculate.
The calculator will instantly show the other three values along with step-by-step formulas.
Circle Calculator
Enter any one measurement of a circle (radius, diameter, circumference, or area) to calculate the rest.
Understanding circles for students
A circle is one of the most important shapes in mathematics, and you’ll use it often in geometry, physics, and real-life problems.
This calculator helps you practise and check your answers quickly, whether you’re in middle school or high school.
Circle formulas you should know
- Radius (r): distance from the centre to any point on the circle.
- Diameter (d): twice the radius (d = 2r).
- Circumference (C): the perimeter or distance around the circle (C = 2πr or C = πd).
- Area (A): space inside the circle (A = πr²).
Remember: π (pi) is approximately 3.1416, but for homework you might be asked to use π = 22/7 or leave answers in terms of π.
Circle diagram with labels
Here’s a simple diagram to help you visualise the main parts of a circle:
This diagram shows the radius, the diameter, and the circumference of the circle.
Why circles matter in school
Circles appear in almost every subject: from measuring wheels in physics experiments, to calculating land areas in geography, and even in computer graphics.
Knowing how to switch between radius, diameter, circumference, and area gives you confidence in solving exam questions and real-world problems.
Tips for students
- Always write down the formula before solving a problem — teachers look for working steps, not just the answer.
- Label your radius and diameter clearly in diagrams to avoid confusion.
- If you forget, remember: the diameter is just two radii back-to-back.
- Practise converting between units (cm, m, mm) since exams often test unit changes.
Using this calculator is a great way to double-check your homework or prepare for tests.
Circle Calculator FAQs
Yes. The circumference of a circle is the same as its perimeter—it is the distance all the way around the circle.
Remember these: Diameter = 2 × Radius, Circumference = π × Diameter, and Area = π × Radius². If you know these three, you can solve any circle problem.
Use A = πr². Square the radius (r × r) and then multiply by π (around 3.1416).
The area is always in square units, such as cm², m², or in², because it measures the space inside the circle.
Both are common. Some teachers prefer 3.14, others use 22/7, and sometimes exams ask you to leave the answer in terms of π.