Circle Calculator
Area, circumference and diameter from any one value.
Enter your values
Choose one of: Radius, Diameter, Circumference, Area.
Enter the value you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Area
- 78.539816
- Circumference
- 31.415927
- Radius
- 5
- Diameter
- 10
- Sphere volume with this radius
- 523.598776
Working
- 1A = π × 5² = 78.5398
Quick answer
Circle area is πr² and circumference is 2πr, so any one measurement determines all the others.
A = πr², C = 2πr
At a glance
| What it does | Area, circumference and diameter from any one value. |
|---|---|
| Category | Math |
| Inputs needed | What do you know?, Value |
| Main output | Area |
| Formula | A = πr², C = 2πr |
| Cost | Free — no sign-up, no download |
How to use the Circle Calculator
- 1Choose which measurement you already have.
- 2Enter its value.
- 3Read every other circle property.
Inputs explained
- What do you know?
- Choose one of: Radius, Diameter, Circumference, Area.
- Value
- Enter the value you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- What do you know?Radius
- Value5
Results
- Area78.539816
- Circumference31.415927
- Radius5
- Diameter10
- Sphere volume with this radius523.598776
- A = π × 5² = 78.5398
Frequently asked questions
Why is π irrational?
Its decimal expansion never repeats or terminates — proved by Lambert in 1761.
How do I find area from circumference?
Divide circumference by 2π to get the radius, then square it and multiply by π.
What do I need to enter into the Circle Calculator?
Just 2 values: what do you know?, value. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Circle Calculator work out the answer?
Circle area is πr² and circumference is 2πr, so any one measurement determines all the others. It applies the formula A = πr², C = 2πr and shows the working so you can check each step by hand.
Is the Circle Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.