Pi is the mathematical constant representing the ratio of a circle's circumference to its diameter. It is approximately 3.14159265 and is irrational (non-terminating, non-repeating).

What units does this calculator use?

The calculator is unit-agnostic. If radius is in meters, circumference is in meters and area is in square meters. Use consistent units throughout.

How do I find the radius from the area?

Divide the area by π, then take the square root: r = √(A/π). For example, area = 50: r = √(50/π) ≈ 3.99.

What is the difference between circumference and perimeter?

Circumference is the specific term for the perimeter of a circle. Both refer to the total length of the boundary.

Can I enter a decimal value?

Yes. All inputs accept decimal values. The calculator works with any positive real number.

Math

↵ to calculate

Circle Calculator

Calculate radius, diameter, area, and circumference of a circle from any one known value.

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How to use this calculator

👉 Fill in the boxes below and your answer appears instantly — no maths needed, we do it all for you! 🎉

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In plain English — what does this do?

🔢 This tool does the maths for you! Just type in your numbers, and it gives you the answer right away. No need to count on your fingers or use a pen and paper.

📌 Direct Answer & Summary

The circle calculator computes all four fundamental circle measurements — radius, diameter, area, and circumference — from any single input value. Enter any one measurement and instantly get the other three.

I know the

Radius Diameter Area Circumference

Radius

What is Circle Calculator?

How to use it

1️⃣ Select which value you know: radius, diameter, area, or circumference.
2️⃣ Enter the known value in the input field.
3️⃣ Click Calculate to see all four measurements.
4️⃣ Results update automatically as you type.
5️⃣ All results use π ≈ 3.14159265358979.

Formula

Area = π × r². Circumference = 2 × π × r. Diameter = 2r. Radius from area: r = √(A/π). Radius from circumference: r = C/(2π).

💡 See it in action — a real example

If radius = 5: Diameter = 10. Area = π × 25 = 78.54. Circumference = 2π × 5 = 31.42.

❓ Common questions

What is pi (π)?: Pi is the mathematical constant representing the ratio of a circle's circumference to its diameter. It is approximately 3.14159265 and is irrational (non-terminating, non-repeating).
What units does this calculator use?: The calculator is unit-agnostic. If radius is in meters, circumference is in meters and area is in square meters. Use consistent units throughout.
How do I find the radius from the area?: Divide the area by π, then take the square root: r = √(A/π). For example, area = 50: r = √(50/π) ≈ 3.99.
What is the difference between circumference and perimeter?: Circumference is the specific term for the perimeter of a circle. Both refer to the total length of the boundary.
Can I enter a decimal value?: Yes. All inputs accept decimal values. The calculator works with any positive real number.

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