Circle Calculator
Find radius, diameter, circumference, and area
What is the Circle Calculator?
The circle calculator finds any missing measurement when you know just one. Give it the radius, and it returns the diameter, circumference, and area. Give it the circumference, and it works backwards to the radius and area. This comes up in geometry class, in construction (calculating how much material wraps around a pipe or drum), and in any design project where you're working with circular shapes. CalciHub's version accepts any of the four measurements as the starting point.
How Does It Work?
All circle measurements connect through one constant: π (pi), approximately 3.14159. The radius is the foundation — everything else is derived from it. Area grows with the square of the radius, so doubling the radius quadruples the area. Circumference grows linearly with radius, so doubling the radius exactly doubles the circumference.
Diameter: d = 2r Circumference: C = 2πr = πd Area: A = πr²
• r = radius (distance from center to edge)
• d = diameter (distance across the circle through the center)
• C = circumference (the perimeter of the circle)
• A = area (the space inside the circle)
• π ≈ 3.14159
How to Use CalciHub's Circle Calculator
1. Choose which measurement you already know: radius, diameter, circumference, or area.
2. Enter that value in the input field.
3. Click Calculate — all four measurements appear in the results.
Tip: If you're working from circumference, make sure you have the full circle perimeter, not just a partial arc length.
A Quick Example
A circular table has a diameter of 120 cm. Enter 120 as the diameter. The calculator gives: radius = 60 cm, circumference = 2π × 60 ≈ 376.99 cm, and area = π × 60² ≈ 11,309.73 cm². If you need to put a trim strip around the edge, you need just under 377 cm of material. If you're covering the top, you need roughly 11,310 cm² of material — or about 1.13 square meters.
Area and circumference scale very differently with size — a circle twice as wide needs twice the trim but four times the covering material.
Frequently Asked Questions
Pi is irrational — its decimal expansion never ends or repeats. The first few digits are 3.14159265358979. For most practical calculations, 3.14159 or the π key on your calculator is accurate enough.