Circle Formulas You Actually Need
Every circle measurement flows from the radius. Double it for the diameter, multiply by 2π for the circumference, square it and multiply by π for the area. Know any one of the four and you can unlock the rest — that is exactly what this calculator does.
The Four Formulas
- Diameter: d = 2r
- Circumference: C = 2πr = πd
- Area: A = πr²
- Going backwards: r = d/2 = C/(2π) = √(A/π)
Where Circles Show Up
Wheels, pipes, pizzas, running tracks, and cylinders all reduce to circle math. Engineers size pipes by diameter, bakers scale recipes by area (a 12-inch pizza has 44% more area than a 10-inch one), and runners convert track laps via circumference.
Working with other shapes? Try our area and perimeter calculator.