Circle Calculator
From a single radius, get the area, the circumference, and the diameter — the three numbers that describe any circle.
One radius, three answers
Enter the radius and the calculator returns the area (πr²), the circumference (2πr), and the diameter (2r) at once.
Keep units consistent
The radius is unit-agnostic — your answers come back in the same unit (squared for the area), so don't mix centimetres with inches.
What is a circle calculator?
Radius in, full circle out
A circle calculator turns one measurement — the radius, the distance from the centre to the edge — into the three numbers that describe the whole circle: its area, its circumference, and its diameter. Each one is fixed once you know the radius, because every circle shares the same constant π (pi). That makes the radius the single input you need for pizza and table sizing, pipe and circle cutting, garden beds, running tracks, and any geometry homework where a circle shows up.
Enter the radius in any length unit to get the area, circumference, and diameter instantly.
Three short formulas, all built from the radius and the constant π (about 3.14159).
area = π × r²The diameter is simply twice the radius (2 × r), and the circumference — the distance around the edge — is 2 × π × r. The area, the space enclosed, is π × r²: square the radius first, then multiply by π. Because the radius is squared, the area grows much faster than the circumference as the circle gets bigger.
Suppose you have a circle with a radius of 5.
Diameter
2 × 5 = 10 — the straight-line distance across the circle.
Circumference
2 × π × 5 = 31.415927 — the distance once around the edge.
Area
π × 5² = π × 25 = 78.539816 square units — the space inside.
The three outputs answer three different everyday questions. The diameter (10 for a radius of 5) is what you measure across a round table or a pipe. The circumference (about 31.415927) is the length you would need to wrap a ribbon around the edge or the distance covered in one lap of a circular track. The area (about 78.539816 square units) is the surface you cover — the dough on a pizza, the soil in a round garden bed, the paint on a circular wall. The key insight is that area scales with the radius squared: double the radius from 5 to 10 and the area jumps fourfold, from 78.539816 to 314.159265, while the circumference only doubles. That is why a 16-inch pizza has far more than twice the food of an 8-inch one. π is the thread tying it all together — the same constant ratio links the radius to both the perimeter and the area of every circle, large or small.
The formulas are exact, but a couple of practical points are worth keeping in mind.
Perfect circles and consistent units
These formulas describe a perfect, flat circle. Real objects — a slightly oval table, a pipe with wall thickness, a garden bed with irregular edges — will differ a little from the computed value. The radius is also unit-agnostic, so the answers are only meaningful if you keep one unit throughout: a radius in centimetres gives a circumference in centimetres and an area in square centimetres, never a mix.