Circle Calculator

Known value

Value

Result

Radius5
Diameter10
Circumference31.415927
Area78.539816

One known value is enough

Enter any one property of a circle and get the other three. Works from radius, diameter, circumference or area.

A circle is fully described by a single number, so knowing any one of radius, diameter, circumference or area pins down the rest.

Watch the difference between the two main formulas: circumference is 2πr while area is πr². Because area squares the radius, doubling the radius doubles the perimeter but quadruples the area.

What this calculator shows

Radius, diameter, circumference and area together
Calculation starting from whichever value you actually know
Enough precision for construction and engineering work

What to keep in mind

Uses π to full floating-point precision, not the 3.14 approximation.
These are flat circles; a sphere's surface area and volume follow different formulas.

FAQs

What is the area of a circle from its diameter?

Halve the diameter to get the radius, then square it and multiply by π. A 10 cm diameter gives a radius of 5 and an area of 78.54 cm².

Why does π appear in both formulas?

Because π is defined as the ratio of circumference to diameter. Everything circular inherits it, area included.

How do I find the radius from the area?

Divide the area by π, then take the square root. An area of 50 gives a radius of about 3.99.

Is 3.14 accurate enough?

For a garden bed, yes. On a 100-metre circumference it is out by about 5 cm, which starts to matter in engineering and machining.

Worked example

A pizza comparison

Input: Radius 5

Output: Circumference 31.42 · Area 78.54

Note: This is why a 16-inch pizza is not twice a 12-inch one — it is about 1.8 times the area, since area grows with the square of the radius.

Useful links

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Circle Calculator

Result

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