Circle Calculator

Written byOleh Soloninupdated 27 September 2026

Known value

Value

Units

Result

Area78.539816
Circumference31.415927
Working
C = 2πr = 2 · π · 5 = 10π ≈ 31.4159; S = πr² = π · 5² = 25π ≈ 78.5398
Radius5
Diameter10

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.

How to use

Choose which measurement you know: radius, diameter, circumference or area.
Enter its value.
Read all four measurements, worked out from the one you supplied.

Why area grows faster than the edge

Circumference is 2πr and area is πr². The radius enters the first formula once and the second twice, so doubling it doubles the edge but quadruples the area: a circle of radius 5 has an area of 78.54, while radius 10 gives 314.16.

That difference decides practical questions. A pizza of twice the diameter is four times the food, and a pipe of twice the bore has four times the cross-section — four times the flow only at the same flow speed — which is why sizes are quoted by diameter but capacity scales by its square.

Worked out with this calculator; doubling the radius doubles the circumference and quadruples the area.
Radius	Diameter	Circumference	Area
1	2	6.2832	3.1416
5	10	31.4159	78.5398
10	20	62.8319	314.1593

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.

What is the area of a circle with radius 5?

78.54, from πr². The circumference at the same radius is 31.42, from 2πr.

How much does area grow when the radius doubles?

Four times over, because the radius is squared. Radius 5 gives 78.54 and radius 10 gives 314.16, while the circumference only doubles.

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.

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