Circle Calculator
From the radius, diameter, circumference or area, find all the other values of the circle.
Tip: One value (radius, diameter, circumference or area) is enough to find the others.
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About this digital tool
The circle calculator finds all the measurements of a circle from one known: the radius (r), diameter (d), circumference (C) or area (A). Choose what you know, enter the value and get the other three instantly, without having to rearrange the formulas yourself.
The relationships are: d = 2r, C = 2 × π × r and A = π × r². For a circle with radius 5: the diameter is 10, the circumference 2 × π × 5 ≈ 31.42 and the area π × 25 ≈ 78.54. If the area is known, the radius is found as √(A ÷ π). It is used for wheels, pipes, round tables, gardens and any geometry task where π ≈ 3.14159 comes up.
How to use
- Choose which value you know.
- Enter that value.
- See the radius, diameter, circumference and area.
Frequently asked questions
How do I find the radius from the circumference?
Radius = circumference ÷ (2π). E.g. circumference 31.42: 31.42 ÷ 6.283 ≈ 5.
How is the circumference of a circle calculated?
C = 2 × π × r, or C = π × d. E.g. for r = 7: C = 2 × 3.14159 × 7 ≈ 43.98.
How do I find the radius from the area?
r = √(A ÷ π). E.g. for A = 78.54: 78.54 ÷ 3.14159 = 25 and √25 = 5.
What is the ratio of circumference to diameter?
Always π ≈ 3.14159, for any circle. That is why a diameter of 10 gives a circumference of about 31.42.