Circle Calculator
Know one measurement of a circle? Get the radius, diameter, circumference and area.
How to use the Circle Calculator
- Choose the i know the.
- Enter the value – Any unit; area is in that unit squared.
- Press Calculate. The result updates automatically as you type.
About the Circle Calculator
Every measurement of a circle depends on a single number, the radius. Once you have it, the diameter is twice as long, the circumference is about 6.283 times as long, and the area is π times the radius squared. That is why one known value is enough to find the rest.
Practical uses include sizing a round table cloth, finding how much edging a circular flower bed needs (circumference), how much mulch covers it (area), or working out a pipe's diameter by wrapping a tape measure around it and dividing by π. Note that area grows with the square of the radius: doubling a pizza's diameter from 8 to 16 inches gives four times as much pizza.
Formula
d = 2r C = 2πr = πd A = πr²
Working backward: r = C ÷ 2π and r = √(A ÷ π). π ≈ 3.14159265; the calculator uses the full double-precision value.
Example calculation
Using the default values:
- I know the = Radius
- Value = 5
the calculator returns:
- Radius: 5
- Diameter: 10
- Circumference: 31.415927
- Area: 78.539816 square units
Frequently asked questions
How do I find the circumference from the diameter?
Multiply the diameter by π. A 10 in circle has a circumference of about 31.42 in.
How do I find the radius from the area?
Divide the area by π and take the square root. An area of 50 ft² gives r = √(50 ÷ π) ≈ 3.99 ft.
What is the area of a circle with radius 5?
A = π × 5² = 25π ≈ 78.54 square units.
Last updated: · Category: Math calculators