Circle Calculator
Know one measurement of a circle and need the rest? Enter the radius, diameter, circumference or area and this calculator returns the other three, in whatever units you choose.
- Radius
- 5 in
- Diameter
- 10 in
- Circumference
- 31.4159 in
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Choose which measurement you know.
- Enter its value and unit.
- Read the radius, diameter, circumference and area, each in the unit you pick.
Worked examples
A 5-inch radius: 10 in across, 31.42 in around, 78.54 in² (0.0507 m²)
- Diameter
- 10 in
- Circumference
- 31.4159 in
- Area
- 78.5398 in²
A circle 10 m around
- Radius
- 1.5915 m
- Diameter
- 3.1831 m
- Area
- 7.9577 m²
A circle covering 50 m²
- Radius
- 3.9894 m
- Diameter
- 7.9788 m
- Circumference
- 25.0663 m
The parts of a circle
The radius runs from the center to the edge. The diameter runs edge to edge through the center and is twice the radius. The circumference is the distance around. π (pi) is the ratio of circumference to diameter, about 3.14159, and is the same for every circle.
Doubling the radius
Doubling the radius doubles the circumference but quadruples the area, because area depends on the radius squared. That is why a 16-inch pizza is nearly 1.8 times the size of a 12-inch one, not 1.33 times.
Questions people ask
What are the measurements of a circle with a 5-inch radius?
Diameter 10 in, circumference 31.42 in, and area 78.54 in².
How do I find the radius from the circumference?
Divide the circumference by 2π (about 6.2832). A circle 44 inches around has a radius of 7.00 inches.
How do I find the diameter from the area?
Divide the area by π, take the square root, and double it. An area of 100 in² gives a diameter of 11.28 in.