Circle Calculator
This circle calculator finds the radius, diameter, circumference, and area of a circle from any one of them. Choose which value you know, enter it, and the circle calculator returns the other three. It works as an area of a circle solver, a circumference calculator, and a diameter to circumference converter.
Results
The Circle Calculator is part of our Geometry collection. Browse more geometry to size, convert, and check your work alongside this tool.
What the circle calculator shows
A circle is fully described by a single measurement: once you know the radius, the diameter, circumference, and area all follow. The circle calculator takes whichever of the four you have and returns the rest. That makes it a quick area calculator for circles and a converter between diameter and circumference.
Circle calculator formulas
π is about 3.14159. The circle perimeter is the same thing as the circumference.
Area of a circle solver example
A circle with a circumference of 50 cm:
A circle with a radius of 5 has a diameter of 10, a circumference of 31.4159, and an area of 78.5398.
How to use the circle calculator
Choose radius, diameter, circumference, or area, then enter the value. The circle calculator returns the radius, diameter, circumference, and area. Units carry through: enter centimetres and you get centimetres and square centimetres.
Reading the circle calculator results
Area grows with the square of the radius, so doubling the diameter quadruples the area. Circumference grows in step with the diameter. When measuring a real object, a tape around it gives the circumference, which is often easier than measuring straight across.
When to calculate circumference
- Finding the area of a round table, slab, or garden bed
- Converting a measured circumference to diameter
- Sizing pipes, wheels, and lids
- Geometry homework with the circle calculator
Common circle calculator mistakes
- Using the diameter in A = πr². Halve it first.
- Using 3 for π. It adds a 4.5 percent error.
- Mixing units. Area is in square units.
Tips for accurate results
- Measure circumference with a tight, level tape.
- Use full π, not 3 or 3.14, for precision.
- Keep units consistent.
Area of a circle with diameter
Many real measurements give the diameter, not the radius. To find the area of a circle with diameter, halve the diameter and square it, then multiply by π: a 10 inch circle has an area of π × 5² = 78.54 square inches. A 20 inch circle has four times that, 314.16 square inches. For other flat shapes such as triangles, the triangle area calculator works from the side lengths.
Diameter to circumference converter
Diameter and circumference are linked by π alone, so converting between them is a single multiplication or division. A pipe with a 30 cm diameter has a 94.25 cm circumference, and a 94.25 cm tape measurement means a 30 cm diameter.
Circle calculator reference
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.2832 | 3.1416 |
| 5 | 10 | 31.4159 | 78.5398 |
| 7.9577 | 15.9155 | 50 | 198.9437 |
| 10 | 20 | 62.8319 | 314.1593 |
Circle Calculator FAQ
What is the area of a circle formula?
A = πr². With the diameter, A = π(d/2)².
How do I find the circumference from the diameter?
Multiply the diameter by π: C = πd.
How do I find the diameter from the circumference?
Divide the circumference by π: d = C ÷ π.
Is the perimeter of a circle the same as the circumference?
Yes, they are the same measurement.