Circle Calculator
Calculate radius, diameter, circumference, and area with a labeled diagram
About this calculator
The Circle Calculator finds radius, diameter, circumference, and area from any single measurement. Enter one value and instantly see all four results with a labeled diagram. Useful for geometry, engineering, and any everyday problem involving circular shapes.
How to use the circle calculator
- Choose which value you know: radius, diameter, circumference, or area.
- Enter that value into the input field.
- Read all four measurements in the result, labeled on the diagram.
- Share the URL to send the exact same calculation to someone else.
Common examples
- Radius 5 cm → diameter 10 cm, circumference 31.42 cm, area 78.54 cm²
- Diameter 12 in → radius 6 in, circumference 37.70 in, area 113.10 in²
- Circumference 100 m → radius 15.92 m, diameter 31.83 m, area 795.77 m²
- Area 50 ft² → radius 3.99 ft, diameter 7.98 ft, circumference 25.07 ft
- Radius 1 (unit circle) → diameter 2, circumference ≈ 6.2832, area ≈ 3.1416
Frequently asked questions
What is the formula for the area of a circle?
Area = π × r², where r is the radius. For example, a circle with radius 5 has area π × 25 ≈ 78.54.
What is the formula for the circumference of a circle?
Circumference = 2 × π × r, or equivalently π × d where d is the diameter. The constant π is approximately 3.14159.
Can I enter any of the four measurements?
Yes. Choose which value you want to enter (radius, diameter, circumference, or area) and the calculator derives the other three automatically. Only one input is active at a time to keep the inputs consistent.
Does it accept zero or negative values?
No. A circle must have a positive size, so zero and negative inputs are treated as invalid and show an explanatory message rather than a numeric result.