About this Circle Calculator
Circles are defined by a single dimension: the radius. From this one number, you can derive the diameter, circumference (perimeter), and area.
Who this calculator is for
- Students practicing circle geometry with radius, diameter, circumference, and area.
- DIYers, builders, and planners measuring circular objects, tables, pipes, or garden beds.
- Anyone who wants to move quickly from radius to the other core circle measurements.
When to use it
- Use this calculator when you know the radius and need area, circumference, and diameter together.
- Use it before switching to area, surface area, or volume problems involving cylinders, cones, or spheres.
- Use it to check circle formulas without doing repeated Pi calculations by hand.
How to use this calculator
- Enter the Radius (distance from center to edge).
Key assumptions
- The radius is entered in a linear unit, and outputs use the matching linear or square unit.
- Radius is treated as a non-negative length.
- Pi is approximated with JavaScript's built-in Math.PI constant.
Accuracy and testing
Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.
How this calculator works
Diameter = 2 × r
Circumference = 2 × π × r
Area = π × r²
Inputs explained
Radius
The distance from the center of the circle to any point on its edge.
Example scenarios
Radius 5
Diameter: 10. Circumference: ~31.42. Area: ~78.54.
Key Features
- Instant Calculation
- Private & Secure
- Mobile Optimized