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Circle Calculator
Computes the area, circumference, diameter and radius of a circle from either its radius or its diameter. Lengths are unit-agnostic: the area is in the square of the input unit.
When to use
You know a circle's radius or diameter and need its area, circumference or the other dimension.
Do not use when: The shape is an ellipse (use ellipse), a solid ball (use sphere) or a disc with thickness (use cylinder).
Formula
r = radius (or diameter / 2); diameter = 2·r; circumference = 2·π·r; area = π·r²
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
radius | number | units | no | Radius of the circle. Give either radius or diameter. Range: > 0 |
diameter | number | units | no | Diameter of the circle (2 × radius). Used when radius is not given. Range: > 0 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
radius | number | units | Radius r (as given, or diameter / 2). |
diameter | number | units | 2·r. |
circumference | number | units | 2·π·r. |
area | number | units² | π·r², in the square of the input unit. |
Example
Radius 3: {"radius":3} → {"radius":3,"diameter":6,"circumference":18.8496,"area":28.2743}
Diameter 10: {"diameter":10} → {"radius":5,"circumference":31.4159,"area":78.5398}
GET https://tttkmbb.com/api/v1/calculate/circle?radius=3
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/circle(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/circle · Markdown: https://tttkmbb.com/geometry/circle.md · JSON definition: https://tttkmbb.com/geometry/circle.json
- MCP: server
https://tttkmbb.com/mcp, toolcalculate_circle_properties - OpenAPI operationId:
calculate_circle_properties - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- Wikipedia – Circle (reference)
- Wolfram MathWorld – Circle (reference)
FAQ
What units are the results in?
The same unit as the input for lengths and its square for the area: a radius in cm gives a circumference in cm and an area in cm².
What if I give both radius and diameter?
They must agree (diameter = 2 × radius); otherwise the calculator returns an INVALID_PARAMETER error rather than guessing which one is right.
How do I get the radius from the area?
r = √(area / π). Enter that radius here to get the remaining properties.
Related calculators
- Sphere Calculator — Volume and surface area of a ball with the same radius.
- Cylinder Calculator — Volume of a circular disc or tube with this cross-section.
- Ellipse Calculator — Area and perimeter when the two axes differ.