Circle Calculator
Find a circle’s diameter, circumference and area from its radius. Choose regional length units and see a labeled circle and conversion table.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Convert the radius to meters internally. Double it for the diameter, multiply by 2π for the circumference, and square it before multiplying by π for the area. Squared-length conversions require squaring the unit factor.
d = 2r; C = 2πr; A = πr²
Worked example
With radius 5 cm, diameter = 10 cm, circumference = 10π ≈ 31.4159 cm, and area = 25π ≈ 78.5398 cm².
Assumptions and input details
Use the internal radius of a circular opening when calculating its usable area. The diagram is illustrative. A circle with radius zero has zero diameter, circumference and area.
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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A little space to keep the tools free.