The calculation, without hidden assumptions
Every circle measurement is tied to one radius. Choose whichever value you actually know — radius, diameter, circumference or area — and this calculator resolves the radius first, then reports the diameter, circumference and area together with the substituted working so each number can be checked.
How to use this calculator
Choose the known value: radius, diameter, circumference or area.
Enter that value as a positive number in any consistent unit.
Review the radius, diameter, circumference and area, plus the working.
Where people use it
- •Laying out round tables, patios or planters
- •Fabrication and cutting of circular parts
- •Converting between measured circumference and diameter
Example: radius 5
Diameter 10, circumference about 31.416 and area about 78.540. Entering the circumference 31.416 as the known value returns the same radius of 5.
What the result does not assume
- •Length results use the entered unit; the area uses the corresponding square unit.
- •Results use full-precision π, so rounded hand calculations with 3.14 can differ in the last digits.
Frequently asked questions
How is circumference calculated?+
Circumference is 2πr, or π times the diameter.
How do I find the radius from the area?+
Divide the area by π and take the square root. Select Area as the known value and the calculator does this for you.
What is the difference between diameter and radius?+
The diameter crosses the whole circle through the centre and is exactly twice the radius.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
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Keep the result connected to the real job
MAXScanner keeps the formula and assumptions beside the result so you can verify the number before using it in a drawing, estimate, specification, worksheet or document. Where a supplier, manufacturer, drawing or applicable standard owns a requirement, that source remains authoritative.