Math & science
Circle Calculator
Calculate circle diameter, circumference, and area from a radius.
Interactive calculator
Calculate circle area and circumference
Enter a radius to calculate the diameter, circumference, and area of a circle.
Results
Diameter
15
Circumference
47.12389
Area
176.714587
How to use it
- 01Enter the radius of the circle.
- 02Review the diameter, circumference, and area.
- 03Use the result in geometry problems, material planning, or layout work.
Result guide
- Diameter is always twice the radius.
- Circumference gives the boundary length around the circle.
- Area measures the total enclosed surface inside the circle.
About this calculator
Circle calculations are common in geometry, construction, machining, and schoolwork because radius-based measurements often need to be translated into area or perimeter quickly.
A focused circle page works well because one input can unlock the three most common outputs people usually need.
Frequently asked questions
Why does the formula use pi?
Because pi is the constant ratio that connects a circle's diameter, circumference, and area.
Can I start from diameter instead of radius?
You can, but this page expects radius, so you would first divide the diameter by 2.
Where is this useful outside school math?
It is useful in layout work, machining, material estimation, and any task involving circular parts or spaces.
Method and limitations
This calculator uses the formula and assumptions described on this page. We do not claim independent professional review. Direct references will be listed here when they support a specific rule or data source used by this calculator.
Results are estimates for general information. Verify current rules and assumptions before making financial, tax, or health decisions.
Read our Editorial and Verification Policy →Related calculators
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