Free Circle Calculator
Find radius, diameter, circumference and area.
What is the Circle Calculator?
Radius, diameter, circumference and area are all locked together by π — know any one and the other three follow. That comes up constantly, from working out how much edging a round garden bed needs to sizing a pipe or a pizza.
This calculator lets you enter whichever value you have and instantly gives the other three, calculated precisely.
Last updated: Aug 12, 2026
What does this tool do?
You pick which measurement you know — radius, diameter, circumference or area — enter it, and the tool derives the rest using π.
Because any single value determines a circle completely, one input is all it needs to fill in everything else.
Key features
Any input
Start from radius, diameter, circumference or area.
All four outputs
Get the other three every time.
Precise
Uses the full precision of π.
Instant
Results update as you type.
Private
Calculations run in your browser.
Example
Common use cases
- Students — Solve geometry problems.
- DIYers — Measure round tables, beds or patios.
- Engineers — Size pipes, discs and openings.
- Designers — Work with circular shapes precisely.
- Gardeners — Calculate edging or coverage for round beds.
- Teachers — Demonstrate the π relationships.
- Makers — Plan circular cuts and parts.
- Anyone with a circle — Get every measurement from one.
Benefits
- One value gives you all four measurements.
- It works from whichever number you actually have.
- It's precise, not a rounded estimate.
- Nothing you enter leaves your device.
Tips
- Diameter is twice the radius; radius is half the diameter.
- Circumference is π times the diameter.
- Area is π times the radius squared.
- Keep units consistent — the outputs match your input's units (area is squared).
- To find radius from area, take the square root of area over π.
- Round the final answer to the precision your task needs.
- For a semicircle, halve the area and adjust the perimeter.
- π is irrational, so results are precise decimals, not exact fractions.
Common mistakes to avoid
Mixing radius and diameter
Fix: Confirm which one your value is — they differ by a factor of two.
Forgetting area is squared units
Fix: Area comes out in square units, unlike the linear measurements.
Rounding π too early
Fix: The tool uses full-precision π; round only the final answer.
Inconsistent units
Fix: Use one unit throughout so the outputs make sense.
How it works
- 1
Pick a known value
Radius, diameter, circumference or area.
- 2
See the rest
The other three are calculated instantly.
Frequently asked questions
Any single value — radius, diameter, circumference or area — and the tool computes the rest.
It uses the full precision of π for accurate results.
Any single value — radius, diameter, circumference or area — and the tool computes the rest.
It uses the full precision of π for accurate results.
Take the square root of the area divided by π. The tool does this automatically when you enter area.
Whatever you enter — outputs match your input's units, with area in squared units.
Compute the full circle, then halve the area and adjust the perimeter for a semicircle.
No. Everything is calculated in your browser.
Conclusion
Any one circle measurement unlocks the other three here, precisely and instantly. Pair it with the area and length converters when you need the results in other units.