Circle Calculator – Find Area, Radius and Circumference
Find radius, diameter, circumference and area of a circle from any one value.
About the Circle Calculator
What is the Circle Calculator?
A circle calculator computes radius, diameter, circumference and area from whichever single value you already have.
Why use this calculator?
The four quantities share two tidy formulas, so any one unlocks all four — exactly the kind of derivation this tool removes.
How to use it
- Pick which property you know.
- Enter its value.
- Read all four related properties.
Formula
Circumference = 2πr; Area = πr².
All results use the same value of π internally, so they are consistent with each other.
Frequently Asked Questions
Is π rational?
No — π is irrational with infinitely many non-repeating decimals, usually approximated to 3.14159.
Why is area πr² not πd²?
Because area depends on radius squared; expressing it with diameter gives π/4·d² instead.
What order of operations does this use?
It follows standard PEMDAS/BODMAS rules: parentheses, exponents, multiplication and division, then addition and subtraction.
How precise are the answers?
The calculator uses double-precision floating-point math (about 15 significant digits) and rounds the display. For exact symbolic results use dedicated algebra software.
Can I enter negative numbers or decimals?
Yes. Negative values, decimals and very large or small numbers are all supported unless a field says otherwise.
What does it mean if I get an error?
Errors usually mean an invalid input, such as division by zero or a negative value where only positives make sense. Check your entries and try again.
Which units should I use?
Use one consistent unit for every input. The result will be in that unit (or its square or cube for area and volume).
How is area different from perimeter?
Perimeter is the distance around a shape; area is the space it covers. Perimeter is measured in units, area in square units.
Does the orientation of the shape matter?
No. Area, perimeter and volume depend only on the dimensions, not on how the shape is rotated or positioned.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.