⭕Circle Calculator
Enter any one value — radius, diameter, circumference, or area — to find the rest, or switch to arc/sector mode.
Free Circle Calculator
Know just one measurement of a circle? Our free circle calculator finds the rest instantly. Enter the radius, diameter, circumference, or area — any one value — and get all four calculated immediately, shown both as exact π-expressions (like 25π) and decimals, with a to-scale diagram.
Need arc length or sector area instead? Switch to arc/sector mode, enter a radius and a central angle, and get the arc length, sector area, and chord length for that slice of the circle, with the sector highlighted visually.
Every formula used is shown step by step, including the algebra to reverse-solve for radius when you start from area or circumference. No sign-up, no downloads — just enter what you know and solve. See the About page or the FAQ for more.
About the Circle Calculator
Features
Solve from any value
Radius, diameter, circumference, or area — enter one, get all four.
Arc & sector mode
Radius and central angle give arc length, sector area, and chord length.
Exact π display
Results shown as exact π-expressions alongside decimals.
To-scale diagram
The circle or sector is drawn to scale with labels.
Formulas used
C = 2πr, A = πr², reverse-solved as r = C/(2π) or r = √(A/π) when starting from circumference or area. Arc/sector mode uses arc length = rθ, sector area = ½r²θ, and chord = 2r·sin(θ/2) with θ in radians.
Frequently Asked Questions
What can I enter?
Any one of radius, diameter, circumference, or area — the rest are calculated automatically.
How does it find radius from area?
It rearranges A = πr² to r = √(A/π) and solves.
Does it show exact values?
Yes, results are shown as exact π-expressions (e.g. 25π) alongside decimals.
What is arc/sector mode?
Given a radius and central angle, it calculates arc length, sector area, and chord length for that slice.
Does it show the formulas used?
Yes, each calculation shows the formula and the algebra involved.
What angle units does arc mode accept?
Both degrees and radians, selectable via toggle.
What if I enter a negative or zero value?
It's rejected with a clear message, since radius/diameter/area/circumference must be positive.
Does it work with any unit (cm, inches, etc.)?
Yes, the calculator is unit-agnostic — just be consistent with your own units.
Is it free?
Completely free, no sign-up or limits.
Does it work on mobile?
Yes, fully responsive.