Circle Calculator

Circle Calculator finds all four basic measurements of a circle — radius, diameter, circumference and area — from whichever one you know. It applies the standard formulas with π, labels lengths in your unit and area in square units, lists the formulas it used and draws a diagram labelled with your starting value.

Working backwards from a circumference or an area to the radius is where most mistakes happen, especially with square roots and units. Getting every measurement at once, with units handled correctly, is useful for DIY projects, garden beds, pipes, round tables and school geometry.

1. Choose the measurement you know: radius, diameter, circumference or area.
2. Enter the length unit, such as cm, m or in (optional).
3. Enter the known value — for an area, in square units.
4. Press Calculate circle.
5. Read all four measurements, check the formulas and diagram, then copy or download the summary.

• Start from radius, diameter, circumference or area
• Standard formulas: d = 2r, C = 2πr, A = πr², and their inverses
• Lengths in your unit and area in square units — never cubic units
• Formulas and substituted values listed for every result
• Responsive diagram labelled with the known measurement
• Decimal input with clear errors for zero, negative and oversized values

• Measure the diameter of real objects across the widest point; it is easier than finding the centre.
• Enter an area in square units of the same unit you set, for example cm² with cm.
• Round the results only at the end, to the precision your project needs.
• Convert units first with a length or area converter if your measurements are mixed.