Circle Radius or Diameter Calculator
Recover a circle’s radius and diameter from a known area or circumference. Choose which measurement you have before entering its value.
How to use Circle Radius or Diameter Calculator
1. Choose Measurement Unit, then enter Known Value without a unit suffix. 2. Choose the matching calculation option. Use the selected ordinary, square or cubic unit for each measurement as explained below; changing the selector relabels inputs and does not convert them. 3. Select Calculate, or press Enter, and read radius and diameter. 4. Select Select Copy result to copy the answer, or Reset to clear the form.
Why use this tool?
Turn a stated circular area or measured distance around an item into dimensions you can use in a design or comparison.
How the calculation works
From area A, radius = √(A ÷ π). From circumference C, radius = C ÷ (2π). Diameter = 2 × radius. Results use up to six decimal places. Tiny nonzero results below 0.000001 use scientific notation with six significant digits. Browser floating-point arithmetic can introduce small rounding differences. Inputs and calculated magnitudes must not exceed 9,007,199,254,740,991. Negative physical measurements and arithmetic underflow are rejected. Zero dimensions are treated as a degenerate limit where the formula allows them; divisors and sides of an actual triangle must be positive. Formula reference: https://openstax.org/books/prealgebra-2e/pages/c-geometric-formulas
Example
Known Value = 100, Known Measurement = Area and Measurement Unit = cm mean 100 cm². The radius is 5.641896 cm and diameter is 11.283792 cm.
Practical tip
When Area is selected, enter square units matching the selected length unit: cm² for cm, or m² for m.
Frequently asked questions
Which units should the known value use?
Use squared units for Area and ordinary length units for Circumference. With cm selected, enter cm² or cm respectively.
Why do I get a different answer after changing Known Measurement?
The same number describes different quantities. An area of 100 cm² and a circumference of 100 cm belong to different circles.
Does changing Measurement Unit convert my numbers?
No. The selector states the unit of the numbers you enter and labels the answer. Convert measurements first when changing units. Area inputs use square units and volume inputs use cubic units; result precision follows the rounding method above.