Last updated October 10, 2026 · Report an error
Every measurement of a circle — radius, diameter, circumference, and area — can be derived from any one of the others using formulas built on π (pi).
This calculator lets you start from whichever value you already know and instantly get the rest, with the full derivation shown.
How it works
Choose which measurement you already know (radius, diameter, circumference, or area) and enter its value. The calculator solves for the radius first, then uses the standard circle formulas to find everything else.
- Enter i know the.
- Enter value.
- Click Calculate to see your results.
Examples
Finding the area of a circular table
A circular table with a 5-foot radius has a diameter of 10 feet, a circumference of about 31.42 feet, and an area of about 78.54 square feet.
Who should use it
- Finding the material needed to cover a circular area (like a tabletop or pool cover).
- Working backward from a known circumference (like a pipe or wheel) to find its radius or area.
Industry applications
- Geometry and mathematics education
- Construction, manufacturing, and engineering
Advantages
- Works from any of the four common circle measurements as a starting point.
- Shows the full derivation from your known value to every other measurement.
Common mistakes to avoid
- Using the diameter where the formula calls for the radius (or vice versa), which throws off every other calculation by a factor of 2.
- Rounding pi too aggressively (e.g. using 3.14 instead of a more precise value) for calculations that need more accuracy.
Best practices
- When precision matters, avoid rounding intermediate results — round only the final answer, which is exactly how this calculator handles the math internally.
Tips
- If you only have a tape measure and can wrap it around a circular object, you've measured the circumference — use that as your starting value.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
The radius is the distance from the center of a circle to its edge; the diameter is the distance all the way across through the center, which is always exactly twice the radius.
Pi is the ratio of a circle's circumference to its diameter — approximately 3.14159, and the same for every circle regardless of size.
Yes — the calculator solves for the radius first (r = √(Area / π)), then derives the diameter and circumference from that radius.