Circle Calculator 🔒 Your data never leaves your browser.
Calculate radius, diameter, circumference and area from any one value.
All four measurements
About this tool
This circle calculator works out every standard circle measurement — radius, diameter, circumference, and area — from whichever single value you happen to have. It uses the same formulas taught in geometry class: diameter = 2r, circumference = 2πr, and area = πr², where r is the radius. Pick which measurement you know from the dropdown, type in the number, and all four results update instantly.
Most circle calculators only accept a radius. This one is different: since it can start from any of the four measurements, you can go straight from a tape-measured circumference to the area, or from a known area to the diameter, without first manually solving for the radius in between. That makes it handy for real-world jobs like sizing a round tabletop from its circumference, or checking a pipe's cross-sectional area once you've measured across its opening.
Frequently asked questions
Why does entering the area give a slightly different radius than I expected?
This is almost always just rounding in the number you typed in — area involves π (an irrational number with infinite non-repeating decimals), so if you enter an area that was itself rounded (e.g. "78.54" instead of the exact 78.53981633974483...), reversing the calculation (radius = √(area/π)) will produce a radius that's extremely close to, but not bit-for-bit identical to, your original value. This is expected floating-point/rounding behavior, not a bug — the difference should only show up in decimal places far beyond what's practically meaningful.