Circle Calculator

Calculate circle properties from any known value. Enter one value below:

How the Solver Picks a Starting Value

This calculator only needs one measurement to derive the other three, because all four circle properties are mathematically linked through the radius. Enter a radius directly, or enter a diameter, circumference, or area instead - the script back-calculates the radius first (diameter / 2, circumference / 2 pi, or the square root of area / pi), then derives everything else from that radius.

Worked Example

Enter a radius of 7 (unit doesn't matter for the math). Diameter = 2 x 7 = 14. Circumference = 2 pi x 7 = 43.9823. Area = pi x 7 squared = pi x 49 = 153.9380. If you instead entered a circumference of 31.4159, the tool would back-solve radius = 31.4159 / 2 pi = 5.0000 and produce the same set of results from there.

Why Only One Field Should Be Filled

Typing a value into any field clears the other three, because the calculator assumes exactly one known value per calculation. If two conflicting values were both present, the geometry wouldn't be internally consistent - so the script enforces a single source of truth on every keystroke.

Units

Selecting a unit from the dropdown only changes the label attached to each result (mm, cm, m, in, ft, or yd) - it does not convert between unit systems. Area results get a squared unit label (for example, cm squared) automatically.

Precision

All four outputs display to four decimal places. For circumference and area this means the result carries more precision than most everyday radius measurements justify, so treat the last one or two digits as computational rather than measured accuracy.