Circle Calculator

How to calculate

1. Choose the known measurement. Area uses squared units; other measurements use linear units.
2. Enter a positive decimal and choose mm, cm, m, in or ft. Calculate explicitly.
3. Change output units or display precision without rounding intermediate values, then copy a summary or download unrounded JSON. Reset and input-type changes offer Undo.

Worked diameter example

A diameter of 10 cm has radius 5 cm and approximate area 78.53981633974483 cm². Radius is half the diameter. Area counts a two-dimensional region, so its unit is squared.

Formulas and conversion

d = 2r; C = 2πr; A = πr². Diameter input uses r = d/2, circumference r = C/(2π), and area r = √(A/π). Runtime Math.PI and floating-point arithmetic are used: all numerical results are approximate. Factors to meters are mm 0.001, cm 0.01, m 1, in 0.0254 and ft 0.3048. Area multiplies or divides by the square of the factor: 1 in² = 0.00064516 m².

Input grammar, range and exports

A dot decimal such as 1.5 or .5, optionally prefixed with + and followed by e/E and a signed integer exponent, is accepted; 1e-3 is valid. Surrounding whitespace is ignored but preserved in the original JSON input. Commas and thousands separators are rejected. Maximum 100 characters. The derived radius must be 1e-9 to 1e9 meters inclusive; zero, negatives, underflow, Infinity and NaN are invalid.

Display uses 2–10 significant digits, default 6; scientific notation is copyable. Output-unit changes always use the unrounded canonical radius in meters, never a displayed value. JSON includes original text/type/unit, canonical radius, output unit, display precision and unrounded finite values with explicit linear or squared unit labels. Changing input marks existing results outdated and disables exports. The diagram is illustrative, not to scale. Refresh clears drafts.