Random number 1–12

Twelve is the number civilisation kept coming back to. Twelve months, twelve hours on a clock face, twelve signs of the zodiac, twelve in a dozen, twelve inches in a foot.

It survived because it divides well. Twelve splits into halves, thirds, quarters and sixths with nothing left over, which is more than ten manages, and that made it the practical base for anyone sharing goods before decimals took over.

So a 1–12 draw is rarely abstract. It usually means a month, an hour, a sign or a group.

Every tap calls crypto.getRandomValues, the browser's cryptographic generator, and asks it for one unsigned 32-bit value. That lands somewhere between 0 and 4,294,967,295, and it then has to be folded down to the 12 numbers in this range.

Folding it down with a plain remainder is where most generators go wrong, an error known as modulo bias. 12 does not divide 4,294,967,296 evenly: 4 values are left over at the top, and wrapping them around would make the first 4 numbers in the range fractionally likelier than the rest. This tool throws away any raw value at or above the last exact multiple of 12 and draws a fresh one instead. That is rejection sampling. It fires about 1 in 1,073,741,824 draws, and it is the difference between 8.33% exactly and 8.33% approximately.

Ask for more than one number and the tool keeps drawing until it holds that many distinct values, then sorts them ascending. Nothing is seeded, nothing is sent anywhere, and each draw is independent of the one before it. The roll history under the tool lives in your browser's local storage, where the clear button empties it. The daily lucky numbers elsewhere on this site work the opposite way on purpose: they use a seeded generator so the same date always returns the same set. How random is random sets the two side by side.

Each number lands at 8.33 percent, and the tail of that decimal is not a rounding error in the draw. Twelve leaves four values over when it divides the 32-bit draw space, and those four are discarded and redrawn rather than folded in, which is what keeps 1 and 12 exactly as likely as 6. What twelve cannot fix is the real-world skew: births cluster in late summer, so a random month is not a random birthday.