Roman Numeral Converter

Split the year into thousands, hundreds, tens and units, convert each part on its own, then write them left to right. 2026 is 2000 plus 20 plus 6, so MM plus XX plus VI: MMXXVI. 1984 is 1000 plus 900 plus 80 plus 4, so M plus CM plus LXXX plus IV: MCMLXXXIV. The hundreds are where people slip — 900 is CM rather than DCCCC, and 400 is CD rather than CCCC. Every year from 1000 to 1999 opens with a single M and every year from 2000 to 2999 with MM, which is why recent dates look so much shorter than nineteenth-century ones.

Because the system adds values rather than filling positions. A numeral is a running total of the letters in it, so an empty column never arises and there is nothing for a zero to hold in place. Our own digits need one for exactly that reason: 105 and 15 differ only by a symbol standing in an empty tens column. Medieval writers using Roman numerals for calendar tables did borrow the Latin nulla, sometimes abbreviated to N, when they needed to record nothing, but it never became one of the letters. The same absence rules out negative numbers and decimals; Roman fractions were a separate twelfths notation of dots and an S for a half.

With the plain letters, 3,999 — MMMCMXCIX — because M is the largest symbol available and no symbol is repeated more than three times. Larger figures needed a multiplier. A vinculum, a bar drawn over a numeral, multiplies it by a thousand, so a barred V is 5,000 and a barred X is 10,000; the older apostrophus system enclosed numerals in bracket-like marks for the same purpose. Neither is an ordinary letter, so neither survives being typed, copied or pasted as plain text, and most converters stop at 3,999 rather than pretend otherwise. Past that point, ordinary digits are the honest answer.

Six, and only six: IV, IX, XL, XC, CD and CM. The underlying rule is that I, X and C may each be placed before the next two larger symbols — I before V and X, X before L and C, C before D and M — while V, L and D are never subtracted at all. So 49 is XLIX rather than IL, and 95 is XCV rather than VC. None of the six is ever doubled either, which is why 8 is VIII and not IIX. Keep to the rule and every number has exactly one spelling, which is what makes conversion reversible in both directions; break it and you get strings that are readable but non-standard.

Almost every traditional dial with Roman numerals uses IIII at four o'clock, and the habit is older than the rule it breaks. The explanations usually offered are visual balance, since IIII on the left mirrors the weight of VIII on the right; legibility, because four upright strokes read faster at an angle than a subtractive pair; and casting, since a set of numerals made from moulds of I, V and X divides neatly when four is IIII. The story that Louis XIV insisted on it is almost certainly invention. Either way, IIII is correct by convention on a dial and IV is correct in writing.

Work from the left in descending groups rather than letter by letter. Take the Ms first, since each is a thousand, then scan for a C or an X sitting immediately before a larger letter, because that pair is a single value and reading its two letters separately is where most errors come from. MCMLXXXIV breaks into M for 1000, CM for 900, LXXX for 80 and IV for 4, giving 1984. The check that catches mistakes is that values should never increase as you move right unless you are inside one of the six subtractive pairs. If they do, you have either split the string wrongly or you are looking at a non-standard spelling.

Not usefully. Without place value there are no columns to line up and no carrying, which is why Roman practice was to work the sum on a counting board or abacus and write only the answer in numerals. Sorting is worse. Sorted as text, C comes before I comes before M, so a run of chapter numbers or monarch numerals ends up in an order with no relation to size, and IX lands next to I rather than after VIII. If numerals are going into a spreadsheet or a database, store the ordinary number in its own column and treat the numeral as a label for display.

No. Both directions are a walk down a short table of values held in the page's own code, recalculated as you type, with no request made and nothing written down — there is no list of recent conversions and no field remembered between visits, so refreshing or closing the tab empties it. That matters for the mundane reason that people paste more than a bare number into a box like this: a case reference, a serial, a line lifted out of a document. Whatever ends up in the field stays on your own machine, and once the page has loaded it behaves the same with the network disconnected.