How to use the Roman numeral converter
- Choose Number → Roman or Roman → number.
- Enter a whole number from 1 to 3,999, or type a Roman numeral in upper or lower case.
- To go above 3,999, tick the overline box. Numbers up to 3,999,999 are then written with a bar over the thousands.
- Read the answer and the working. If a numeral breaks the rules, the message says which rule and shows the standard form.
How Roman numerals work
Seven letters stand for fixed values, and a number is written by adding them from largest to smallest:
When a smaller letter comes before a larger one, it's subtracted instead of added. Only six pairs do this, and each place value (thousands, hundreds, tens, ones) is written separately. The calculator converts one place at a time: it looks up the Roman form of the thousands, then the hundreds, tens and ones, and joins them.
Worked example
The default number is 2026:
- Thousands: 2 × 1,000 = 2,000, written MM
- Hundreds: 0, so nothing is written
- Tens: 2 × 10 = 20, written XX
- Ones: 6, written VI (5 + 1)
- Together: MMXXVI
Going the other way, the default numeral MCMXCIV splits into M (1,000), CM (1,000 − 100 = 900), XC (100 − 10 = 90) and IV (5 − 1 = 4). The total is 1,000 + 900 + 90 + 4 = 1994.
The rules for a valid numeral
| Rule | Valid | Not valid |
|---|---|---|
| I, X, C and M repeat at most three times | III, XXX | IIII, XXXX |
| V, L and D never repeat | X, C | VV, LL |
| V, L and D are never subtracted | V, XLV | VX, LC |
| I goes only before V and X; X only before L and C; C only before D and M | IX, XC, CM | IC, IL, XM |
| Only one smaller letter can be subtracted | VIII, XVIII | IIX, XXC |
These are the modern standard rules taught in schools and used for dates and outlines. The converter reads a broken numeral loosely too, so for IIII it tells you the intended value is 4 and that the standard form is IV.
Numbers above 3,999
The classic system has no letter above M, and four Ms in a row breaks the repeat rule, so 3,999 (MMMCMXCIX) is the largest standard numeral. For bigger numbers, a bar over a letter, called a vinculum, multiplies it by 1,000. With the box ticked, 12,345 becomes X̅I̅I̅CCCXLV: the overlined XII is 12,000 and CCCXLV is 345. Writers in Roman and medieval times used several different conventions for large numbers, so treat this as one common modern choice rather than the only correct form.
Frequently asked questions
Why do some clocks show 4 as IIII?
It's a long tradition among clockmakers. One popular explanation is that IIII visually balances the VIII on the opposite side of the dial. Outside clock faces, the standard form is IV.
How do I write 1999 in Roman numerals?
Write each place separately: 1,000 is M, 900 is CM, 90 is XC and 9 is IX, so 1999 is MCMXCIX. The shortcut IMM is not allowed, because I can only come before V or X.
Is there a Roman numeral for zero?
No. The Roman system had no symbol for zero, because it counted things rather than using place value. Medieval scholars who needed one wrote the word nulla, meaning "none", or the letter N.
How do I write this year in Roman numerals?
2026 is MMXXVI: two thousands (MM), two tens (XX) and six (VI). Next year, 2027, is MMXXVII. Movie credits, cornerstones and Super Bowl numbers all follow the same rules, so you can check any of them here.
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