Roman Numeral Converter: Numbers to Roman and Back

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Roman Numeral Converter

Numbers to Roman numerals and back, using the standard subtractive rules. Anything that is not a valid numeral is rejected rather than quietly guessed at, so you can trust a result that comes back.

2026 = MMXXVI

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Numbers either side

NumberRoman numeralBroken down

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How to use this tool

Type a number in the left box or a numeral in the right box. Whichever you touch, the other follows. The breakdown underneath shows which symbols were used and what each is worth, which is the part that actually teaches you to read them rather than just look them up.

If you type something that is not a valid numeral the tool says so rather than producing a number anyway. That matters more than it sounds, because most online converters will happily accept IIII and hand back a four, which hides the mistake instead of catching it.

The rules, in the order they matter

There are seven symbols. I is one, V is five, X is ten, L is fifty, C is a hundred, D is five hundred and M is a thousand. Everything else follows from four rules.

Write symbols largest first and add them up. So MDCLXVI is 1000 plus 500 plus 100 plus 50 plus 10 plus 5 plus 1, which is 1666, and it happens to use every symbol exactly once in descending order.

Repeat a symbol at most three times. III is three, IIII is not four. This is the rule that forces the subtractive pairs to exist.

When a smaller symbol comes before a larger one, subtract it. There are exactly six legal pairs: IV is four, IX is nine, XL is forty, XC is ninety, CD is four hundred and CM is nine hundred. No others are allowed. IM is not a thousand less one, and IC is not ninety nine.

Only I, X and C can be subtracted, and only from the next two symbols up. That is why V, L and D never appear before a larger symbol, and why they are never repeated either. Two fives would just be a ten.

Why the system stops at 3999

M is the largest symbol, and you may write at most three of them. That caps the standard system at MMMCMXCIX, which is 3999. There is no MMMM.

The Romans did have a way past this. A bar written over a numeral, called a vinculum, multiplied it by a thousand, so a barred V was five thousand. That notation cannot be typed in ordinary text, which is why virtually every converter, including this one, stops at 3999 rather than inventing something.

A worked check you can repeat on paper

Take 1994. Start with the thousands: one M leaves 994. Nine hundred cannot be written as DCCCC because that repeats C four times, so use the subtractive pair CM, leaving 94. Ninety is likewise XC rather than LXXXX, leaving 4. Four is IV. Put them together and you have MCMXCIV.

Read it back to check. M is 1000. CM is 900 because C comes before M. XC is 90. IV is 4. Total 1994. The three subtractive pairs in a row are what make this year a common example.

Mistakes that catch people out

Writing IIII for four. It appears on clock faces, and that is a genuine typographic tradition rather than correct notation. Clockmakers used it for visual balance against the VIII opposite. In writing, four is IV.

Inventing subtractive pairs. IM, IC and VC look plausible and none of them are valid. Only I, X and C subtract, and only from the next two symbols up.

Repeating V, L or D. VV is not ten, it is a mistake. Those three symbols appear at most once in any numeral, because doubling them always produces a value that already has its own symbol.

Expecting a zero. There is no Roman symbol for nothing. The system counts rather than places, so it never needed one, which is also why arithmetic in Roman numerals is so unpleasant.

Numbers in Roman numerals

The numbers people look up most, including every year in recent memory. Each opens the converter on that value with its own address.

1 in Roman numerals2 in Roman numerals3 in Roman numerals4 in Roman numerals5 in Roman numerals6 in Roman numerals7 in Roman numerals8 in Roman numerals9 in Roman numerals10 in Roman numerals11 in Roman numerals12 in Roman numerals13 in Roman numerals14 in Roman numerals15 in Roman numerals16 in Roman numerals17 in Roman numerals18 in Roman numerals19 in Roman numerals20 in Roman numerals21 in Roman numerals22 in Roman numerals23 in Roman numerals24 in Roman numerals25 in Roman numerals26 in Roman numerals27 in Roman numerals28 in Roman numerals29 in Roman numerals30 in Roman numerals31 in Roman numerals32 in Roman numerals33 in Roman numerals34 in Roman numerals35 in Roman numerals36 in Roman numerals37 in Roman numerals38 in Roman numerals39 in Roman numerals40 in Roman numerals41 in Roman numerals42 in Roman numerals43 in Roman numerals44 in Roman numerals45 in Roman numerals46 in Roman numerals47 in Roman numerals48 in Roman numerals49 in Roman numerals50 in Roman numerals51 in Roman numerals52 in Roman numerals53 in Roman numerals54 in Roman numerals55 in Roman numerals56 in Roman numerals57 in Roman numerals58 in Roman numerals59 in Roman numerals60 in Roman numerals61 in Roman numerals62 in Roman numerals63 in Roman numerals64 in Roman numerals65 in Roman numerals66 in Roman numerals67 in Roman numerals68 in Roman numerals69 in Roman numerals70 in Roman numerals71 in Roman numerals72 in Roman numerals73 in Roman numerals74 in Roman numerals75 in Roman numerals76 in Roman numerals77 in Roman numerals78 in Roman numerals79 in Roman numerals80 in Roman numerals81 in Roman numerals82 in Roman numerals83 in Roman numerals84 in Roman numerals85 in Roman numerals86 in Roman numerals87 in Roman numerals88 in Roman numerals89 in Roman numerals90 in Roman numerals91 in Roman numerals92 in Roman numerals93 in Roman numerals94 in Roman numerals95 in Roman numerals96 in Roman numerals97 in Roman numerals98 in Roman numerals99 in Roman numerals100 in Roman numerals110 in Roman numerals120 in Roman numerals150 in Roman numerals200 in Roman numerals250 in Roman numerals300 in Roman numerals400 in Roman numerals500 in Roman numerals600 in Roman numerals700 in Roman numerals800 in Roman numerals900 in Roman numerals1000 in Roman numerals1100 in Roman numerals1200 in Roman numerals1500 in Roman numerals1600 in Roman numerals1700 in Roman numerals1800 in Roman numerals1900 in Roman numerals1950 in Roman numerals1960 in Roman numerals1970 in Roman numerals1975 in Roman numerals1980 in Roman numerals1985 in Roman numerals1990 in Roman numerals1995 in Roman numerals1999 in Roman numerals2000 in Roman numerals2005 in Roman numerals2010 in Roman numerals2015 in Roman numerals2018 in Roman numerals2020 in Roman numerals2021 in Roman numerals2022 in Roman numerals2023 in Roman numerals2024 in Roman numerals2025 in Roman numerals2026 in Roman numerals2027 in Roman numerals2028 in Roman numerals2030 in Roman numerals2050 in Roman numerals2100 in Roman numerals3000 in Roman numerals3999 in Roman numerals

How this was checked

The converter was round tripped across the entire range. Every number from 1 to 3999 was converted to a numeral and parsed back, and all 3999 returned the original value. That is the strongest test available here, because it would catch any symbol table error or any greedy matching mistake immediately.

The parser was then tested on things it should refuse: IIII, VV, XXXX, IM, IC, VX, non letters and an empty box. All eight were rejected. The mechanism is worth explaining, because it is simple and it is what most converters skip. After parsing a numeral to a number, the tool converts that number back to a numeral and checks it matches what you typed. If it does not, the input was not canonical and it is refused. That single check enforces every rule at once without needing a separate rule engine.

Range guards were checked separately: zero, 4000 and a fractional number are all refused rather than producing nonsense.

Related Calculators and Reference

See also the Number to Words Converter for writing the same number out in English rather than Roman symbols.