Roman Numeral Validator
Type or paste a numeral. If something is off, you see which letter breaks which rule, and the standard form.
The rules the validator checks
- Only seven letters: I, V, X, L, C, D and M. Anything else, including the digit 1, fails.
- I, X, C and M repeat at most three times in a row: III is fine, four I's are not.
- V, L and D never repeat: 10 is X, not VV.
- Only six subtractive pairs: IV, IX, XL, XC, CD and CM. A letter subtracts only from one five or ten times bigger.
- One smaller letter at most before a bigger one: 8 is VIII, not IIX.
- Values go down from left to right, place by place: thousands, hundreds, tens, units.
Numerals people check
These come up in searches. Each row is checked by the same code as the box above.
- XIIvalidStandard form: XII = 12
- MCMXCvalidStandard form: MCMXC = 1990
- MCMXCIXvalidStandard form: MCMXCIX = 1999
- ICinvalidOnly I, X and C subtract, and only from the next two symbols (IV, IX, XL, XC, CD, CM).Standard form: XCIX = 99
- VXinvalidOnly I, X and C subtract, and only from the next two symbols (IV, IX, XL, XC, CD, CM).Standard form: V = 5
- ILinvalidOnly I, X and C subtract, and only from the next two symbols (IV, IX, XL, XC, CD, CM).Standard form: XLIX = 49
- XLXXinvalidThe symbols are out of order.Standard form: LX = 60
- XXCXinvalidOnly one smaller symbol can come before a larger one.Standard form: CX = 110
- XVXXinvalidOnly I, X and C subtract, and only from the next two symbols (IV, IX, XL, XC, CD, CM).Standard form: XXV = 25
- IIIIinvalidA symbol can repeat at most three times in a row.Standard form: IV = 4
- MMXV1invalidThe digit 1 is in place of the letter I.Standard form: MMXVI = 2016
Why a numeral can add up and still be wrong
XLXX looks like 40 + 20 = 60, but the tens are written twice. Each place gets one group of letters, so 60 is LX. The same happens with XXCX and XVXX: the letters add up to a number, but no standard numeral is written that way.
Because of this, every number from 1 to 3,999 has exactly one standard form. The validator compares what you typed with that form.
IIII on clocks and other old forms
Clock faces very often show four o'clock as IIII instead of IV, although some, like the clock of the Elizabeth Tower in London, use IV (source: en.wikipedia.org). The validator marks IIII as non-standard and says so. The guide to IIII on clocks covers the explanations that have been proposed.
Common questions
Is IIII wrong?
It is not the standard form, which is IV. It is common on clock faces, so on a clock it is a convention rather than a mistake.
Is IL a valid way to write 49?
No. I can only come before V and X. 49 is 40 + 9: XL + IX = XLIX.
Why is my numeral invalid if it adds up?
Because each place (thousands, hundreds, tens, units) has to be written once, in order. A numeral that writes the tens twice, like XLXX, adds up but is not standard.
Can I check lowercase numerals?
Yes. Lowercase letters are read as capitals, and spaces are ignored.
Does it check numerals above 3,999?
No. The validator covers the standard range, 1 to MMMCMXCIX. For numbers with a bar over them, use the converter.