Take the largest Roman symbol that fits and repeat: 1994 is M + CM + XC + IV, or MCMXCIV. Type a numeral to convert it back; the calculator accepts only the standard form and tells you what is wrong with anything else.
The formula
The symbols
I 1 · V 5 · X 10 · L 50 · C 100 · D 500 · M 1000The six subtractive pairs
IV 4 · IX 9 · XL 40 · XC 90 · CD 400 · CM 900Number to numeral
repeatedly write the largest symbol or pair that fits, and subtract its valueNumeral to number
add the values; a symbol before a larger one is subtractedThe numeral for a number is built one decimal place at a time: thousands, hundreds, tens, then ones.
Worked example
Joined together, that is MCMXCIV. Read back: 1000 + 900 + 90 + 4 = 1994. And the longest numeral in range, MMMDCCCLXXXVIII, is 3888.
When to use it, and the mistakes to avoid
Use it for dates on buildings, film copyright years, book chapters and prefaces, outlines, clock faces, monarchs and events such as Super Bowls, and to check a numeral you have read.
- Repeating a symbol four times. 4 is IV and 40 is XL, not IIII or XXXX.
- Subtracting the wrong symbol. Only I, X and C are subtracted, and IC is not 99: that is XCIX.
- Writing 0. There is no Roman zero.
- Expecting numbers above 3999. The standard form stops there.
- Mixing up the order. A smaller symbol after a larger one adds (VI is 6), but before it subtracts (IV is 4).
FAQ
How do I write a number as a Roman numeral?
Take the largest Roman symbol that fits, write it down, subtract its value, and repeat. 1994 is 1000 (M), then 900 (CM), then 90 (XC), then 4 (IV), which gives MCMXCIV. The calculator shows each symbol and its value.
Why is 4 written IV and not IIII?
In the standard form, a smaller symbol placed before a larger one is subtracted, so IV is 5 − 1. The standard form also never repeats a symbol more than three times in a row, which rules out IIII, and it uses only six such pairs: IV, IX, XL, XC, CD and CM. Some clock faces still use IIII by tradition, but it is not the standard numeral, and this calculator reports it as invalid and shows IV.
Why does the converter stop at 3999?
Because standard Roman numerals have no symbol above M (1000) and M is repeated at most three times, so the largest number is MMMCMXCIX, which is 3999. Larger numbers used a bar over a numeral to multiply it by 1000, which is a separate notation this page does not use.
Is there a Roman numeral for zero?
No. The Romans had no symbol for zero, and the numerals were used for counting and labelling, not for arithmetic with place value. That is why the converter starts at 1 and reports 0 as out of range.
Which Roman numerals are invalid?
Any that break the standard form. A symbol cannot be repeated more than three times (IIII, XXXX), V, L and D cannot be repeated or subtracted (VV, VX), and only I, X and C can be subtracted, and only from the next two larger symbols (IC and XM are not valid). When a numeral has a clear value but is not standard, the calculator tells you the standard form.