Roman Numeral Converter
Seven letters carry value: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000.
Both directions, one page
Switch the mode to go either way. Number-to-Roman takes a value from 1 to 3999 and builds the numeral. Roman-to-number takes the letters and reads them back as a figure.
When you hand it a numeral to decode, the converter does not just translate blindly. It checks that the letters form a valid numeral first, so a typo gets caught rather than quietly returning a wrong number.
Reading the result
Going one way, the result is the assembled numeral. Going the other, it is the decimal value. Either way the input you gave is echoed back so you can confirm the converter read what you meant.
A numeral that breaks the rules, like one with five identical letters in a row, returns a plain error instead of a number. That refusal is a feature: it stops a malformed numeral from looking legitimate.
Seven letters and the subtractive rule
The whole system rests on seven letters: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. Most of the time you add them up from left to right, largest first.
The twist is subtraction. When a smaller letter sits directly before a larger one, you subtract it, which is how IV becomes four and IX becomes nine. Only certain pairs are allowed, and that restriction is exactly what the validity check enforces.
2024 and 1984 worked out
To build 2024, you take two thousands, MM, then twenty as XX, then four as IV, giving MMXXIV. Each chunk is the largest numeral that still fits the remainder.
Reading the other way, MCMLXXXIV unpacks as M for 1000, CM for 900, LXXX for 80 and IV for 4, which adds to 1984. The subtractive pairs CM and IV are doing the heavy lifting.
The rules people break
The common errors all come from ignoring the subtractive rule. Four is IV, not IIII. Forty is XL, not XXXX. Ninety is XC, not LXXXX. The same letter is never meant to repeat four times in a row.
There is also a ceiling. The standard seven letters only reach 3999, written MMMCMXCIX. Going higher historically meant drawing a bar over a letter to multiply it by a thousand, which sits outside the ordinary set this converter uses.
Where Roman numerals still turn up
They mark the year on film credits and building cornerstones, number the Super Bowl, and label book chapters, clock faces and the front matter of a long document.
Most of those are years or small counts, comfortably inside the 1 to 3999 range, which is why a converter capped there handles almost everything people actually run into.
Roman Numeral Converter FAQ
How do I write a number in Roman numerals?
Select the number-to-Roman direction and enter a value from 1 to 3999.
The converter assembles the letters for you using the standard subtractive rules, so 49 comes out as XLIX rather than IL.
What is the largest standard Roman numeral?
With the basic seven letters it is 3999, written MMMCMXCIX.
Numbers beyond that need a bar drawn over a letter to multiply it by a thousand, which the standard set does not include.
Why is four written IV and not IIII?
Roman numerals use subtraction for these cases. A smaller letter placed before a larger one is subtracted, so IV is five minus one.
The same idea makes nine IX and forty XL.
Is there a zero in Roman numerals?
No. The system has no symbol for zero, which is one of the reasons it was eventually set aside for arithmetic in favor of the decimal digits we use now.
How do I read a Roman numeral date?
Switch to the Roman-to-number direction and type the numeral exactly as you see it.
Film credits, clock faces and cornerstones often write the year this way, and the converter turns it straight back into digits.