Roman Numeral to Decimal Converter

What the Roman numeral to decimal converter does

This reads a Roman numeral and tells you the ordinary number it stands for. Type the numeral in, press the button, and copy out the value.

Reading Roman numerals is mostly a matter of adding up letter values, with one small twist for the spots where a subtraction is hiding. Learn to spot those and the rest is just arithmetic.

How to use it

1. Enter your Roman numeral. Type it in, for example MMXXIV. Capitals or lowercase both work.
2. Press Convert. The decimal value appears in the output box.
3. Click the result to copy it. Tap the output to send it to your clipboard.

Press Reset to clear both boxes.

The symbol values

Each letter in a Roman numeral has a fixed value, and there are only seven to know:

• I is 1, V is 5, X is 10
• L is 50, C is 100
• D is 500, M is 1000

That is the entire key. Everything else is deciding whether to add a value or take it away.

The one rule: add, but sometimes subtract

Read the numeral from left to right and add the values as you go. The single exception is this: when a smaller symbol sits directly in front of a larger one, you subtract the smaller instead of adding it. So IV is not 1 then 5 making 6; it is 5 minus 1, which is 4. The same goes for IX being 9, XL being 40, and so on. If each symbol is the same size or larger than the one after it, you just add all the way along.

Spotting a subtraction

There are only six places a subtraction can happen, and they all follow the same shape, a smaller symbol before a bigger one: IV is 4 and IX is 9, XL is 40 and XC is 90, CD is 400 and CM is 900. When you scan a numeral, look for one of these pairs. Treat the pair as a single chunk with its combined value, add up all the chunks and lone symbols, and you have the number. Anything that is not one of those six pairs is a plain addition.

A worked example with a real numeral

Let us read MMXXIV. Break it into its pieces from the left:

• MM is two 1000s, so 2000
• XX is two 10s, so 20
• IV is one of the subtraction pairs, so 4

Add the pieces: 2000 + 20 + 4 = 2024. The only spot needing care was IV at the end, where the I sits before the V and so is subtracted rather than added.

Questions people ask

How do I read a Roman numeral?

Add the letter values from left to right, but when a smaller symbol comes just before a larger one, subtract it. Pairs like IV and IX are subtractions; everything else is added.

How do I know when to subtract?

Only when a smaller symbol sits directly before a larger one, which happens in just six pairs: IV, IX, XL, XC, CD, and CM. Outside those, you always add.

Does capitalization matter?

No. You can enter the numeral in capitals or lowercase, and it reads the same either way.

What is MMXXIV?

2024. MM is 2000, XX is 20, and IV is 4, which add up to 2024.

References

1. Roman numerals. Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/wiki/Roman_numerals