Roman numerals explained: rules, chart and how to convert

6 min read

Roman numerals still turn up on clocks, film credits, book chapters and sporting events. The system looks puzzling at first, but it comes down to seven letters and two simple rules.

The seven letters

Every Roman numeral is built from these letters:

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

Rule 1: add when the smaller letter comes after

Letters are written from largest to smallest and added together. VI is 5 + 1 = 6. XXX is 10 + 10 + 10 = 30. MDC is 1000 + 500 + 100 = 1600.

A letter is not repeated more than three times in a row. That is why 4 is not IIII in standard writing.

Rule 2: subtract when the smaller letter comes before

When a smaller letter is placed before a larger one, you subtract it. IV is 5 − 1 = 4 and IX is 10 − 1 = 9.

Only six pairs are used in the standard system: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900).

Quick chart

1 I     6 VI     20 XX     100 C
2 II    7 VII    30 XXX    400 CD
3 III   8 VIII   40 XL     500 D
4 IV    9 IX     50 L      900 CM
5 V     10 X     90 XC     1000 M

Converting a number to Roman numerals

Split the number into thousands, hundreds, tens and ones, then write each part on its own and join them. For 2026:

2000 is MM.
0 hundreds gives nothing.
20 is XX.
6 is VI.
Join them: MMXXVI.

Converting Roman numerals to a number

Read from left to right. If a letter is smaller than the one after it, subtract it. Otherwise add it. For MCMXCIV:

M is 1000. CM is 900 (1000 − 100). XC is 90 (100 − 10). IV is 4 (5 − 1). Add them up: 1000 + 900 + 90 + 4 = 1994.

Writing years

Years follow the same method. 2000 is MM, 2025 is MMXXV and 2026 is MMXXVI. The year 1999 is MCMXCIX, which is 1000 + 900 + 90 + 9.

Films and TV shows often show their copyright year in Roman numerals at the end of the credits, which is a classic place to practise.

Why there is no zero and no number above 3999

The Roman system has no symbol for zero, because it was designed for counting and recording, not for calculation. Numbers above 3999 have no standard form, because there is no letter above M and no letter may repeat more than three times.

The largest standard numeral is MMMCMXCIX, which is 3999. Some historical writing uses a bar over a letter to multiply it by a thousand, but that is not part of the everyday system.

Where you still see them

Clock faces, where 4 is often written IIII for visual balance
Chapter numbers, prefaces and outlines in books and documents
Names of monarchs and popes, such as Elizabeth II
Sports events and film sequels, such as Super Bowl LX
Building cornerstones and monument dates

Checking your answer

The Roman numeral converter on Toolstia converts both ways and rejects invalid forms such as IIII or IC, so you can test your own conversions. It accepts lowercase and works for any number from 1 to 3999.