Roman Numerals Lesson: Definition, Rules & Examples

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Updated: Apr 14, 2025

Lesson

Lesson Overview

Roman Numeral Symbols and Their Values
Writing Numbers with the Additive Rule
The Subtractive Rule in Roman Numerals
Basic Rules for Writing Roman Numerals
Converting Roman Numerals to Hindu-Arabic Numbers
Converting Hindu-Arabic Numbers to Roman Numerals
Roman Numerals Chart (1 to 100)
Complex Roman Numeral Examples

Before the numbers we use today (1, 2, 3…) were created, ancient people needed a way to count, trade, and keep records. The Romans, who lived over 2,000 years ago, created their own system using letters from the alphabet instead of digits. This system is called the Roman numeral system.

Roman numerals are still used today in clocks, book chapters, movie release years, and event names (like the Olympics). This lesson will help you learn how Roman numerals work, what each letter means, how to build numbers using the correct rules, and how to convert between Roman and regular (Hindu-Arabic) numerals.

Roman Numeral Symbols and Their Values

Roman numerals use seven capital letters to represent different values. These letters are combined in different ways to make other numbers.

Roman Numeral	Value
I	1
V	5
X	10
L	50
C	100
D	500
M	1000

These are the building blocks of Roman numerals. By putting them together in the correct way, we can write any number.

You can add or subtract these values depending on how they are arranged.

Writing Numbers with the Additive Rule

When a smaller numeral is placed after a larger numeral, you add the values.

This is the most common rule in Roman numerals.

Examples:

VI = 5 + 1 = 6
XII = 10 + 1 + 1 = 12
XX = 10 + 10 = 20
LXX = 50 + 10 + 10 = 70
MCC = 1000 + 100 + 100 = 1200

You can repeat numerals, but only up to three times in a row.

Repeated Numerals	Value
III	3
XX	20
CCC	300

You cannot write four of the same letter in a row. So instead of writing IIII, the Romans used another rule.

The Subtractive Rule in Roman Numerals

If a smaller numeral is placed before a larger one, you subtract the smaller value.

This rule helps make numbers shorter and easier to write.

Common subtractive pairs:

Roman Numeral	Value	Explanation
IV	4	5 − 1 = 4
IX	9	10 − 1 = 9
XL	40	50 − 10 = 40
XC	90	100 − 10 = 90
CD	400	500 − 100 = 400
CM	900	1000 − 100 = 900

Using the subtractive rule helps avoid writing the same numeral too many times.

Examples:

90 is not LXXXX → it's written as XC
900 is not DCCCC → it's written as CM

Take This Quiz:

Counting in Classics: Roman Numerals 1-20 Quiz

Basic Rules for Writing Roman Numerals

To write Roman numerals correctly, follow these important rules:

Start with the largest numeral possible
- Write the numeral with the highest value first
Use additive combinations for numbers like 6, 15, or 300
- VI, XV, CCC
Use subtractive combinations for numbers like 4, 9, 40, or 900
- IV, IX, XL, CM
Never repeat the same numeral more than three times in a row