Roman Numeral Converter
No popups. No autoplay ads. Accurate answers with formulas shown.
Specialty Tools
Roman Numeral Converter
Convert between Roman numerals and decimal numbers instantly. Supports values from 1 to 3,999.
Step-by-Step Conversion
Convert decimal 42 to Roman numeral:
42 โฅ 40 โ XL, remainder 2 2 โฅ 1 โ I, remainder 1 1 โฅ 1 โ I, remainder 0
Result: XLII
Roman Numeral Reference Table
| Roman Numeral | Decimal Number |
|---|---|
| M | 1000 |
| CM | 900 |
| D | 500 |
| CD | 400 |
| C | 100 |
| XC | 90 |
| L | 50 |
| XL | 40 |
| X | 10 |
| IX | 9 |
| V | 5 |
| IV | 4 |
| I | 1 |
Conversion Rules
Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), M (1000).
Symbols are generally written from largest to smallest, left to right, and their values are added together.
When a smaller value appears before a larger value, it is subtracted: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900.
No symbol repeats more than three times in a row. V, L, and D never repeat.
A Brief History of Roman Numerals
Roman numerals evolved from Etruscan number symbols and were the standard numerical system throughout the Roman Empire (27 BC โ 476 AD). The system was used for trade, construction, military record-keeping, and monumental inscriptions. Unlike our modern positional decimal system, Roman numerals are additive and subtractive โ there is no zero, and the value of a number is determined by adding or subtracting symbol values rather than multiplying by position. Despite being replaced by Hindu-Arabic numerals for calculation by the 14th century, Roman numerals remain embedded in modern culture: clock faces, Super Bowl numbering, papal names, copyright dates, and building cornerstones.
The Subtractive Principle Explained
The subtractive principle is what makes Roman numerals concise. Instead of writing IIII for 4, the Romans wrote IV โ placing I (1) before V (5) to mean 5 โ 1 = 4. Only specific subtractive combinations are valid: I before V or X (giving 4 and 9), X before L or C (giving 40 and 90), and C before D or M (giving 400 and 900). The number 1994 demonstrates this elegantly: MCMXCIV = M (1000) + CM (900) + XC (90) + IV (4). No symbol repeats more than three times consecutively, and the symbols V, L, and D never repeat because doubling them would equal the next higher symbol (VV = X, LL = C, DD = M).
Common Uses of Roman Numerals Today
Roman numerals appear in contexts where tradition, formality, or visual distinction matters. In publishing, they number front matter (preface pages i, ii, iii) separately from the body text. In law and outlines, they organize hierarchical structures (I, II, III for major sections, then A, B, C). In chemistry, Roman numerals indicate oxidation states of metal ions (Fe(III) for iron with a +3 charge). Watches and clocks traditionally use Roman numerals, with most using IIII instead of IV for the 4 o'clock position โ a convention dating to the 14th century, possibly for visual symmetry with the VIII on the opposite side.
Frequently Asked Questions
What are Roman numerals?
Roman numerals are a numeral system originating from ancient Rome. They use combinations of seven Latin letters โ I (1), V (5), X (10), L (50), C (100), D (500), and M (1000) โ to represent numbers. The system was used throughout the Roman Empire and remains widely used today for clocks, book chapters, movie sequels, and formal numbering.
How do you convert Roman numerals to numbers?
Read the Roman numeral from left to right. Add the value of each symbol to the total. However, if a smaller value appears before a larger value, subtract the smaller from the larger (subtractive notation). For example, XIV = 10 + (โ1 + 5) = 14, and MCMXCIV = 1000 + (โ100 + 1000) + (โ10 + 100) + (โ1 + 5) = 1994.
What is the largest number you can write in Roman numerals?
Using standard Roman numerals (without vinculum or other extensions), the largest number is 3,999, written as MMMCMXCIX. For numbers above 3,999, historical systems used a bar over symbols (vinculum) to multiply values by 1,000, but this converter supports the standard range of 1 to 3,999.