The seven symbols
Every roman numeral is built from just seven letters, each standing for a fixed value:
| Symbol | Value |
|---|---|
| I | 1 |
| V | 5 |
| X | 10 |
| L | 50 |
| C | 100 |
| D | 500 |
| M | 1000 |
How the rules combine them
A numeral standing after a bigger one gets added to it — VI is 5 + 1 = 6. A numeral standing before a bigger one gets subtracted instead — IV is 5 − 1 = 4. Only six subtractive pairs are allowed in standard notation: IV, IX, XL, XC, CD, and CM. I, X, C, and M can repeat up to three times in a row (III is 3, CCC is 300), but V, L, and D never repeat at all — that's why 4 has to be written IV rather than IIII, even though IIII shows up on some clock faces as a stylistic tradition rather than standard notation.
This is exactly what the converter's step-by-step view walks through: for a number, it works from the largest symbol down, taking as much value as it can at each step. For a roman numeral, it does the reverse — matching the longest valid symbol or pair at each position and adding up the values.
In short:
- Additive Rule: Smaller symbols following larger symbols are added (e.g.,
VI = 5 + 1 = 6). - Subtractive Rule: Smaller symbols preceding larger symbols are subtracted (e.g.,
IV = 5 - 1 = 4).
To learn all four conversion rules or explore advanced methods like the Apostrophus system and Vinculum overlines for large numbers, visit our detailed Roman Numerals Guide.
Numbers beyond 3999
Standard roman numeral notation tops out at 3999, written MMMCMXCIX — there's no seven-letter symbol for anything bigger, so this converter's valid range stops there too. Historically, Romans did have a way to write larger numbers: drawing a horizontal line (called a vinculum) over a symbol multiplied its value by 1,000. A line over V (5) meant 5,000; a line over X (10) meant 10,000; a line over M (1,000) meant 1,000,000. In practice this notation was inconsistent, rarely needed for everyday Roman use, and isn't something you'll run into outside of historical inscriptions and academic texts — which is part of why it never became as standardized as the basic seven symbols, and why calculators (including this one) generally don't attempt to support it.
Roman numeral dates
A roman numeral date writes the month, day, and year as three roman numerals in a row, month first — the same order as MM/DD/YYYY. December 25, 2024 becomes XII·XXV·MMXXIV. This converter accepts a dash, dot, slash, backslash, pipe, or plain space between the three parts, so XII-XXV-MMXXIV, XII/XXV/MMXXIV, and XII XXV MMXXIV all work identically. Since the year has to fit standard roman numeral notation, this converter only handles years from 1 to 3999.
Frequently asked questions
Why did my roman numeral get rejected as "non-standard"?
It's probably technically readable but not written the way standard rules require — IIII instead of IV for 4, or VX instead of XV for 15, for example. The error message shows you the correct standard form for the value you were going for.
Is there a zero in roman numerals?
No — the Romans had no symbol for zero as a number. The concept of zero as a placeholder value came later, popularized by Indian and Arabic mathematicians.
What's the largest number I can convert here?
3999, written MMMCMXCIX — the largest value expressible in standard roman numeral notation without resorting to historical extensions like the vinculum (overline) system.