Roman Numeral Rules
Roman numerals stay readable when a small set of rules is followed. These conventions define the modern “canonical” form used by converters, charts, and most reference materials.
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Write larger values before smaller ones
In additive form, place the largest value first and continue downward. CLXVI is 100 + 50 + 10 + 5 + 1 = 166. Writing smaller values first usually changes the meaning or creates an invalid string under modern rules.
When a smaller symbol appears immediately before a larger one, that pair is subtractive—for example XC (90) inside XCVI (96). Learn the full list in Roman numeral subtraction rules.
Repetition limits
- I, X, C, and M may repeat up to three times in a row in standard form: III = 3, XXX = 30, CCC = 300, MMM = 3000.
- V, L, and D are not repeated. 10 is X, not VV; 100 is C, not LL.
- Four identical symbols in a row are avoided in modern standard notation: write IV (not IIII), XL (not XXXX), CD (not CCCC), and CM (not DCCCC).
Clock faces are a notable exception where IIII sometimes appears for 4—see Roman numerals on clocks.
Only six subtractive pairs
Modern standard Roman numerals allow exactly these subtractive combinations:
| Pair | Value | Not used |
|---|---|---|
| IV | 4 | IL for 49 |
| IX | 9 | IC for 99 |
| XL | 40 | XD for 490 |
| XC | 90 | IM for 999 |
| CD | 400 | VM, XM, etc. |
| CM | 900 | Other one-off shortcuts |
A symbol may subtract only from the next two higher denominations of the appropriate scale (I from V or X; X from L or C; C from D or M). Forms such as IC, IL, or XM are not standard.
Canonical form versus “readable” variants
Several strings can add up to the same number if you ignore style rules. Converters on this site prefer the canonical spelling produced by the usual greedy algorithm: for example, 1999 → MCMXCIX, not a looser historic variant.
When you decode an inscription, you may still see older or decorative spellings. The decoder helps identify the numeric value; then compare it with the modern form on the matching number page (for example 2026 in Roman numerals).
Range, zero, and large numbers
Standard practice for this site covers 1–3999. There is no classical zero symbol among the seven letters—see Roman numeral zero. Numbers at or above 4000 typically need extended notation such as a vinculum, covered in large Roman numerals.
For learning aids, use the 1–100 chart, then practice with how to write Roman numerals.
FAQ
Can V, L, or D be repeated?
Not in modern standard Roman numerals. Use X instead of VV, C instead of LL, and M (or CM/CD patterns) instead of repeating D.
Is IIII a valid Roman numeral?
IIII is common on clock faces for 4, but the modern standard written form is IV. Both are widely recognized in their contexts.
Why is 99 written XCIX and not IC?
Standard subtraction only allows I before V or X. Ninety-nine is 90 + 9, written XC + IX → XCIX.
What is the largest number this site converts?
3999, written MMMCMXCIX. Larger values require extended notation beyond the seven standard symbols.