Roman Numeral Converter
Numbers to Roman numerals and back, with validation and a breakdown.
Use via API
Convert a whole number to Roman numerals (1 to 3999, or up to 3,999,999 with vinculum overlines) or Roman numerals to a number. The direction is detected from the input. Send JSON, get JSON back.
POST https://pithlab.com/api/v1/tools/roman-numeral-converter
curl https://pithlab.com/api/v1/tools/roman-numeral-converter \
-H "Authorization: Bearer $PITHLAB_KEY" \
-H "Content-Type: application/json" \
-d '{"input":"2026"}'About the Roman Numeral Converter
Convert numbers to Roman numerals and back, from 1 to 3,999, with strict checks that explain why forms like IIII, VX or IC are wrong, and a breakdown.
How to use the Roman Numeral Converter
- Type a number (like 2026) or a Roman numeral (like MCMXCIV). The direction is detected automatically, and letter case doesn't matter.
- Read the conversion and the place-by-place breakdown. Invalid numerals get an explanation and the correct form.
- For numbers above 3,999, turn on vinculum notation to get overlined numerals worth 1,000 times as much.
Examples
Input
2026
Output
2,026 = MMXXVI
Input
MCMXCIV
Output
MCMXCIV = 1,994
Input
1234567 (vinculum)
Output
1,234,567 = M̅C̅C̅X̅X̅X̅I̅V̅DLXVII
Input
IC
Output
IC isn't a valid Roman numeral. I can only be subtracted from V and X, so IC isn't allowed. Read loosely it would be 99, which is written XCIX.
Frequently asked questions
How do Roman numerals work?
Seven letters stand for values: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1,000. Write each place from largest to smallest and add them up: MMXXVI is 1,000 + 1,000 + 10 + 10 + 5 + 1 = 2,026. A smaller letter before a larger one is subtracted, as in IV (4) and CM (900).
Why is IIII or IC rejected?
Modern standard form only allows six subtractive pairs: IV, IX, XL, XC, CD and CM. I, X and C can repeat up to three times; V, L and D never repeat or get subtracted. So 4 is IV not IIII, and 99 is XCIX not IC. (You'll still see IIII on some clock faces, but it isn't the standard form.)
Why do Roman numerals stop at 3,999?
Without extra notation the largest numeral is MMMCMXCIX, because M can't repeat four times. The Romans had ways to write larger numbers, the best known being the vinculum: a bar over a numeral multiplies it by 1,000, so V̅ is 5,000. This tool can write numbers up to 3,999,999 that way, using a combining overline character, but it only reads standard numerals.
Is there a Roman numeral for zero?
No. The Roman system had no zero digit. Some medieval writers used N (for nulla, “none”) to mean zero, but it isn't part of the numeral system.