Advanced options
What The Roman Numeral Converter Works Out
- Result: the number written in Roman numerals, or a Roman numeral read back as a number. The seven symbols are I, V, X, L, C, D and M for 1, 5, 10, 50, 100, 500 and 1,000, written largest first and added together.
- Breakdown: each symbol group and its value, so you can see how 1990 becomes M (1000) + CM (900) + XC (90). A smaller symbol before a larger one subtracts: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900.
- Rules of standard form: only I, X and C subtract, and only from the next two steps up; no symbol repeats more than three times; and the system tops out at 3,999 (MMMCMXCIX), since larger numbers historically used an overline to multiply by a thousand.
- Clock-face form (advanced): most clock and watch dials write 4 as IIII rather than IV, for visual balance against VIII opposite and, by tradition, to avoid starting with the first letters of the god Jupiter's name (IVPPITER).
- Lowercase form (advanced): small Roman numerals (i, ii, iii, iv) are the convention for book prefaces and front matter, sub-clauses in contracts and statutes, and outline numbering.
- Standard form check (advanced): when reading a numeral, the converter tests whether it is written the standard way. IIII and IC both read as numbers, but 4 is normally IV and 99 is XCIX, and most exam mark schemes expect the standard form.
- Symbols used (advanced): how many characters the numeral needs. Roman numerals get long quickly: 3888 (MMMDCCCLXXXVIII) is the longest standard numeral at 15 symbols.
- Arithmetic (advanced): add or subtract a second number, entered as either Roman or decimal, with the answer in both forms. The Romans had no zero and no negative numbers, so the converter says so when a subtraction drops below one.
- Neighbouring numbers (advanced): a table of the five numbers either side, handy for checking a run of film dates, monarchs (Charles III) or Super Bowl numbers.