The system in one breath: seven letters — I, V, X, L, C, D, M for 1, 5, 10, 50, 100, 500, 1,000 — written largest first and added up, except that a smaller letter before a larger one subtracts: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400, CM is 900. Only those six subtractive pairs are standard. The ceiling is MMMCMXCIX, 3,999 — past it, medieval scribes drew a bar over letters to multiply them by 1,000: X is 10,000.
Convert any number up to 3,999,999 into Roman numerals with vinculum overline notation, letter counts, subtractive pair detection, and a place-by-place construction breakdown.
The one reading rule: scan left to right; whenever a letter is smaller than the one after it, it counts as negative. That's the whole grammar. It also means the parser here is deliberately forgiving — carved inscriptions and old clock faces freely wrote IIII for 4 and LXXXX for 90, and real Romans wouldn't have blinked. When you feed it such a form, the value is read faithfully and the modern standard spelling is offered beside it.
Convert Roman numerals to numbers with a letter-by-letter reading breakdown, automatic detection of non-standard forms with the canonical spelling, subtraction counting, and a century check for years.
The secret of Roman arithmetic: there isn't any — and that's the insight. The numerals were for writing down results; the calculating happened on a counting board or a pocket abacus, sliding pebbles (calculi — yes, that's where "calculate" comes from). But addition happens to work beautifully on paper anyway: pour the letters of both numbers together, sort them, and trade five of a kind upward — no memorized sums, ever. Multiplication used halving and doubling, which needs nothing but addition. Long division? That's why they had professionals.
Add, subtract, and multiply Roman numerals with authentic methods — addition by merging and regrouping letters, multiplication by duplation with halving and doubling steps, plus modern-number checks.
Right now, in the old alphabet — with the traditional IIII at four o'clock, just like real dials (the History tab explains why). When a count hits zero, we borrow the medieval N for nulla: Rome had no zero.
Letter counts for every year from 1 to 2100 — a jagged mountain range with peaks wherever 8s and 3s pile up. The all-time summit: 1888, thirteen letters. The modern valley: the year 2000, a serene MM.
Spotting them in the wild: film studios traditionally date credits in Roman numerals (an old habit — partly gravitas, partly, legend says, to soften how dated a rerun looks). The Super Bowl counts itself in them — the February 2026 game is Super Bowl LX. Monarchs and popes number themselves the same way, and clock faces keep the oldest quirk of all, which has its own chapter on the History tab.
A live Roman numeral clock with the traditional IIII, year conversion for movie credits, copyrights, and inscriptions, centuries, famous years from MDCCLXXVI to MM, and Super Bowl numbering.
The sawtooth of numeral lengths from I to C. Watch the subtractive pairs bite: 8 costs four letters (VIII) but 9 costs two (IX) — thrift, fourteen centuries before it was standardized.
The history of Roman numerals — why there is no zero, the IIII clock-face mystery, calculating with the Roman abacus, the medieval standardization of subtractive notation, and where the numerals survive today.