Converting numbers to and from Roman numerals

2026-10-04 · 2 min read

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A copyright year reads MMXXIV, a clock face shows IV or sometimes IIII, a book's front matter is numbered in lowercase roman — and unlike decimal, there's no place-value system underneath it. Roman numerals are built from seven fixed symbols (I=1, V=5, X=10, L=50, C=100, D=500, M=1000) combined by addition and one subtraction rule.

The method

Work from the largest value down to the smallest, in this order, subtracting each value from the number as many times as it fits and writing its symbol each time: 1000(M), 900(CM), 500(D), 400(CD), 100(C), 90(XC), 50(L), 40(XL), 10(X), 9(IX), 5(V), 4(IV), 1(I). The 900/400/90/40/9/4 entries are the "subtraction" shortcuts (one smaller symbol placed before a larger one means subtract it) — without them you'd write four or nine of the same symbol in a row, which Roman numerals avoid.

A real example: converting 1994 to Roman numerals

Working down the list: 1994 has one 1000 → M, remainder 994. It has one 900 → CM, remainder 94. It has one 90 → XC, remainder 4. It has one 4 → IV, remainder 0. Reading the symbols in order: MCMXCIV — the numeral famously stamped on 1994 film reels.

A real example: converting 2026 to Roman numerals

2026 has two 1000s → MM, remainder 26. It has no 900, 500, 400 or 100. It has two 10s → XX, remainder 6. It has one 5 → V, remainder 1. It has one 1 → I. Reading in order: MMXXVI.

A real example: reading CDXLIV back to a number

Going the other direction, scan left to right and add each symbol's value, but subtract instead when a smaller symbol sits directly before a larger one. CD is C(100) before D(500), so it's 500-100=400. XL is X(10) before L(50), so it's 50-10=40. IV is I(1) before V(5), so it's 5-1=4. Total: 400+40+4=444. So CDXLIV is 444.

Why there's no zero and no numbers above 3999

The Romans had no symbol for zero in this notation, so there's no way to write it, and standard practice never repeats a symbol more than three times in a row, which caps the largest ordinary numeral at MMMCMXCIX (3999) — one more and you'd need a fourth M. Larger numbers historically used a bar over a symbol to mean ×1,000, but that's outside the numerals most tools and style guides support.

Run your own numbers

The free Roman Numerals converter turns a number into Roman numerals and reads Roman numerals back into a number instantly, entirely on your device.