Type numbers or numerals in either box. Every number in the text converts.
Seven symbols do all the work: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000. Write them largest first and add them up: MDCLXVI is 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1666.
The one twist is subtraction. A smaller symbol written before a larger one is taken away, in six fixed pairs: IV 4, IX 9, XL 40, XC 90, CD 400, CM 900. So MCMXCIV is 1000 + 900 + 90 + 4 = 1994.
Break the number into thousands, hundreds, tens and units, and write each part on its own. 2026 is 2000 + 20 + 6 = MM + XX + VI = MMXXVI. A symbol never repeats more than three times in standard form, which is why the largest standard numeral is 3999, MMMCMXCIX.
The converter handles whole passages. Paste a block of text and every number or numeral in it converts, while ordinary words stay as they are.
Roman numerals hide in plain sight: chapter headings, clock faces, film copyright dates, monuments. They also hide inside words. Read the capitals of a sentence for I, V, X, L, C, D and M and see whether they add up to something. A converted number often goes on to a letter-number step, where XIV is 14, and 14 is N.
I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Symbols add up from largest to smallest, except that a smaller one placed before a larger one is subtracted: IV is 4 and XC is 90.
In standard form, 3999 (MMMCMXCIX). Larger numbers used a bar over a numeral to multiply it by 1,000, which plain text can't show.
It's non-standard but real: many clock faces use IIII for 4. The converter reads it and notes that the standard form is IV.