How Roman numerals work
Seven letters carry the values: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1,000. Values are written from largest to smallest and added: MDCLXVI is 1,000 + 500 + 100 + 50 + 10 + 5 + 1 = 1,666. To avoid four identical letters in a row, a smaller letter placed before a larger one is subtracted: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400 and CM = 900.
Only these six pairs subtract. I goes only before V and X, X only before L and C, and C only before D and M. V, L and D are never repeated and never subtracted. With these rules every number from 1 to 3,999 has exactly one standard form, and that is the form this converter writes and checks.
Errors explained, not just rejected
When a numeral breaks a rule, the tool says which one and suggests the standard form. IIII becomes IV, VV becomes X, IC becomes XCIX and IIX becomes VIII. Lowercase letters, spaces and the special Roman numeral characters (such as the single character for XII) are accepted.
Clock faces often show IIII instead of IV. That is a long design tradition, not an error on the clock, but in writing IV is the standard form.
Large numbers with the overline
There is no zero and no standard way to write numbers above 3,999 with the seven letters alone. The Romans drew a line over a numeral to multiply it by 1,000, so V with a line is 5,000 and X with a line is 10,000. Turn on the overline option to convert numbers up to 3,999,999. The line is a combining Unicode character, so it shows in most apps, but check how it looks before you print or engrave it.
Dates and charts
The Date tab turns a day into Roman numerals for wedding rings, tattoos, invitations or building plaques. Choose day-month-year, month-day-year or year-month-day, a separator such as a dot or a middle dot, and a full or two-digit year. Each part is shown separately so you can check it. The Chart tab lists any range, for example 1 to 100 or the years 1900 to 2100, ready to copy or download as a CSV file.