Number Converters · Browser · Free

Roman Numerals Converter

Turn a Roman numeral into a decimal number, or a decimal number into a Roman numeral, in a single form. Choose the direction, fill in the field, and press Convert.

Both sides of the swap in one form

A Roman numeral and a decimal number are two ways of writing the same value, and this tool changes one into the other. The Conversion field picks which way round the change goes: Roman numeral to decimal, or Decimal to Roman numeral.

There is a field for each side of that change — Roman numeral and Decimal number — and a Convert button to make the change happen. Nothing is worked out until you press the button, so you can set the form up at your own pace.

Reading the result, copying it, starting over

Once you press Convert, the answer is shown in cards below the button, each one naming what it holds — so a run of the tool gives back Decimal number: 14, or Roman numeral: XIV.

Copy writes a result and its unit to the clipboard, ready to paste wherever you need it. Clear resets the form, which is handy when you want to switch direction and take a different value through the same two fields.

The conversion formula

Because the two spellings stand for one value, every result can be written as an equation, and that is how the worked examples read: XIV = 14, MCMXCIX = 1999 and MMXXVI = 2026, or the other way round as 14 = XIV and 1999 = MCMXCIX.

On the numeral side the letters are read left to right as a running total. A letter is added to the total, except when it is written in front of a bigger letter, in which case it is counted off the bigger one instead — that is the rule behind IX = 9, where the small letter in front lowers the value rather than raising it.

The two directions undo each other, so a value sent one way comes back the same the other: XIV gives Decimal number: 14, and sending 14 back the other way gives Roman numeral: XIV. Larger numbers follow the same reading, as 1999 = MCMXCIX and MMXXVI = 2026 show.