Once you get past 20 or so, converting Roman numerals to Arabic (or vice versa) can be tricky. But you can save time and improve accuracy with a pair of Excel functions.
To convert an Arabic number to its classic Roman numeral, use:
Naturally, you can place a number directly into the parentheses or choose any cell in your worksheet to reference - as long as the value is between 1 and 3999.
Turning Roman numbers into Arabic is slightly trickier, but still pretty simple. Paste in this formula:
But don't hit Enter. Instead, use Ctrl-Shift-Enter. This makes it an array formula, which should handily make the conversion as long as your Roman numeral value is in the same 1 to 3999 range.
Here are some examples of these functions in action:
Doesn't seem to work in openofffice Calc, probably because this sense of INDIRECT() is an undocumented dingus in Excel.