Roman Numeral Translator:
The Romans had no negative numbers, and could express only a limited range of fractions with numeric symbols: 1/12 to 11/12. (The American pound is divided into 16 ounces, but the Roman libra was divided into 12 unciae.)
That means that while any properly-formed Roman number can be expressed in Arabic numbers, the converse is not true. The Arabic → Roman calculator can only handle numbers from 1/12 up through 99,9999,999 and 11/12ths. Just put the whole number into the first slot, the twelfths into the second, and press the button or type 'Enter' to calculate. You may use commas or not, as you please, for whole numbers greater than 1,000.
The Roman → Arabic calculator is more difficult to operate correctly, since Roman numerals are less familiar and harder to explain. (See PDF handout here.) A little practice with the Arabic → Roman calculator should help you understand what to put into the Roman → Arabic. You can use either vertical or slanted slash marks (|| or //) to delimit Roman thousands: the first is more Roman, the second may be easier not to mix up with capital Is. For Roman hundreds of thousands you will have to use square brackets ([ ]): the program will turn these into vertical slashes and an overline.
[What about small Roman numbers? xviii - they work now. Talk about how lack of digamma, qoppa, sampi, and backwards C prevents some other features.]