I've put together a conversion calculator for positive integers less than 4,000. The calculator will either convert a number to an appropriate Roman Numeral or convert a Roman Numeral with a positive numeric value less than 4,000 back to a number.
Let me preface this by saying I'm aware of the existing calculators out there that do 1 way conversion, and some that do both way conversions, however, I took this calculator a few steps further and provided step by step math work and instructions on the actual conversion for each letter/number. For the conversion to Roman Numerals, I set this up to take a decreasing total all the way down to zero while building each letter. For the conversion back to a number, I took an increasing total until we get to our answer.
It is located on the "Other Calculations" page, and searchable by converts, positive, integer, roman, numeral, and 4000. As always, please alert me to any errors you find or enhancements that you would like to see.
Roman Numeral Conversions