Replace the capital letter I with the Arabic number 1. I equals 1, and each letter in Roman numbers can be used up to three times. Hence, II equals 2; III equals 3. Thus, I, II, III equals 1, 2, 3. Lower case letters may be used as well; thus, i, ii, iii equals 1, 2, 3. The basic rule in Roman numbers is that the letter equals the number's value, so three letters "I" in a row would mean "3."
Replace the letter V with the number 5. Put the letter I before the V to give you 4; thus, IV equals 4. Another rule for Roman number is that you can only subtract the letters that represent powers of 10, so it is common to see the letter "I" before the letter "V," which indicates you subtract 1 from that number; thus 5 - 1 = 4. Put the I after the V to get the number 6; thus, VI equals 6, VII equals 7 and VIII equals 8. Remember, you cannot use a letter in Roman numerals more than three times in a row.
Replace the letter X for the number 10. Put the letter I before the X to give you 9; thus, IX equals 9. Another rule for Roman numbers is that you can use a letter to subtract only one time. Thus, you cannot write IIX to indicate an 8. So the pattern for converting Roman numbers to Arabic numbers 10 through 15 is X equals 10, XI equals 11, XII equals 12, XIII equals 13, XIV equals 14 and XV equals 15. It is generally a simple principle to follow and only gets complex when you are putting together letters to represent four digit numbers and above.