Write out the Roman numeral you are attempting to convert on paper, leaving spaces between each numeral.
Write out the basic values for Roman numerals below the number, to make converting easier. I is equal to 1, V is five, X is 10, L is 50, C is 100, D is 500 and M is 1,000.
Circle any areas where a smaller Roman numeral is placed before a bigger Roman numeral. For example, if you are converting the Roman numeral CMXLVIII, you would circle the C and the M at the beginning and the X and the L following those numbers.
Write an equation for any numerals that you circled in the previous step. First, convert your Roman numerals to the numbers you wrote below the Roman numeral. When converting CMXLVIII, you would write out 100 for C and 1,000 for M. Next, create your equation by subtracting the smaller number from the larger number. In this example, you would write out 1,000 - 100 = 900. Your second equation, for XL, would read 50 - 10 = 40. Roman numerals are traditionally written with the largest numbers coming first and moving down to the smaller numbers. If a smaller number is placed in front of a larger number, this is an indicator to subtract.
Convert the remaining Roman numerals that you did not circle to the Arabic counterpart. In the example of CMXLVIII, you would write 5 for V and a 1 for each I. Write down the Arabic numerals on your sheet of paper.
Write an addition equation, using each of the Arabic numerals you received when converting the Roman numerals. For the Roman numerals you circled, add in the final number of the subtraction equation. For the Roman numeral CMXLVIII, your equation would look like this: 900+40+5+1+1+1.
Add up your equation using a calculator or do the math in your head. Your final answer, in this case 948, is the Roman numeral converted into Arabic decimals.