First, you determine the handicap differential for each round of golf you played, up to the last 20 rounds. Subtract the USGA course rating from your score. Take the difference and multiply it by the standard USGA slope rating, which is 113. Then divide that number by the course's slope rating. The result is the handicap differential for that round. Calculate the differential for each of your scores.
To calculate your handicap index, you average a certain number of your best handicap differential scores, depending on how many scores you have accumulated. Ideally, you want 20 scores, so you can use the lowest 10 of the last 20. To average the differentials, add them together and divide by 10. Then multiply the average by .96 and delete all the numbers after the tenths digit. The result is your handicap index. If you have fewer than 20 scores, average this many differentials: 19 scores, use the nine lowest differentials; 18, eight; 17, seven; 15 or 16, six; 13 or 14, five; 11 or 12, four; nine or 10, three; and seven or eight, two. If you have only five or six scores, you will use just one differential, so there's no need to average.
Converting Index to Handicap
To determine your course handicap, find the slope rating of the set of tees you will be playing. This number is usually listed on the scorecard. Multiply your handicap index by that slope rating and then divide the result by 113. The answer is your course handicap.