There are lots of numbers on a golf scorecard, but there are two numbers most golfers don't understand: golf course rating and golf course slope (or, simply, rating and slope). You may have never looked too hard at slope and rating and you might be wondering what they are, what they tell you and how they're determined.
What is golf course rating and how to calculate golf course rating
Of the two, slope and rating, course rating is the most likely to indicate a course's true difficulty. A golf course rating tells you what a scratch golfer would be expected to shoot from the tees they're playing on a particular course under normal conditions. The rating is different for men and women.
The USGA defines a scratch golfer as (for men) "a player who can play to a Course Handicap of zero on any and all rated golf courses. A male scratch golfer, for rating purposes, can hit tee shots an average of 250 yards and can reach a 470-yard hole in two shots." A scratch woman can "can hit tee shots an average of 210 yards and can reach a 400-yard hole in two shots at sea level."
Courses are rated every 10 years (or within five years of opening), and the rater takes into account a properly measured course, prevailing winds, roll out of the golf ball, elevation changes, doglegs and lay-ups, hazards, altitude, fairway width, green size, rough, bunkers, out of bounds markers, trees, green speeds and other psychological factors.
Course rating expressed as a number with a single decimal point. For example, a particularly tough course could have a par of 72 and a rating from the back tees of 74.3. An easier course would likely have a rating from their back tees that is lower than par.
What is golf course slope and how to calculate slope rating
The golf course slope is a measure of a course's difficulty, but it is not considered the better of the two measurements. Golf course slope is actually derived from the golf course rating. Slope is a measure of a golf course's difficulty in a relative comparison of a scratch golfer to a bogey golfer (someone who shoots around 90 for 18 holes). It is always a number between 55 and 155, with 113 being the "standard" slope.
You calculate slope rating by finding the bogey rating, which is like the course rating, but measured for a bogey golfer. Then subtract the course rating from that figure. Then multiply that figure by 5.381 for men and 4.24 for women. Then round up to the nearest whole number.