Swing weight is a confusing concept in many ways. It isn't really a weight and it isn't measured in ounces or grams. Two clubs that are very different from each other in total weight might have the same swing weight, while two similar clubs might have different swing weights. Not all club makers agree that swing weight is a useful measurement, but all club makers know how to measure swing weight. It's the de facto standard for matching all the clubs in a set.
Swing weight isn't an actual weight. Rather, it's a comparison of the weight distribution between the grip end and the head end of a golf club. It's a measurement intended to help a club maker ensure that all the clubs in a given set feel the same when you swing them.
The terms "head heavy" and "head light" are often used when referring to swing weight. Although they are not precisely defined, they accurately sum up the purpose of swing weights: to help you identify how heavy the club head feels when you swing the club. By adjusting the swing weight, the club maker can help slow down an overly quick swing tempo (with a heavier swing weight) or even reduce fatigue late in the round (with a lighter swing weight).
The most common swing weight scale is called a Lorythmic measurement. For this measurement, the club is balanced on a fulcrum, or pivot point, located 14 inches from the grip end of the club. The resulting swing weight is notated by a letter, from A through G, followed by a number between 0 and 9. The lightest swing weight is A0 and the heaviest is G0. The most common swing weights for off-the-shelf club sets range from C9 to D2.
Club makers use a specially made scale that resembles a hospital scale in some ways. The scale has a base with a pivot point at one end and a centering marker at the other. A bar marked with the swing weight notations rests on the pivot point, and a pointer on one end of the bar points at the centering marker. The marker is a vertical piece with a line to indicate when the bar is level. The club's grip is attached to the bar 14 inches from the pivot point, and a small weight is slid along the bar until the bar's pointer aligns with the centering marker. The swing weight is then read from the bar.
Newer digital swing weight scales streamline this process by eliminating the sliding weight entirely.
Club makers use many different methods to make sure all the clubs in a set feel the same way when you swing them, such as shaft frequency matching or total weight matching. However, swing weight matching is one of the simplest methods. If all of your clubs feel the same, it's much easier to make the same swing each time regardless of which club you use. By matching the swing weights in a set of clubs, a club maker can improve your consistency.