What it measures
Near the ball, the clubhead travels along something close to a large tilted hoop. Swing plane is the angle that hoop leans at compared with the ground, in degrees. A hoop lying flat on the floor would read 0 and a hoop standing straight up would read 90. Real full swings land in between, typically from the mid 40s with a driver to around 60 with shorter irons.
The number is always positive and works the same way for right and left-handed golfers. It only describes tilt. Which way the hoop is pointed, left or right of the target, is a separate number called swing direction. It is also not the same as the shaft lines coaches draw on a video frame, which capture one moment in two dimensions. Swing plane describes the arc from roughly knee height in the downswing to roughly knee height in the follow-through, so it reflects the overall shape of the bottom of the swing rather than the shaft angle at one instant.
Typical numbers
A typical driver swing plane is between about 45 and 50 degrees. Amateur men average close to 48 to 49 degrees from scratch players through average handicaps, and amateur women read slightly lower, in the mid to high 40s. In other words, skill level barely moves the average, while consistency from swing to swing matters more.
Irons read higher because the clubs are shorter and more upright and you stand closer to the ball. There is no standard tour average by iron, but a 6 iron reading above about 54 often means something has gone wrong, and roughly 60 degrees works as an upper guide for a 6 or 7 iron. Wedges tend to read higher still. Your height and posture shift all of these, so treat any figure as a range.
What changes it
Club length and built lie angle are the biggest factors: shorter clubs produce a steeper plane. Taller golfers and golfers who stand closer to the ball tend to read higher, and standing farther away or with more knee flex tends to read lower. Arms that swing more around the body flatten it.
Through impact, the plane rises when the hips move toward the ball and the hands lift away from the body, a pattern often called standing up or early extension. An inconsistent setup, such as changing ball position or distance from the ball shot to shot, usually shows up as a plane that moves around.
What it changes
Swing plane has no direct effect on the ball, but it controls how other delivery numbers relate. On any tilted arc, hitting down moves the club path to the right and hitting up moves it to the left for a right-handed golfer. How much depends on the plane: on a flat arc near 45 degrees, each degree of attack angle shifts the path by about a degree, while on a steeper arc near 60 degrees the shift is closer to six tenths of a degree.
A plane that climbs through impact often brings the hands up, the toe up and the heel into the ground, which shows as a more upright dynamic lie, heel-side strikes and pulled shots. A plane that changes from swing to swing makes the face angle harder to control.
How to use it
Do not chase a single ideal number. Look at your average and, more importantly, how much it varies with each club. Swing plane is not a good guide for fitting lie angle because it describes motion over time, so use dynamic lie for that.
Bay drill: lay an alignment stick on the mat to fix ball position and your distance from the ball, then hit ten 7 irons with the same routine and note the plane on each. If the spread is wide or the number climbs on your poor strikes, rehearse keeping your backside back toward your heels through impact and see whether the plane settles and your strikes improve.