A putting stroke does not travel in a straight line and it is not supposed to. You are standing to the side of the ball swinging a club hung from your shoulders on an inclined plane, and the geometry of that arrangement produces an arc. The head moves inward on the way back, returns to the ball, and moves inward again on the way through. The face rotates open relative to the target line going back and closes coming through.
How much it does all that is what we mean by arc, and the amount varies far more between golfers than most people expect. It is not a skill measure. Excellent putters exist at every point on the range and so do poor ones.
What Sets Your Arc
Four things, none of which are easy to change and none of which should be changed to suit a putter you already own.
- Posture, specifically how far you bend from the hips
- Where your eyes sit relative to the ball, inside or over it
- How your arms hang and whether the elbows stay connected
- Shoulder plane, which follows from spine angle
Because those four are structural, arc class turns out to be remarkably stable. Golfers we first measured six or seven years ago re-measure within the same class today, with the exception of players who have had a genuine physical change or been through substantial coaching on their setup.
How the Gate Measures It
The laser gate sits at ball level and reads the head's path through a defined window either side of impact, while the overhead camera records face angle relative to the path at two hundred and forty frames per second. Between the two we get total face rotation across the stroke, expressed in degrees, plus the lateral path deviation at the extremes.
We take the reading from a set of putts at a real distance — typically twelve feet — rather than the short strokes golfers produce when they know they are being measured. A three-foot stroke understates arc substantially, which is one reason the shop-carpet assessment of "you look pretty straight" is so unreliable.
The measurement has to happen at a distance the golfer actually cares about. Nobody putts the way they putt when they know a camera is watching a two-footer.
The Four Classes
We work in four bands, defined by total face rotation across the measured stroke.
Straight is under two degrees of rotation. Genuinely rare — perhaps one golfer in nine measures here. It usually comes with a very upright posture and eyes directly over or slightly outside the ball. It asks for a face balanced head, and it is the only class where a large high-MOI mallet is straightforwardly the right answer.
Slight is two to five degrees. The most populated band. Suits a slight toe hang head, typically a mid-sized mallet with a short slant neck.
Moderate is five to nine degrees. Common among players who set up with the eyes clearly inside the ball. Wants a moderate toe hang, which in practice means a blade or compact mallet with a plumber's neck.
Strong is over nine degrees, with pronounced rotational release. Suits a strong toe hang blade, flow neck or heel-shafted design. Golfers in this class fitted with face balanced mallets tend to be the most dramatically improved by a correct fitting, because the mismatch they have been fighting is the largest.
What This Does Not Mean
Knowing your arc class does not tell you your stroke is good. It tells you what the club in your hands needs to be so that the stroke you have can repeat without compensation. A golfer with a strong arc and a correctly matched putter still has to read greens and control pace.
It also does not mean you should try to change class. We have watched golfers attempt to flatten their arc to justify a mallet they liked the look of, and it produces a worse outcome than either the original stroke or a correctly matched head would have. Fit the equipment to the person.
The Short Version
Arc is measurable, stable and almost universally misjudged when self-assessed. Five minutes on the gate produces a class, and the class narrows the entire putter market down to a shortlist you can then choose from on feel and looks with a clear conscience.