Your Stroke Arc Has a Radius

The putter head is on the end of a shaft. The shaft is held by arms attached to a body that rotates, even slightly, during a stroke. Unless you are making a genuinely tiny stroke, the head has to travel on a curve, because that is what an object on the end of a rotating lever does.

Which makes "straight back, straight through" a description of a very short putt rather than a stroke style. On a three-footer the arc exists but its departure from the target line is small enough to be invisible. On a thirty-footer it is several inches, for everybody, including players who are certain they do not have one.

What the Overhead Trace Measures

We film a full stroke from directly above and track the head through it. Two numbers come out.

The first is arc width: how far inside the target line the head travels at the top of the backstroke, measured in inches, on a stroke sized for a ten-foot putt. This normalises for stroke length, which matters because a longer putt produces a wider arc for the same technique and comparing raw numbers across distances is meaningless.

The second is face rotation, which we discuss separately, and the relationship between the two is the interesting part.

Arc width and face rotation should move together. A wide arc with a face that barely rotates means the hands are actively holding the face square against the geometry of the stroke, and that holding is the thing that fails under pressure.

Typical Numbers

From our own lab records on ten-foot strokes, arc width falls into a fairly narrow band. Under about half an inch is a genuinely minimal arc and is rare. Half an inch to an inch and a quarter covers most players. Beyond about an inch and three-quarters is a pronounced arc, usually produced by a player who stands further from the ball with the eyes well inside the line.

Setup drives this more than technique does. Eye position relative to the ball is the strongest single predictor we see. Eyes directly over the ball tend to produce a narrow arc; eyes well inside produce a wide one. Which is a useful thing to know, because eye position is a function of putter length and posture, and both of those are adjustable.

Why Arc Width Is a Fitting Input, Not a Fault

There is a persistent idea that a narrow arc is more repeatable and therefore better. The data does not support treating it as a target. What correlates with consistent start lines is not arc width itself but the agreement between arc width, face rotation and the balance properties of the putter.

A player with a wide arc and matching face rotation, putting a full toe hang head, produces beautifully consistent start lines and never thinks about the face. The same player with a face-balanced mallet has to manage the face manually all the way through, and manual face management is exactly what deserts people on the eighteenth green.

So we take the arc as given and fit to it. The exception is when the arc is being produced by a setup problem — a putter that is two inches too long, forcing the player upright and the eyes inside the line. In that case we fix the length, remeasure, and fit to the corrected arc.

Consistency Matters More Than Shape

The number I actually care about most is not arc width but its variability. Twenty putts producing arc widths between nine-tenths of an inch and an inch and a tenth is a stroke that will respond well to fitting. Twenty putts producing anything between half an inch and two inches is a stroke that has not settled, and no putter will make it consistent.

When we see that second pattern, we say so. It is one of the situations where the honest answer is that a lesson will do more than a fitting, and that fitting afterwards will be worth more because there will be something stable to fit to.

What You Can Observe Without Equipment

• Lay a chalk line or an alignment stick on the ground parallel to your target line, a few inches outside the ball.
• Make normal ten-foot strokes and watch, in peripheral vision, whether the head stays parallel to the stick or moves away from it going back.
• Do not try to change what you see. The exercise is diagnostic, and trying to straighten an arc that your body geometry produces naturally is a reliable way to make putting harder.
• Repeat from thirty feet. If the arc looked absent at ten feet and obvious at thirty, that is normal and expected.

The Summary I Give in the Lab

Your arc is a consequence of your build, your posture and the length of the club you are holding. It is not a flaw and it is generally not worth rebuilding. What is worth doing is measuring it once, properly, and then buying a putter whose balance agrees with it — which is a decision you make approximately three times in a golfing life and which most people currently make by picking up whatever looks nice on the rack.