Reading Your Stroke Arc From Two Strips of Tape
You do not need a camera rig to find out how curved your stroke is. You need a straight edge, two marks and a willingness to accept the answer.
Golfers are told constantly that their stroke is either straight-back-straight-through or on an arc. This framing is a nuisance for two reasons. First, every stroke is on an arc, because the club is attached to a body that rotates. Second, the useful question is not which category you fall into but how much curve there is, and that is a quantity, not a label.
The quantity we care about is how far the club head travels inside the target line at a fixed distance back from the ball. Give us that one measurement and we can compare two golfers meaningfully. Tell us that somebody has "a bit of an arc" and we have learnt nothing we can act on.
The Setup
Everything here can be done at home on a strip of practice surface or on a real green early in the morning when nobody is watching.
- Lay a straight edge on the ground running along your intended target line. A chalk line, an alignment stick or the edge of a yardstick all work. It must sit just outside where the toe of the putter passes so it does not interfere with the stroke.
- Mark the ball position on the straight edge. That is your zero.
- Put a second mark on the ground twelve inches back along the line from the ball. This is where you are going to take the reading.
- Make ten normal strokes to a target eight feet away, with a strip of tape stuck to the surface at the twelve-inch mark and a light dusting of foot spray on the tape so the passing club leaves a trace.
- Measure the perpendicular distance from the straight edge to the centre of the trace on the tape.
That perpendicular distance is your inside offset at twelve inches. For most golfers it lands between an eighth of an inch and five eighths. Anything above three quarters usually means the stroke is being driven by shoulder turn far more than by the arms, and anything under an eighth is a stroke being held unnaturally straight by conscious effort.
Turning It Into a Radius
If you want the arc expressed as a radius rather than an offset, the geometry is straightforward. For a chord length c along the target line and a sagitta — the inside offset — of s, the radius is approximately c squared divided by eight s, plus s over two.
| Inside Offset at 12 in | Approximate Arc Radius | Reads As |
|---|---|---|
| 0.13 in | ~139 in | Very shallow |
| 0.25 in | ~72 in | Shallow |
| 0.38 in | ~48 in | Moderate |
| 0.50 in | ~36 in | Pronounced |
| 0.75 in | ~24 in | Strong |
Do not chase the third decimal place. Hand measurement on a strip of tape is good to perhaps a sixteenth of an inch, which is more than enough resolution to place somebody in a bracket and nowhere near enough to justify quoting a radius to the nearest inch.
What the Number Is Good For
Chiefly, it sets expectations about face rotation. A club travelling on a tighter radius has to rotate more relative to the target line simply because it is turning more sharply, and a golfer with a pronounced arc who has been trying to hold the face square to the target has been fighting geometry the whole time.
It also informs alignment aids. Wide sight lines running front to back suit shallow strokes and look misleading to somebody with a strong arc, because the line never appears parallel to anything for more than an instant. Golfers frequently choose a head for its sight line and then find the face is doing something unexpected, and the arc is why.
And it changes how we read the sole tape from the lie test. A strongly arced stroke reaches its low point differently and puts wear slightly further toward the heel, so the two measurements have to be interpreted together rather than one at a time.
What It Is Not Good For
It is not a diagnosis. Arc size is not correlated with putting ability in any way we have been able to detect across our own session records, and we have looked. There are excellent putters at both ends of the range and the middle is not a virtuous place to be.
It is also not stable across the whole session. Nearly everyone's arc widens as putt length grows, which is exactly what you would expect from a longer stroke on the same body. Measure at one distance, record the distance alongside the number, and do not compare a twelve-foot reading to somebody else's four-foot reading.
Lastly, it is not something to change. We measure arc so that the hardware can be matched to it. If somebody wants to alter the stroke itself, that is a conversation for a coach, and it is a conversation that should happen before a putter gets built rather than after.