Green Reading Lab
Set the slope, the green speed and where the ball sits. See the path, where to aim, and how pace changes the break.
What this answers
Where should you aim this putt, and how fast must it start?
- Set where the ball sits around the hole, then the putt’s length, slope, green speed and pace.
- Follow the putt from above: it starts toward the aim mark and curls down the slope into the hole.
- Read the aim, break and starting speed in the tiles. Around the hole shows putts from every side.
Reopen it any time with ? Guide
- AimAimWhere the start line passes the hole: inches left or right of its center. The line below measures it from the hole’s edge instead.
- 8.2in left
- 6.1 in outside the left edge
- ModelGrober 8.1 in
- BreakBreakThe farthest the ball’s path strays from the straight line between the ball and the hole.
- 2.7in
- From the straight line
- Model
- Starting speedStarting speedHow fast the ball must leave the putter, in feet per second, to reach the hole at the chosen pace. The line below gives its start direction from the line to the hole.
- 6.8ft/s
- Starts 3.9° left of the hole
- Model
- Level-green lengthLevel-green lengthThe level putt, at the same pace, that needs the same starting speed. Longer than this putt means it is uphill; shorter means downhill.
- 11.4ft
- Uphill: same stroke on a level green
- Model
The chart shows Grober’s 10 ft example putt. The controls need JavaScript.
Show the numbers as a table
| Putt length | 10.0 ft | Grober, Fig. 1 |
|---|---|---|
| Position | 8 o'clock | Grober, Fig. 1 |
| Slope | 2.0% | Grober, Fig. 1 (20 ft-%) |
| Green speed | 10.0 ft | USGA medium, regular play |
| Pace | Finishes 18 in past | Grober’s convention |
| Aim | 8.2 inches left of the hole's center | Model (Grober’s figure: about 8.1 in) |
| Aim, from the edge | 6.1 in outside the left edge | Model |
| Break | 2.7 in | Model |
| Start line | 3.9° left of the hole | Model |
| Starting speed | 6.8 ft/s | Model |
| Level-green length | 11.4 ft: the same stroke on a level green | Model |
| Speed at the hole’s edge | 2.6 ft/s | Model |
| Fastest a centered putt drops | 5.4 ft/s | Model, from Holmes (1991) via Penner (2002) |
| Time to the hole | 3.5 s | Model |
| Along the line | Off the line |
|---|---|
| 0.0 ft | 0.0 in |
| 1.4 ft | 1.1 in |
| 2.7 ft | 1.8 in |
| 4.0 ft | 2.4 in |
| 5.1 ft | 2.6 in |
| 6.2 ft | 2.7 in |
| 7.2 ft | 2.5 in |
| 8.0 ft | 2.1 in |
| 8.8 ft | 1.5 in |
| 9.4 ft | 0.9 in |
| 10.0 ft | 0.0 in |
Model: R. D. Grober, “The Geometry of Putting on a Planar Surface” (2011). Capture limit: Penner 2002.
For learning and practice. During a round, the Rules of Golf limit equipment and green-reading materials: Rule 4.3 and the 2019 limits on green-reading materials.
For your gear
Reading the line is half of it
Starting the ball on your line
A small error in the start line grows into inches at the hole. A putter that suits your stroke makes that line easier to repeat.
Putter FinderA head you can aim
Choose the head shape you find easiest to aim, then check that its balance suits the way you move it.
Blade or mallet? Choosing a putterEyes over the line
Putter length changes your posture and where your eyes sit, which changes how the line looks to you.
How to choose putter length
Method
How it is calculated
The ball rolls on a perfectly flat, tilted green. Five sevenths of gravity’s pull down the slope acts on a rolling ball, and the grass slows it by a constant amount set by the green speed. The equations and the 18-inch pace come from Robert Grober’s paper, “The Geometry of Putting on a Planar Surface”, which builds on Penner’s physics of putting.
Checked against the paper’s own figure of the target diamond, 30 putts from 5 to 15 ft, the model is within 0.24 in everywhere, 0.07 in on average. It ignores grain, undulation and the ball’s first skid off the face.
The full method, the checks and the sources are on the Golf Lab methods page.