Breakdown of the launch (hop and skid)
While the ball is airborne there is no contact force, so seen from above it travels perfectly straight. After landing it has almost no spin, so it slides: sliding friction slows the ball while spinning it up. Once the slip disappears the ball rolls purely. The slope pushes the ball sideways throughout, but while it is moving this fast the path barely curves.
Calculating
Green and ball position1–3
The stroke4–6
Where your aim line crosses the line through the cup perpendicular to the ball–cup line. + = right of the cup.
Equivalent angle 5.71°
Stroke dispersion (skill level)7–8
The chance of holing out is computed assuming this dispersion, and the pace and aim that maximise it are recommended.
Wind9–10
Wind is weaker close to the ground: the ball feels about 35% of the speed entered. A headwind counts for more than a tailwind, because the ball is moving into it.
Advanced green settings11–16
Calculating…
This is a tool that calculates how the ball rolls from the conditions you enter. It is not equipment for striking a ball and it does not measure anything. The rolling friction coefficient is calibrated numerically from the Stimpmeter value, and the trajectory is obtained by integrating gravity, friction and grain on the sloped surface. A putt is holed if, from the moment the ball centre crosses the rim, it can fall by one ball radius before reaching the far rim (critical speed about 1.63 m/s dead centre).
The slope is treated as a single uniform plane at the average gradient. The complex undulations of a real green are not modelled.
What this tool does
Enter the distance, slope and green speed, and it works out how much the putt breaks and which pace and aim hole it. No sign-up, no install — every calculation runs inside your browser.
Put a number on the break
Turn “a couple of cups out to the right” into a figure computed from the slope angle and the green speed. Flip the side slope and compare how a left-to-right putt breaks against a right-to-left one.
See how uphill and downhill change the pace
The same distance needs more pace uphill and less downhill. Move the front-to-back slope and watch how far the holing pace shifts — and why a downhill putt refuses to stop, as a balance of gravity against friction.
Compare green speeds
Change the Stimpmeter reading and both the break and the pace needed move together. You can follow exactly how a faster green makes the ball break more.
See the window that goes in
The probability of holing is computed for every combination of pace and aim, so you can see how far off you can be and still hole it. Change the dispersion (your skill level) and watch that window narrow.
Check the path in 3D
The ball’s curving path to the hole is drawn in 3D. From the side you can follow the hop and skid right after impact, and the transition into pure rolling.
Frequently asked questions
How do I find the slope angle of a green?
A slope app or your phone’s level will measure it. As a guide, 1% (about 0.6°) looks almost flat, 3% (about 1.7°) is clearly visible, and 5% (about 2.9°) is steep enough that a hole could not be cut there.
What should I enter if I do not know the green speed?
Everyday courses run about 8–10 feet; greens prepared for competition run 10–12. If in doubt, leave it at the default of 9.5 and move it either side to see what a faster or slower green would do.
How much does wind affect a putt?
On a flat 5 m putt (Stimp 9.5), a 5 m/s headwind leaves the ball about 50 cm shorter, a tailwind carries it about 20 cm farther, and a wind straight across moves it about 13 cm sideways. The headwind counts for more because the ball is moving into it, so the wind it meets is stronger. Wind weakens near the ground, so the calculation takes the speed at ball height as about 35% of the figure entered. That share is uncertain, and the size of the effect can vary by around 30% either way.
If I hit it exactly as calculated, will it go in?
No. The slope is treated as one uniform plane at the average gradient, so real undulations, uneven grain and the difference between morning and afternoon speed are not included. Use the numbers to check your judgement, not to replace it.
Does it work offline?
Once you have opened it, it keeps calculating without a connection. Everything runs in the browser, and nothing you enter is sent anywhere.
Can I show someone the conditions I used?
The “Share conditions” button at the top copies a URL that opens with your current inputs.