See the break you can't read.

It's built for golfers who stand over a putt and honestly can't tell how much it will break. If you keep missing on the low side, the problem usually isn't your stroke: it's that slope is very hard to see. In a study of nearly 1,300 putts by Eric Alpenfels and Bob Christina, 96% of golfers under-read their putts by more than 60% (GOLF.com's summary). A green reading app measures what your eyes miss.

Why guessing the slope costs you putts

Most hole locations sit on slopes of 1 to 3%. You can barely see 1%, and 2% feels gentle underfoot. But the difference is big once the ball is rolling. Here's one 10-foot putt at Stimp 10, from Plumb Bob's putt simulator:

Read a 2% slope as 1% and you start the ball 5½ inches too low. That's more than a hole-width. It misses on the low side even with a perfect stroke. Downhill makes it worse: the same putt running 2% downhill needs 15.2 inches, and 2% uphill only 8.6. For the full story, see why you miss putts on the low side.

It takes about twenty seconds. Behind the scenes, Plumb Bob maps the slope along and around your line. Then it rolls simulated putts over that surface at your green speed until it finds the one that drops, paced to finish about 17 inches past the hole if it missed. You get:

• Aim: how far left or right of the cup to start it, in inches, with an aim spot on the grass.
• Break: in cups, the way you'd say it to your playing partner.
• Plays like: uphill and downhill turned into the distance to hit it. An 11-footer that plays 12.
• Slope map: a top-down view of the fall line, with a confidence rating so you know when to trust the read.

Once you've seen a few reads, compare them with what you'd have guessed. That's where the app pays off: you learn what a 2% slope looks like and how much it really breaks, so you read better without the phone too. Our four-step routine for beginners shows how to practise it.

The three kinds of green reading app

“Green reading app” covers tools that work in very different ways. What matters is where the slope number comes from:

Any of them beats a guess on the practice green. If you want to learn to read greens rather than just be told the answer, pick one that shows you the line on the green you're standing on.

Green speed changes the read

The same slope breaks more on a fast green, because the ball rolls slower for longer near the hole. The 10-foot putt across a 2% slope, from the simulator:

So Plumb Bob asks for the green speed. Pick a preset, or roll any ball 8 to 15 feet and it measures the Stimp from how fast the ball slows. Roll one back the other way to cancel out slope. Measuring green speed is free. No app? Here's how to measure green speed without a Stimpmeter.

What it can't do

• It isn't for competition. Rule 4.3 of the Rules of Golf bans devices that measure elevation changes or interpret slope during a round. Using one in a tournament, a league match or a round you post for handicap is a breach. Plumb Bob is for practice, lessons and friendly rounds. See are green reading apps legal?
• It measures slope, not grain. Grain is the direction the grass grows, and no camera can see it. On Bermuda greens it can add or take away break. A two-ball roll test shows it: see how to read grain on greens.
• It can't hit the putt. The line assumes a pace that would finish about 17 inches past. Hit it firmer and it breaks less; leave it short and it breaks more.
• It needs a clean scan. Walk at about half your normal pace and keep the camera on the grass. If the confidence rating is low, walk the line again.

Do you need LiDAR?

No. Plumb Bob works on any iPhone with iOS 17 or later. LiDAR, on iPhone Pro and Pro Max models from the iPhone 12 Pro on, measures the green in depth and gives the most precise reads. Other iPhones build the surface from camera feature points, which is rougher. On those, scan slowly in good light and check the confidence rating.

FAQ

What is a green reading app?

A phone app that works out how a putt will break. Some measure the slope where you set the phone down, some show pre-made slope maps of a course's greens, and some, like Plumb Bob, scan the green with the camera as you walk your line. The good ones turn the slope and green speed into an aim point.

Is there a green reading app for iPhone?

Yes. Plumb Bob runs on any iPhone with iOS 17 or later. iPhone Pro models with LiDAR give the most precise reads; other iPhones work too, with rougher reads, so scan slowly in good light.

Is Plumb Bob on Android?

No. Plumb Bob is iPhone only, because it relies on Apple's AR tracking and, on Pro models, the LiDAR scanner.

Is there a free green reading app?

Plumb Bob gives you 3 full reads free, with no account and no card, and measuring green speed and the demo green stay free after that. Unlimited reads, rounds and stats are Plumb Bob Pro at $9.99 a month or $59.99 a year, with a free week for new subscribers.

Can I use a green reading app in a tournament?

No. Rule 4.3 of the Rules of Golf bans using a device to measure elevation changes or interpret slope during a round, so a green reading app is a breach in competition and in rounds you post for handicap. Use it to practise, in lessons and in friendly rounds.

Does a green reading app read grain?

Plumb Bob doesn't. It measures the slope of the ground, and grain is the direction the grass grows, which a camera can't see. On Bermuda greens grain can add or take away break, so check it with a roll test and adjust.

Does it need signal on the course?

No. Reads are computed on your iPhone, so it works without signal. Only subscriptions and the nearby-course search go online.

How accurate is a green reading app?

It depends on how well it measures the slope and whether it knows the green speed. Plumb Bob gives every read a confidence rating: if it's low, walk the line again more slowly. Set the green speed too, because on a 10-foot putt across a 2% slope the aim moves from 8.7 inches at Stimp 8 to 13.3 inches at Stimp 12.

Learn to read greens yourself

• How to tell which way a putt breaks
• How to plumb bob a putt, and why it shows less break than you need
• How to know how hard to hit a putt
• Putt break calculator: length, slope and green speed in, aim out