Are green reading apps legal? Rule 4.3 in plain English
No, not during a competition round. Here's what Rule 4.3 says about green reading apps, AimPoint, green books and levels, and where you can use them.
September 25, 2026 · 6 min read
Short answer: no, not during a round that counts. Rule 4.3a of the Rules of Golf says you can't use equipment to measure elevation changes or to interpret distance or direction for you. A phone app that reads the slope of a green and tells you where to aim does both. Using one during a competition round is a breach, and so is using one during any round you play under the Rules.
That's the whole answer for a tournament. The rest of this guide covers where the line actually sits, because plenty of green reading is still allowed: on the practice green, in a lesson, in a casual round with friends, and with your own eyes and feet during a competition.
What Rule 4.3 actually says
Rule 4.3 covers the equipment you use during a round. The part that matters for green reading is 4.3a, on distance and directional information. According to the R&A's text of Rule 4, you're allowed to:
- Get information on distance or direction, for example from a distance-measuring device or a compass.
You're not allowed to:
- Measure elevation changes.
- Interpret distance or directional information, for example with a device that recommends a line of play or a club based on where your ball is.
- Use an alignment device to help align the ball.
A green reading app hits the first two points head-on. Measuring the slope of a green is measuring elevation change. Turning that slope into "aim 9 inches left" is interpreting information into a recommended line of play.
What's the penalty?
The first breach of Rule 4.3 carries the general penalty: two strokes in stroke play, loss of the hole in match play. A second breach, after something happened in between (such as playing another hole), means disqualification. You don't get a warning, and it doesn't matter whether you followed the app's advice.
Does it matter if the app is only open, not used?
Having a phone in your bag is fine. Rule 4.3 is about how you use equipment. Checking a text or a weather app during a round is allowed. Opening a green reading app to read your putt is not.
Where green reading apps are allowed
Rule 4.3 applies "during a round". Outside a round the Rules of Golf don't govern what you do with your phone, so all of this is fine:
| Situation | Green reading app allowed? |
|---|---|
| Practice green before or after your round | Yes |
| A practice round, a lesson or a range session | Yes |
| A casual round with friends, not played under the Rules | Yes, if everyone agrees |
| A round you're submitting for a handicap | No: the round has to be played under the Rules |
| Club competition, league night, tournament | No: Rule 4.3 breach |
| Between holes during a competition round | No: the breach applies to the next hole |
One catch on competition days: in stroke play, Rule 5.2b stops you practising on the course before your round, and that includes testing the surface of a putting green by rolling a ball on it. Reading greens on the practice putting green is fine. Walking the course's greens that morning to scan and roll putts is not.
What about AimPoint, green books and your feet?
This is where people get confused, because some green reading methods are completely legal.
AimPoint and plumb bobbing: legal
AimPoint uses your feet to feel the slope and your fingers to pick an aim point. Plumb bobbing uses a hanging putter shaft. Both use your body and your normal equipment, and neither measures anything with a device, so both are allowed in any competition. Some green reading apps, including the best-known ones, are sold as a way to train a method you then use on your own during a round. That's legal, as long as the phone stays in the bag when it counts.
Green books: legal, with limits
Since 2019 the Rules have limited detailed green-reading materials. Any green map used during a round has to be at a limited scale: no more than 3/8 inch to 5 yards, printed in a book no bigger than 4¼ by 7 inches. Handwritten notes are allowed, but only information you or your caddie gathered yourselves. For elite events, a committee can go further with Model Local Rule G-11 and only allow a yardage book it has approved. The USGA's page on Model Local Rule G-11 has the details.
A photo or screenshot of a slope map on your phone counts as green-reading material too. If it's more detailed than the scale limit, you can't use it during a round.
A level, a coin or a ball on the green
A spirit level or a bubble app measures elevation change, so it's a Rule 4.3 breach during a round. Rolling a ball on the green to test the surface during a hole is a separate breach: Rule 13.1e. Standing on your line, crouching behind the ball and feeling the slope through your feet are all fine.
Rangefinders: legal unless the committee says otherwise
Distance-measuring devices have been allowed by default since 2019, unless a committee bans them with a local rule. Slope on a rangefinder is the exception: it measures elevation change, so it has to be switched off during a competition round.
Why the rule exists
The governing bodies' view is that reading a green is a skill, and the Rules should reward the player who does it well. A device that does the reading for you removes that skill. That's also why yardage books were cut down to size. Detailed green maps had turned into a way to skip the read.
For a practising golfer, the opposite is true. A device that shows you the real slope and the real line is one of the fastest ways to build the skill. You learn what 2% of side slope looks like, how much more a downhill putt breaks, and how much a fast green adds. Then you take that feel into competition, where it's yours to use.
How to use a green reading app the right way
This is how we suggest using Plumb Bob or any other green reading app:
- Read it yourself first. Walk the putt, crouch behind the ball and commit to a read: which way and how many cups.
- Check it with the app. See the line drawn on the green, the aim point and how much it breaks. Note where you were off.
- Putt your read and the app's read. Two balls, two lines. The one that drops teaches you something.
- Keep score of your misses. Most golfers under-read break, especially on downhill putts and fast greens. Once you know your pattern, you can correct it without a device.
- Put the phone away when it counts. In competition, use what you learned.
FAQ
Is a green reading app legal in a casual round?
The Rules only govern a round played under the Rules. In a casual round with friends, what you allow is up to you. If you're posting the score for a handicap, though, the round has to be played under the Rules, so leave the app in the bag.
Can I use a green reading app on the practice green before a tournament?
Yes. Rule 4.3 applies during a round, and the practice green is fine before and after. In stroke play, don't practise on the course's own greens before your round on the day of the competition (Rule 5.2b).
Is AimPoint legal?
Yes. AimPoint uses your feet and fingers, not a device, so it's allowed in every competition.
Are green books legal?
Yes, within limits. Green-reading materials used during a round have to meet the scale and size limits (3/8 inch to 5 yards, a book no bigger than 4¼ × 7 inches). Some elite events only allow a committee-approved book.
What is the penalty for using a green reading app in a tournament?
The general penalty for the first breach (two strokes in stroke play, loss of hole in match play) and disqualification for a second breach.
Can I use a slope rangefinder in competition?
Only with the slope function turned off, and only if the committee hasn't banned distance-measuring devices. Slope measures elevation change, which Rule 4.3 prohibits.