Drivers: head size, springiness, forgiveness and testing.

Head size and shape

Part 2 of the Equipment Rules sets the limits for every wood, including the driver:

The head must also be generally plain in shape, rigid and functional, may not resemble another object, and may have only one striking face. Holes through the face, features in front of the face, and electronic or optical devices (beyond limited identification devices) are not allowed.

In a June 2022 notice the R&A said it was researching a lower driver moment of inertia (MOI) limit, looking at values as low as 2,000 g cm squared, to reward hitting the centre of the face. It said changes to head volume, dimensions or geometry were not under consideration. In December 2023 the R&A and USGA said they would keep researching driver forgiveness and might explore regulating design features that reward centre strikes. The current Part 2 limit is still 5900 g cm squared.

The spring-like effect (CT)

The Equipment Rules limit the spring-like effect of a clubhead and measure it with a pendulum test protocol held on file by the R&A and USGA. Golf.com explains that the test measures how long a steel weight stays in contact with the face, a figure called characteristic time, or CT. It gives the limit as 239 microseconds with a test tolerance of 18 microseconds, so a head measuring above 257 microseconds is ruled non-conforming.

• The limit does not apply to putters, and clubs may be tested away from the centre of the face.
• A manufacturer's claim that a club has a spring-like effect can itself make the club non-conforming, according to Part 2.
• Part 1 says a used driver that tests above 257 microseconds in a field pendulum test is deemed non-conforming and may not be used in later stipulated rounds.

Driver creep

ESPN explains that a driver face can "creep" with use, thinning and gaining spring until a club that conformed when new no longer does. The Equipment Rules say wear from normal use does not make a conforming club non-conforming, yet Part 1 also says a used driver that tests above the limit is deemed non-conforming. In December 2023 the R&A and USGA said they would change driver testing to identify earlier the models that are within tolerance when new but at higher risk of going over the limit through normal use.

Testing at elite events

• 2018. Golf Digest reported that the R&A began random driver testing at the 2018 Open at Carnoustie.
• 2025. At the 2025 PGA Championship at Quail Hollow, ESPN reported that the USGA tested about 50 players' drivers at the PGA of America's request. Rory McIlroy's driver was reported non-conforming, and the eventual winner, Scottie Scheffler, said the driver he had planned to use also failed after more than a year of use. ESPN notes that results are normally confidential.

The testing described here took place at elite events. Its relevance is that a driver which conformed when it was bought is not guaranteed to conform after heavy use.

Adjustable drivers

Some drivers have adjustable settings, such as the loft on PING's G440 MAX. Part 2 allows any club to be adjustable for weight. Other adjustments must be evaluated by the R&A or USGA, must need a special tool (a coin or other pocket item does not count), and the club must be essentially unusable unless the mechanism is locked. Every possible setting must conform.

During a round, Rule 4.1a(3) of the 2023 Rules of Golf, in effect from 1 January 2023, forbids making a stroke with a club whose performance you deliberately changed, including by moving an adjustable setting. There is no penalty if you put the setting back before the stroke. Lead tape is a long-standing exception for adding weight, but you may not add or remove it during a round.

Length

A driver may be up to 48 inches long under the Equipment Rules. Model Local Rule G-10 lets a Committee lower that to 46 inches, with a measurement tolerance of 0.20 inches, but only when it adopts the rule, and it is recommended only for professional and elite amateur events. Manufacturers' standard lengths vary: PING, for example, lists its G440 MAX driver at 46 inches with its stock shaft and 45.5 inches with some tour shafts. PING also claims that the lighter build and longer shaft give more clubhead speed and carry; that is the maker's claim.

The conforming driver list

The R&A and USGA keep a List of Conforming Driver Heads, which Part 2 says is updated weekly. Model Local Rule G-1 lets a Committee require every driver used to have a head on that list, with disqualification for a stroke with an unlisted driver and no penalty for carrying one; heads made before 1999 are exempt. G-1 applies only when a Committee adopts it, and it is recommended only for elite and professional events. If you play in such an event, check the list for your exact head model and loft.

A proposal under consultation

Golf.com reported on 23 September 2026 that the R&A and USGA had proposed three new Model Local Rules for the highest professional levels. One would require a driver head model and loft on the conforming list with a maximum CT of 239 microseconds when tested before competition, removing the 18-microsecond tolerance. The earliest effective date reported was January 2028, comments were due by 21 October 2026, and we could not read the primary notice. It is a proposal, not an adopted rule. See the ball testing change for the full set.

Nothing on this page says a particular driver conforms. Check the conforming driver list and the Terms of the Competition before you rely on one in an event.