The short answer: mostly air. Under the rules, a driver head can be as big as 460 cubic centimeters, yet a typical one weighs only about 200 grams. Made from solid titanium alloy, the same shape would weigh about 2 kilograms. So the head is a thin shell. That puts the weight far from the center, where it resists twisting, and leaves the face free to flex, up to a limit the rules set and test with a pendulum.
This article goes with episode 4 of HYTON Golf, our video series that cuts golf open in 3D. Every fact below is linked to its source at the end.
Key numbers
| Number | What it means | Source |
|---|---|---|
| 460 cm³ | Largest driver head volume allowed, plus a 10 cm³ tolerance | USGA Equipment Rules, Part 2 |
| About 200 g | Mass of a standard driver head | Tucker, Kenny, Byrne and Anderson (2009) |
| About 2 kg | What a 460 cm³ head would weigh in solid titanium alloy (our calculation from 4.43 g/cm³) | UPMET Ti-6Al-4V data sheet |
| 5,900 g·cm² | Moment of inertia limit, plus a 100 g·cm² test tolerance | USGA Equipment Rules, Part 2 |
| 239 µs | Characteristic time limit in the central area of a driver face, plus an 18 µs tolerance | USGA and R&A notice (2016) |
| 1.49 | PGA TOUR and LPGA Tour average driver smash factor | TrackMan |
More numbers from the series, each with its source: Golf by the Numbers.
Watch the episode: What's Inside a Driver Head? on YouTube (6:43).
The rule box
- Volume: no more than 460 cubic centimeters (28.06 cubic inches), plus a 10 cubic centimeter tolerance for measurement.
- Heel to toe: no more than 5 inches (127 mm), and longer than face to back.
- Sole to crown: no more than 2.8 inches (71.12 mm).
- How they're measured: the dimensions are taken with the head set at a 60 degree lie angle.
Volume is the tricky one, because a driver head is a lumpy, hollow shape. The USGA uses weighed water displacement: the head is lowered into water until it's just below the surface, with the hosel kept dry, and the change on a scale gives the volume. It's Archimedes' principle.
A short history
Drivers used to be carved from solid wood, often persimmon. By 1997, most top players had switched to metal, and one of the big titanium heads that year measured 290 cubic centimeters. Heads kept growing until the USGA set the 460 cc limit, which took effect on January 1, 2004. That's nearly 60 percent bigger than that 1997 head.
Why hollow: the weight budget
A driver head has a budget of about 200 grams, spread over a shape that big. Solid titanium alloy has a density of about 4.43 grams per cubic centimeter, so a solid 460 cc head would weigh about 2,040 grams (our calculation), roughly ten times too heavy. Making it hollow turns the head into a thin shell, and a shell puts the weight far from the center.
That matters on mishits. When the ball strikes out toward the toe or the heel, the head starts to twist. Mass far from the center resists that twist; engineers call this moment of inertia, and it grows the farther out the mass sits. Think of a spinning figure skater: arms in, she spins fast; arms out, she turns slowly. A high moment of inertia helps keep the face closer to square on an off-center hit. Since 2006, the rules cap it too: no more than 5,900 gram square centimeters, plus a 100 g cm² test tolerance.
The face: a limited spring
On a hollow head, the face is a plate with nothing solid behind it, so at impact it can flex inward and spring back. The rules call this a spring-like effect and limit it. The measure is the coefficient of restitution (COR): how much of the speed of the collision comes back out. A perfectly bouncy collision would score 1.0; the limit for a driver is 0.830.
A simple physics model shows why that number matters. Ball speed divided by clubhead speed equals (1 + COR) divided by (1 + ball mass ÷ head mass). With a 195 gram head, a 45.9 gram ball and a COR of 0.83, that's about 1.48 (our calculation, using Arnold's model). With a perfect 1.0 it would be about 1.62. TrackMan puts the tour average for this ratio, called smash factor, at 1.49 with a driver, so the simple model lands close.
The pendulum test
The original test fired golf balls at the face from an air cannon; mapping a single head that way could take about 45 minutes. Since 2004, the main check has been simpler. A small pendulum swings a steel weight into the face, and sensors time how long the two stay in contact. A stiff face kicks the weight away quickly; a springier face flexes, so the contact lasts a little longer. That time is called characteristic time (CT). In 2008, testers began measuring CT at more points on driver faces. Under a 2016 notice from the USGA and The R&A, a driver with a CT above 239 microseconds (plus an 18 microsecond tolerance) in the central impact area, a strip 1.68 inches wide down the middle of the face, is ruled non-conforming; elsewhere on the face the limit is 257 microseconds, plus the same tolerance. The PGA Tour has tested players' drivers at events since the 2019-20 season.
Inside the lines
Put it together and a driver lives inside a set of numbers: 460 cubic centimeters; 5 inches heel to toe and 2.8 inches tall; a COR of 0.830; a moment of inertia of 5,900 g cm²; and a club no longer than 48 inches (since 2022, elite competitions can choose a local rule that caps it at 46). The ball has a box of its own, as we saw in episode 1: at least 1.68 inches across and no more than 1.62 ounces. Inside those lines, designers decide where every gram goes.
Illustrations by HYTON. Proportions are approximate; wall thickness and face flex are exaggerated. Not affiliated with or endorsed by the USGA or TrackMan.
Sources
- USGA Equipment Rules, Part 2 (Clubs), Rule 4 (Clubhead): volume 460 cm3 (28.06 in3) plus 10 cm3 tolerance; heel to toe no more than 5 in (127 mm); sole to crown no more than 2.8 in (71.12 mm); heel to toe greater than face to back; moment of inertia 5,900 g cm2 plus 100 g cm2 test tolerance; spring-like effect limited by the Pendulum Test Protocol
- USGA TPX3003, Procedure for Measuring the Club Head Size of Wood Clubs: weighed water displacement (Archimedes' principle), hosel not submerged; dimensions measured at a 60 degree lie
- USGA notice, October 29, 2003: Head Size and Club Length Limits: 460 cc plus 10 cc tolerance; 48 inch club length; effective January 1, 2004
- USGA and R&A, Final Rule on Spring-Like Effect (August 6, 2002): COR limit of 0.830
- USGA and R&A notice to manufacturers, January 11, 2016: Spring Effect and Dynamic Properties: for drivers, a characteristic time above 239 microseconds (plus an 18 microsecond tolerance) within the central impact area, 1.68 inches wide, or above 257 microseconds (plus the same tolerance) outside it, is non-conforming
- John Spitzer, A Higher Standard, USGA (January 2, 2025): most pros on metal drivers by 1997; a 290 cc titanium head in 1997; 460 cc is nearly 60 percent larger; MOI limit in 2006
- USGA, History of Equipment Rules: pendulum test for spring-like effect adopted in 2004; CT measured at additional locations on driver heads from 2008; MOI standard in 2006
- Andrew Tursky, GOLF.com (2020), Gear 101: What is a CT test, and how does it differ from a COR test?: air cannon COR test took about 45 minutes to map a head; the USGA moved to the pendulum CT test in 2004; PGA Tour driver testing from the 2019-20 season
- D. N. Arnold (2010), The Science of a Drive: impact model; clubhead 0.195 kg; ball 0.0459 kg
- Tucker, Kenny, Byrne and Anderson (2009), The Effect of Varying Club Head Mass on Velocity and Kinetic Energy, ISBS Conference Proceedings: a standard driver head mass of approximately 0.2 kg
- TrackMan, Smash Factor: PGA TOUR and LPGA Tour driver averages of 1.49
- UPMET, Ti-6Al-4V data sheet: density 0.16 lb/in3 (about 4.43 g/cm3)
- R&A Rules of Equipment, Part 4 (Conformance of Balls), Rules 4.2 and 4.3: weight not greater than 1.620 oz (45.93 g); diameter not less than 1.680 in (42.67 mm)
- Sports Illustrated (2021), USGA and R&A local rule on driver length: Model Local Rule G-10 from January 1, 2022: optional 46 inch limit for elite competitions
Sources last checked October 2026.
Frequently asked questions
Why are driver heads hollow?
A driver head can be as large as 460 cubic centimeters but typically weighs only about 200 grams. A thin hollow shell lets that small mass be spread far from the center, which raises the moment of inertia and helps the head resist twisting on off-center hits. It also leaves the face free to flex at impact, up to the limit in the rules.
What is the maximum size of a driver head?
Under the USGA Equipment Rules, a wood's clubhead volume must not exceed 460 cubic centimeters (28.06 cubic inches), plus a 10 cubic centimeter measurement tolerance. Heel to toe it can be no more than 5 inches, sole to crown no more than 2.8 inches, and heel to toe must be longer than face to back.
How is a driver head's volume measured?
The USGA uses the weighed water displacement method, based on Archimedes' principle. The head is lowered into water until its top is just below the surface, with the hosel kept dry, and the change on a scale gives the volume.
What does COR mean for a driver?
COR is the coefficient of restitution, a measure of how much of the speed of a collision comes back out. The limit for drivers is 0.830. In a simple impact model with a 195 gram head and a 45.9 gram ball, a COR of 0.83 gives a ball speed about 1.48 times the clubhead speed (our calculation).
What is the CT test for drivers?
The characteristic time (CT) test swings a small pendulum into the face and times how long it stays in contact. A springier face keeps contact longer. The pendulum test has been used since 2004. Under a 2016 notice from the USGA and The R&A, a driver with a CT above 239 microseconds (plus an 18 microsecond tolerance) in the central impact area is ruled non-conforming; elsewhere on the face the limit is 257 microseconds.
When did the 460 cc driver limit start?
The USGA announced the final 460 cc head size limit, along with a 48 inch club length limit, in October 2003. The limits took effect on January 1, 2004.
More from HYTON Golf
Every number from the series, with its source, is on Golf by the Numbers. The designs from the videos are in the Wear the Science collection, and every episode is on the HYTON Golf Videos blog.