The standard minimum diameter of a conforming golf ball is 1.68 inches.
The R&A and the United States Golf Association specify that a legal golf ball must measure at least 1.680 inches in diameter. This is technically a minimum rather than an exact required size, but 1.68 inches is the standard figure used in trivia, equipment specifications and everyday golf discussion.
The same rules limit a golf ball’s weight to 1.620 ounces. Balls must also meet requirements for spherical symmetry, velocity, distance and overall performance. Manufacturers therefore design balls within a tightly controlled equipment framework rather than making them in any convenient size.
There is a historical wrinkle behind the measurement. Until 1990, R&A-governed competitions could use a smaller ball, commonly called the British ball, while the USGA-approved ball was the larger American ball. The smaller version offered a slight aerodynamic advantage, especially in strong winds, so the two standards were eventually unified. Modern golf balls also rely on dimples—usually numbering several hundred—to reduce drag and improve lift and flight stability.