Ball compression measures how much a ball deforms under a standardized load, expressed as a number that runs roughly 30 (softest) to 120 (firmest). The standard test presses the ball between plates under about 200 pounds of force and converts the deformation into that number. Low-compression balls (30–60) are the classic recommendation for swing speeds under about 85 mph, mid-compression (60–85) for 85–100 mph, and high-compression (85–110+) for 100 mph and up. In practice the effect on distance is smaller than old marketing suggested — nearly every player, fast or slow, compresses a modern ball enough to access its full speed — so compression today mainly drives feel at impact and, secondarily, spin, rather than being a hard distance rule.