APPENDIX III  The Ball

1. Weight

The weight of the ball shall not be greater than 1.620 ounces avoirdupois (45.93 gm).

2. Size

The diameter of the ball shall not be less than 1.680 inches (42.67 mm). This specification will be satisfied if, under its own weight, a ball falls through a 1.680 inches diameter ring gauge in fewer than 25 out of 100 randomly selected positions, the test being carried out at a temperature of 23 + 1C.

3. Spherical Symmetry

The ball must not be designed, manufactured or intentionally modified to have properties which differ from those of a spherically symmetrical ball.

4. Initial Velocity

The initial velocity of the ball shall not exceed the limit specified (test on file) when measured on apparatus approved by the United States Golf Association.

5. Overall Distance Standard

The combined carry and roll of the ball, when tested on apparatus approved by the United States Golf Association, shall not exceed the distance specified under the conditions set forth in the Overall Distance Standard for golf balls on file with the United States Golf Association.

This information is from the USGA page http://www.usga.org/Rule-Books/Rules-of-Golf/Rules-and-Decisions/