The pilot departed on the two-hour flight with the fuel quantity gauge of each fuel tank indicating about half full, but did not otherwise verify the quantity of fuel present in the tanks prior to the flight. Approximately 20 minutes from the destination airport, the left fuel tank gauge indicated between empty and one-quarter full, and the right tank gauge indicated one-quarter full. On the downwind leg of the airport traffic pattern, the engine began to "sputter" and experienced a total loss of power. The pilot moved the fuel selector to the right tank, but engine power was not restored. The pilot subsequently conducted a forced landing to the grass short of the runway, and the airplane nosed over and came to rest inverted, resulting in substantial damage to the engine firewall. Postaccident examination by a Federal Aviation Administration inspector revealed that the left fuel tank contained no fuel, and the right tank contained approximately one half of a pint of fuel. The tanks were not damaged during the accident sequence, and there was no evidence of fuel spillage at the scene. The pilot reported there were no mechanical malfunctions or anomalies of the airplane that would have precluded normal operation.