The private pilot reported the airplane had fuel exhaustion during cruise flight that resulted in a total loss of engine power and a forced landing in a field. The airplane touched down on the main landing gear and the pilot was able to keep the nose landing gear off the ground until the airplane traversed through a low area in the field, which caused the airplane to pitch down and the nose landing gear to collapse when it impacted the ground. The left wing rear spar was substantially damaged during the forced landing. The pilot did not report any mechanical malfunctions with the airplane that would have precluded normal operation. The pilot reported that he visually inspected the airplane’s two 13-gallon fuel tanks before the flight and had estimated that each fuel tank was about ¾ full (about 19.5 gallons total). He did not utilize a calibrated dipstick to measure the actual fuel remaining in either fuel tank. According to flight track data, the loss of engine power occurred about 1 hour 25 minutes into the flight. The pilot stated that his inaccurate estimation of the fuel level before the flight had resulted in fuel exhaustion during cruise flight and a total loss of engine power.