Field goals are measured as the distance between where the ball will be kicked from the uprights of the goal posts. This distance is 17 yards plus the distance of the line of scrimmage to the goal line. The additional 17 yards is added to account for the 10 yards of the end zone in which the goal posts are at the back of and the seven yards behind the line of scrimmage from which the ball will be kicked from. For example, if the line of scrimmage was at the 40 yard line, the field goal attempt would be a 57-yard attempt because 40, the distance of the line of scrimmage to the goal line, plus 17, the distance of the end zone and the ball behind the line of scrimmage, equals 57.
Some field goal kickers from the special teams unit struggle to make kicks that are further than 30 yards. However, other kickers can consistently kick 50 yard field goals without a problem. Kicks from the 35 yard line seems to be the NFL's average for field goal kicks.
The longest field goal ever kicked was a 64 yard kick by Matt Prater in 2013.