Regulatory basis for reserve fuel may be found in Title 14 of the Code of Federal Regulations parts 91, 121, and 135 §§ 91.167, 121.639, and 135.223.

(91)Civilian- Day VFR (enough to fly to where you are going ...then for 30 min. at normal cruise speed after that)

Night VFR- (enough from A to B plus 45 min)

IFR- (enough to get you there and shoot your approach and then go 45 min to your alternate)