(a) Except as provided in paragraph (b) of this section, no person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to--
(1) Complete the flight to the first airport of intended landing;
(2) Fly from that airport to the alternate airport; and
(3) Fly after that for 45 minutes at normal cruising speed or, for helicopters, fly after that for 30 minutes at normal cruising speed.
(b) Paragraph (a)(2) of this section does not apply if--
(1) Part 97 of this chapter prescribes a standard instrument approach procedure for the first airport of intended landing; and
(2) For at least 1 hour before and 1 hour after the estimated time of arrival at the airport, the weather reports or forecasts or any combination of them indicate--
(i) The ceiling will be at least 2,000 feet above the airport elevation;
and (ii) visibility will be at least 3 statute miles.
FAR 91.151 - Explanation
During the day, no person may begin a flight in an airplane under VFR conditions unless there is enough fuel to fly to the first point of intended landing, assuming normal cruising speed, to fly after that for at least 30 minutes