According to FAR 91.167(a), “no person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to”
- Complete a flight to the first airport of intended landing/touch-and-go;
- Fly from that airport to the chosen alternate airport; and
- Fly after that for 45 minutes at a normal cruising speed.
Item 2 above does not apply in some circumstances. Learn more by reading FAR 91.167(b).