Remaining fuel is on board minus distance over groundspeed times flow
Estimated time en route is remaining distance divided by current groundspeed. Burn is that time times the current fuel flow, and fuel at the destination is fuel on board minus the burn. Endurance is on board divided by flow. Against that arrival fuel the page subtracts a reserve: 30 minutes for VFR day and 45 minutes for VFR night under 14 CFR 91.151, or, for IFR under 91.167, 45 minutes plus the time to an optional alternate at the same groundspeed. Those are the same regulatory times used on this site’s preflight fuel planner; this page is the in-flight remaining-distance check, not a second trip planner. An optional clock time is added to the ETE to print an ETA.
What you should not read into the number
Fuel flow and groundspeed change. Gauges, unusable fuel, a headwind on the last miles, a go-around, and a taxi after landing are not in the arithmetic. Part 91 reserves are planning minima, not a target to arrive with, and this check does not replace a decision to divert while you still have options. For a raw time-speed-distance conversion without fuel, use the TSD calculator; for a point of no return, see the PNR page.
Sources
- 14 CFR 91.151, fuel requirements for flight in VFR conditions.
- 14 CFR 91.167, fuel requirements for flight in IFR conditions.
- FAA Pilot’s Handbook of Aeronautical Knowledge, fuel planning and in-flight endurance.
Frequently asked questions
How is this different from the fuel-planning calculator?
The preflight planner builds a trip with taxi, climb, cruise, and reserves from a planned route. This page starts from the fuel now on board, the distance still ahead, and the groundspeed and flow you are seeing, and asks whether the destination reserve is still intact.
Do I have to use gallons?
No. Use any consistent volume unit for on-board fuel and fuel flow (gallons and gph, pounds and pph, litres and L/h). The surplus is in that same unit.
Is a 30-minute VFR reserve enough to keep flying?
91.151 is a preflight planning minimum, not a target to land with. If this page shows you arriving below that reserve, treat it as a prompt to divert or change the plan, not as a legal interpretation.