What reserve fuel is legally required?

Under common day VFR rules you must land with at least 30 minutes of fuel at normal cruise burn, and 45 minutes for night VFR or IFR. The tool subtracts the reserve you select from usable fuel before computing the endurance and range you can actually plan to.

Should I plan to tanks-dry endurance?

Never. Tanks-dry endurance is shown only as a reference. Always plan your flight to the endurance and range remaining after the reserve, and add allowances for taxi, climb, and any unusable fuel your aircraft cannot draw.

What units should I use?

Use whatever unit your aircraft documents, gallons or litres, as long as usable fuel and burn rate share the same unit. The range output is in nautical miles because the speed input is in knots.

Should I use true airspeed or groundspeed?

Use true airspeed for a still-air estimate, or groundspeed if you have applied a wind correction. A headwind lowers groundspeed and therefore range, so for the binding leg of a flight use the groundspeed you expect on that heading.

Why did the tool warn that reserve exceeds fuel?

If your usable fuel is less than the required reserve, there is no legal fuel to fly any distance and the warning appears. You must add fuel, choose a shorter reserve category if eligible, or not depart with that load.

Aircraft Fuel Burn & Endurance Calculator

Running out of fuel in flight is almost always avoidable with honest planning. This calculator turns your usable fuel, cruise burn rate, and speed into endurance, range, and the time and distance you can plan to once the legal reserve is set aside.

How it works

Endurance and range come straight from the burn rate, then the reserve is removed:

endurance     = usable fuel / burn rate
range         = endurance × speed
reserve fuel  = burn rate × (reserve minutes / 60)
usable to plan = usable fuel − reserve fuel
endurance to reserve = usable to plan / burn rate
range to reserve     = endurance to reserve × speed

The endurance and range you should actually plan to are the figures after the reserve, not the tanks-dry numbers.

Example and notes

With 40 gallons usable, an 8.5 gallon-per-hour burn, and 110 knots true airspeed, total endurance is about 4 hours 42 minutes, but the 45 minute IFR reserve removes roughly 6.4 gallons, leaving about 3 hours 56 minutes and 432 nautical miles to plan to. Always add taxi and climb allowances, and use groundspeed rather than true airspeed on a leg with a strong headwind so the range figure reflects what you will really achieve.