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.

What is the Aircraft Fuel Burn & Endurance Calculator?

Compute flight endurance, usable range, and the 30 or 45 minute VFR/IFR reserve from fuel burn rate, usable fuel quantity, and true airspeed. Pilots use it during preflight fuel planning to confirm legal reserves remain. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Aircraft Fuel Burn & Endurance Calculator

Name: Aircraft Fuel Burn & Endurance Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

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.

Understanding the two reserve categories

The required minimum reserve is not the same for every type of flight:

Day VFR (30 minutes): Under US 14 CFR 91.151, day VFR flights must land with enough fuel for at least 30 minutes of cruise flight at normal consumption.
Night VFR and IFR (45 minutes): Night VFR and IFR flights under 91.151 and 91.167 require 45 minutes of fuel at normal cruise burn (IFR calculated to the destination alternate, then adding the 45-minute reserve on top).

These are legal minimums, not operational goals. Many pilots add personal minimums of an additional 30 minutes or more, especially in areas with limited alternate airports or when flying into forecast IMC.

What “usable fuel” actually means

Usable fuel is the fuel your aircraft can actually burn — it excludes:

Unusable fuel — the amount trapped below fuel pickups that the engine cannot draw on, specified in the POH
Fuel for start and taxi — typically 1 to 3 gallons depending on the aircraft

The POH’s usable fuel figure accounts for the unusable portion, but taxi fuel is usually listed separately. If you enter the POH usable figure here, remember to subtract anticipated taxi and runup fuel mentally.

Worked example

A Cessna 172 with 53 gallons total, 50 gallons usable, a cruise burn of 8.5 gal/h, and a true airspeed of 110 knots:

Total endurance (tanks dry): 50 ÷ 8.5 = 5 h 53 min
45-minute IFR reserve fuel: 8.5 × 0.75 = 6.375 gal
Fuel available to plan: 50 − 6.375 = 43.625 gal
Endurance to reserve: 43.625 ÷ 8.5 = 5 h 8 min
Range to reserve: 5.13 h × 110 kts = 564 nm (still-air, no wind)

On a leg with a 20-knot headwind reducing groundspeed to 90 knots, that same endurance covers only 462 nm of actual ground — a significant difference over a long leg.

Tips for realistic fuel planning

Always plan with groundspeed on the specific leg, not true airspeed. Climb phase burns more fuel per mile than cruise, so add an allowance — a common rule of thumb is to budget for an extra 15 to 20 minutes of cruise consumption to cover the full climb phase. Re-run the calculation at cruise altitude with updated wind data from your weather briefing before committing to a fuel stop decision in flight.