How do you calculate fuel endurance?

Endurance = usable fuel ÷ fuel flow. With 38 gallons usable and a burn rate of 9.5 gallons per hour, endurance is 38 ÷ 9.5 = 4 hours to dry tanks. Subtract the required reserve (30 minutes day VFR, 45 minutes night VFR or IFR) to get your real planning endurance of about 3 hours 15 to 30 minutes.

What are the FAA fuel reserve requirements?

14 CFR 91.151 requires enough fuel to fly to the first point of intended landing plus 30 minutes at cruise power by day, or 45 minutes at night, for VFR. IFR under 91.167 requires fuel to the destination, then to the alternate if one is required, plus 45 minutes at normal cruise. Many pilots plan a personal reserve of a full hour.

What fuel flow number should I use for planning?

Use the POH cruise-performance figure for your planned power setting and altitude, and verify it against what your airplane actually burns (from fill-ups or a totalizer). Real-world burn commonly runs higher than the book because of climbs, richer leaning, and pattern work. If your measured number and the book disagree, plan with the higher one.

Fuel Burn & Endurance Calculator for Pilots

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Endurance = usable fuel ÷ fuel flow. Leg burn = fuel flow × time. 38 gallons at 9.5 GPH is 4 hours to dry tanks; after a 45-minute reserve it is a 3:15 airplane, and a 2:30 leg lands with 45 minutes above the reserve. Fuel exhaustion still makes the NTSB reports every month, and almost every case had this arithmetic available before takeoff.

Fuel flow (GPH)

POH cruise figure for your power setting, or your measured burn if higher

Usable fuel on board (US gal)

Usable, not total: check the POH fuel-system section

Leg time (minutes, optional)

ETE from the time-speed-distance calculator, plus taxi and climb allowance

Required reserve

At normal cruise consumption

Enter your numbers to see the margin.

Endurance (dry tanks) – never plan to this number

Planning endurance – after reserve

Leg burn – US gal

Margin at landing – flight time beyond the reserve

The three fuel questions, one relationship

Volume = rate × time, rearranged three ways, exactly as on an E6-B or CX-3:

Burn: how much does a 1:15 leg take at 9.5 GPH? 9.5 × 1.25 ≈ 11.9 gallons.
Endurance: how long will 38 gallons last at 9.5 GPH? 38 ÷ 9.5 = 4 hours.
Rate: what am I really burning if the last 2:30 took 25 gallons? 25 ÷ 2.5 = 10 GPH. Measured rate beats book rate every time they disagree.

How pilots use it: plan in time, not gallons

Experienced fuel planning runs on the clock, not the gauges. The habit pattern:

Before takeoff: compute endurance from known fuel and honest burn rate. Write down the time the tanks run dry ("wheels-up 14:10, dry tanks 18:10"). That number does not drift the way gauge needles do.
Set a personal landing floor: the FAA minimum reserve (30 min day VFR, 45 night/IFR) is a legal floor, not a plan. A common personal rule is to be on the ground with at least an hour in the tanks.
In flight: at each checkpoint compare planned burn with the totalizer or clock. A stronger-than-forecast headwind shows up here as a later ETA against a fixed dry-tanks time, and that comparison is what triggers a fuel stop while the options are still open.
Groundspeed changes everything: endurance is fixed by fuel flow, but range is endurance × groundspeed. Rerun your reachable-airports thinking whenever measured groundspeed drops.

Worked example

A 172 with 38 gallons usable, burning 9.5 GPH, on a 2:30 leg with a 45-minute reserve:

Endurance to dry tanks: 38 ÷ 9.5 = 4:00
Planning endurance after reserve: 4:00 − 0:45 = 3:15
Leg burn: 9.5 × 2.5 = 23.8 gallons, landing with 14.2 gallons
Margin at landing: 3:15 − 2:30 = 45 minutes beyond the reserve

Legal, and reasonable by day. Add an unforecast 15-knot headwind and a 20-minute reroute, and the margin is gone; that is why the calculator flags anything under an hour.

Fuel weight for W&B

Standard aviation weights: avgas 6.0 lb/gal, jet fuel 6.84 lb/gal, oil 7.5 lb/gal. This calculator shows the weight of your fuel load so you can carry it straight into a weight-and-balance problem.

Common Questions

Is the legal reserve enough?

It is a floor, not a plan. Thirty minutes at cruise power is one missed approach and a short hold, or one closed runway with the next airport 40 miles away. NTSB fuel-exhaustion reports are full of flights that were legal at takeoff. An hour on the ground is a common personal minimum, and this calculator lets you select it.

Why usable fuel instead of total capacity?

Some fuel in every tank cannot reach the engine in flight attitudes; the POH lists it as unusable. A "40-gallon" airplane may have 38 usable. Plan on usable only, and remember that visually checking tanks tells you more than gauges, which are only required to read accurately when empty.

Should I add anything to the leg time?

Yes: start, taxi, run-up, and climb burn more than cruise numbers suggest. A common allowance is an extra 1 to 1.5 gallons for start-taxi-takeoff in a trainer, plus climb fuel from the POH time-fuel-distance-to-climb table. Enter your leg time with those minutes included, or just treat the first gallon as gone before wheels-up.

Related tools and guides

Time-Speed-Distance Calculator

Where the leg time comes from: distance and groundspeed.

Heading & Groundspeed Calculator

The wind triangle that decides how long you are airborne.

The Go/No-Go Decision Framework

Fuel margin is one of the eight questions; here is the whole structure.

Fuel is one of eight risk categories

FlightDecide checks your fuel plan alongside winds, ceilings, visibility, NOTAMs, performance, and W&B for your specific flight window and aircraft, and flags a thin margin before you commit.

Get FlightDecide on the App Store

Educational tool for flight-planning practice. It is advisory only and not a substitute for your POH, an official weather briefing, or your own judgment as pilot in command (14 CFR 91.3). Sources: 14 CFR §§91.151, 91.167; FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Ch. 7. Last reviewed: July 17, 2026.