What a fuel plan actually contains
A fuel plan is not one number. It is a stack of five, and the reason accidents keep happening is that pilots compute the middle one and treat it as the total. The stack is: fuel to start and taxi, the extra the climb costs, the cruise burn to the destination, enough to reach an alternate if you cannot land, and a reserve that must still be in the tanks when the wheels stop.
Only the cruise layer scales with distance. The other four are fixed by the aircraft, the weather and the rule you are flying under, and on a short leg they are a large fraction of the whole. On the 300 NM leg worked below, 30 gallons of cruise fuel becomes 38.5 gallons of required fuel once one gallon of taxi and 45 minutes of reserve are added — 28% more, with no alternate at all. Add a 30-minute alternate at the same 10 gal/hr and the required figure is 43.5 gallons, 45% above the cruise burn.
The second thing a fuel plan produces is a weight. Fuel is heavy: at 6 lb per US gallon, 50 gallons is 300 lb, which is two adults. It is also the only major weight that changes in flight, which is why the fuel figure feeds directly into the weight and balance calculator and why some aircraft are legal at take-off and out of centre of gravity limits on landing, or the reverse.
Each layer, and where its number comes from
Cruise fuel is fuel flow multiplied by time en route, and time en route is distance divided by ground speed. Substituting gives FF × D ÷ GS, which makes the wind's cost visible: a 20 kt headwind on a 140 kt aeroplane raises cruise fuel by 17%, because the leg takes 17% longer. Get the ground speed from the forecast wind with the wind correction angle calculator rather than assuming still air.
Taxi fuel is a fixed allowance, usually quoted in the flight manual as a figure for start, taxi and run-up. It does not scale with the flight, but at a busy airport with a long taxi and a hold it can be several times the book figure.
Climb fuel in this calculator is the extra the climb costs above what those same minutes would have burned in cruise, because the cruise term already accounts for the whole distance at cruise flow. Flight manual climb charts give fuel, time and distance to height; subtract the cruise burn for that time to get the increment. Entering the full climb fuel double-counts and makes you conservative by a gallon or two, which is a safe direction to be wrong in but is worth knowing about.
Alternate fuel is the cruise burn for the flying time from the destination to the alternate, at cruise power. Reserve fuel is a duration converted to volume at the same cruise flow, which is why a reserve is always quoted in minutes rather than gallons: 45 minutes means 45 minutes of your consumption.
Two derived numbers round out the plan. Endurance is the usable fuel on board, less taxi, divided by fuel flow — how long you can stay airborne, ignoring where you go. Specific range is distance divided by cruise fuel only, in nautical miles per gallon, and it is the efficiency figure to compare between power settings and altitudes. Be careful with its denominator: quoting specific range against total fuel including reserves makes an aircraft look worse on short legs than it is.
Worked example: 300 NM at 100 kt in a 10 gal/hr single
You are flying 300 NM. The forecast gives you 100 kt of ground speed, the cruise table says 10 gal/hr, the flight manual allows 1 gallon for start and taxi, you have no alternate, and you want the 45-minute reserve that night VFR and IFR both require. The tanks hold 50 gallons usable, and it is 100LL at 6.0 lb per gallon.
- Time en route. 300 NM ÷ 100 kt = 3.0 hours, or 180 minutes.
- Cruise fuel. 10 gal/hr × 3.0 hr = 30.0 gal.
- Trip fuel. 1.0 taxi + 0 climb + 30.0 cruise = 31.0 gal. This is what you expect to have burned when you shut down.
- Reserve fuel. 45 minutes is 0.75 hours, so 10 × 0.75 = 7.5 gal.
- Total required. 31.0 + 0 + 7.5 = 38.5 gal. That is the minimum that must be in the tanks before you start.
- Fuel on landing. 50.0 usable − 31.0 trip = 19.0 gal, comfortably more than the 7.5 gal reserve.
- Endurance. (50.0 − 1.0) ÷ 10 = 4.9 hours, against 3.0 hours of flying.
- Weight. 38.5 gal × 6.0 lb/gal = 231 lb of fuel to carry, and full tanks would be 300 lb.
- Specific range. 300 NM ÷ 30 gal = 10.0 NM per gallon.
Now change one thing: make it a 20 kt headwind, so ground speed drops to 80 kt. Time en route becomes 3.75 hours, cruise fuel 37.5 gal, total required 46.0 gal, and the fuel remaining on landing falls from 19.0 to 11.5 gal. A 20% speed loss has cut your landing fuel by 40%, because the reserve does not shrink with it. This non-linearity is why headwind legs deserve a recalculation rather than a mental adjustment.
Reading the plan, and what the regulations actually require
Look at the fuel remaining figure first, not the total. The total tells you whether you can start; the remaining figure tells you what margin you have when the plan meets reality. A comfortable plan lands with well over the reserve, so that a diversion, a hold or a 15 kt error in the forecast is absorbed without a decision. A plan that lands with exactly the reserve has no slack at all, because the reserve is the amount you must not touch.
In the United States, 14 CFR 91.151 requires enough fuel to fly to the first point of intended landing and then, at normal cruising speed, for 30 minutes by day or 45 minutes at night under VFR. Under IFR, 91.167 requires fuel to the destination, then to the most distant alternate where one is required, and then 45 minutes at normal cruising speed. Those are minimums for the flight to be legal, not targets. Most instructors, insurers and operators set a personal or company minimum above them — a one-hour landing reserve is a common private-flying standard, and it is the reason the 60-minute option exists in this calculator.
Specific range is the number to watch when you are choosing an altitude or a power setting. Higher and slower is usually more efficient in miles per gallon, but it is also slower, so the fuel saving competes against exposure time and against any headwind that strengthens with height. A quick test: recompute specific range at two candidate altitudes using the manual's fuel flows and the forecast winds for each. If the higher level gives more nautical miles per gallon and a comparable ground speed, take it.
Finally, the fuel plan and the range calculation are different questions. This page answers what does this leg need. The Breguet range equation calculator answers how far could this aircraft go, taking into account that it gets lighter as it burns fuel. For a piston aeroplane on a short leg the constant-fuel-flow assumption used here is close enough; for a long-range jet it is not.
Reserve fuel in gallons by fuel flow and reserve duration
| Cruise fuel flow | 30 min | 45 min | 60 min | Weight of the 45 min reserve (100LL) |
|---|---|---|---|---|
| 8 gal/hr | 4.0 | 6.0 | 8.0 | 36 lb |
| 10 gal/hr | 5.0 | 7.5 | 10.0 | 45 lb |
| 12 gal/hr | 6.0 | 9.0 | 12.0 | 54 lb |
| 15 gal/hr | 7.5 | 11.3 | 15.0 | 68 lb |
| 20 gal/hr | 10.0 | 15.0 | 20.0 | 90 lb |
| 30 gal/hr | 15.0 | 22.5 | 30.0 | 135 lb |
| 90 gal/hr | 45.0 | 67.5 | 90.0 | 405 lb |
Weights use 6.0 lb per US gallon for 100LL. For Jet A at 6.7 lb per US gallon, multiply the gallon figures by 6.7 instead.
Fuel densities and volume conversions
| Quantity | Avgas 100LL | Jet A / Jet A-1 |
|---|---|---|
| Pounds per US gallon | 6.0 | 6.7 |
| Kilograms per litre | 0.72 | 0.80 |
| Pounds per litre | 1.59 | 1.77 |
| Kilograms per US gallon | 2.72 | 3.04 |
One US gallon is 3.7854 litres and one imperial gallon is 1.20095 US gallons. Densities are the nominal figures used for planning; refuellers uplift by volume and correct to mass using the measured density on the delivery ticket.
Ways a fuel plan goes wrong
- Planning on true airspeed rather than ground speed. The single most common error, and it always errs in the unsafe direction on the outbound leg into a headwind.
- Using total capacity instead of usable. Unusable fuel is real and is listed in the flight manual. On some types it is several gallons per tank.
- Assuming the book fuel flow. Book figures come from a new, correctly rigged, properly leaned aeroplane. Fly your own aircraft against the fuel totaliser for a few trips and use the number you actually see.
- Counting the reserve as available. A reserve you plan to burn is not a reserve. If the plan only closes by dipping into it, the leg is too long for the fuel on board.
- Forgetting the taxi at the destination. A long taxi in after landing eats into the reserve you carefully preserved in the air, and it is not in most flight manual figures.
- Ignoring temperature when uplifting by volume. Fuel expands as it warms. Refuelling from a hot bowser and then cold-soaking at altitude changes the mass in the tanks relative to the volume on the ticket, which matters for a jet fuel load computed in pounds.
- Applying a cruise fuel flow to a whole flight that is mostly climb. On a short leg the climb can be half the airborne time, and climb flow is much higher. Use the climb chart rather than a single average.
The reserve is a floor, not a plan
Fuel exhaustion and fuel starvation remain among the most preventable causes of accidents in general aviation, and the pattern is consistent: the plan was legal on paper and had no margin for a diversion, a hold, or a forecast that was 15 knots optimistic. Treat the regulatory reserve as the point at which the flight has already failed, and build your planning minimum above it. If the calculator shows you landing with the reserve and little more, the answer is a fuel stop, not a tighter lean setting.
How this fits with the rest of the flight plan
Fuel planning sits downstream of navigation and upstream of loading. You need the route and the forecast winds before you can produce a ground speed, which is why the flight time and ETA calculator comes first in a normal planning sequence. The fuel figure it produces then becomes a weight, which goes into the loading calculation and can force a decision — carry full tanks and leave a bag behind, or carry less fuel and add a stop.
Cruise altitude ties the two ends together. Climbing higher costs fuel and time on the way up but reduces fuel flow at cruise, and where the trade breaks even depends on your climb fuel, the fuel flows at each level and the wind at each level — so settle it by running this page twice with the two sets of figures rather than by rule of thumb. Descent planning matters less than pilots think for fuel, but it is not free: starting down early means minutes at low level at high fuel flow, which is why the top of descent calculator belongs in the same conversation.
For turbine operations the arithmetic here is the same but the inputs come from a computerised flight plan rather than a paper chart, and the regulatory framework is heavier — contingency fuel, taxi fuel, final reserve and extra fuel are separate named quantities under commercial rules. The structure of the stack does not change; only the names and the minimum values do.
Key terms
- Usable fuel
- The fuel the engine can actually draw under all approved flight attitudes, which is less than tank capacity. Only this figure belongs in a fuel plan.
- Trip fuel
- Taxi, climb and cruise fuel to the destination — what you expect to have burned on shutdown, excluding alternate and reserve.
- Endurance
- How long the aircraft can remain airborne on the fuel aboard at a given fuel flow, independent of distance covered.
- Specific range
- Distance flown per unit of fuel burned in the cruise, in nautical miles per gallon. Higher is more efficient, and it varies with altitude, weight and power setting.
- Final reserve
- Fuel that must remain in the tanks on landing, expressed as minutes of flight at normal cruising consumption rather than as a fixed volume.