Completing the fuel block on a flight plan means adding up burn for every leg and then layering on the regulatory reserves. This calculator does both: it works out time and fuel for each leg from distance, groundspeed, and burn rate, then sums the trip and adds taxi, reserve, alternate, and contingency allowances to give the total minimum fuel required on board.
How it works
For each leg the time and fuel are:
leg time (hr) = distance (nm) / groundspeed (kt)
leg fuel = leg time (hr) × burn rate (per hr)The trip fuel is the sum of all leg fuels. Reserve fuel is the reserve minutes expressed in hours multiplied by the cruise burn rate of the final (or a representative) leg. The total required fuel is:
total = taxi + trip + reserve + alternate + contingencywhere contingency can be entered as an absolute amount or computed as a percentage of trip fuel.
Example and tips
Three legs of 120 nm at 120 kt, 90 nm at 110 kt, and 60 nm at 100 kt, each burning 10 gph, give leg fuels of 10.0, 8.2, and 6.0 gallons — a trip total of 24.2 gallons. Add 1 gallon taxi, a 45-minute reserve (7.5 gallons), 5 gallons to the alternate, and 5 percent contingency, and the minimum fuel on board is about 38.9 gallons. Always cross-check the result against your aircraft’s usable fuel capacity — if the requirement exceeds capacity, the leg structure or the route needs to change.
FAR fuel reserve requirements at a glance
The minimum reserves depend on the type of flight:
| Flight rule | Condition | Reserve requirement (FAR Part 91) |
|---|---|---|
| VFR | Day | 30 minutes at normal cruise to destination |
| VFR | Night | 45 minutes at normal cruise to destination |
| IFR | To alternate required | Fuel to destination + alternate + 45 minutes at normal cruise |
| IFR | No alternate (1-2-3 rule) | Fuel to destination + 45 minutes at normal cruise |
Part 135 (charter/commuter) and Part 121 (airline) operations carry additional requirements. The reserves above are minimums — most pilots and operators carry more.
What groundspeed to use for each leg
Groundspeed is not airspeed. For realistic leg planning:
- Use your aircraft’s true airspeed for the altitude and density altitude expected on that leg
- Adjust for forecast winds: add a headwind component, subtract a tailwind component
- If uncertain about the winds, be conservative — plan for a headwind and treat any tailwind as a bonus
For flights into or out of mountainous terrain, plan reduced groundspeed for the climb legs where indicated airspeed stays constant but groundspeed is lower than cruise.
Burn rate accuracy
Using a single burn rate for an entire leg is a simplification. In reality:
- Climb burns significantly more fuel per hour than cruise — often 20–40% more
- Descent at reduced power burns less
- Economy cruise and best-power cruise settings have different burn rates even at the same altitude
For conservative planning, use the higher of your leaned cruise burn or your climb fuel flow. The difference is small on short legs and matters more on long cross-country flights where the climb phase is a smaller fraction of the total.
Using the result
Once you have the total minimum fuel, compare it to your aircraft’s usable fuel:
- If the requirement fits comfortably within usable capacity, your route works as planned
- If it is tight (within 5–10% of usable), consider adding a fuel stop or shortening legs
- If it exceeds usable fuel, the route requires an intermediate stop regardless of other considerations
Always record your planned fuel on board (FOB) on the flight plan, note the fuel required per leg in your kneeboard or EFB, and check actual consumption against the plan at each checkpoint.