One question, five answers
Every fuel plan that works answers the same question: does this trip — the real route, out and back, on the actual day — fit inside the fuel aboard, with a reserve left untouched? Boaters answer it five ways: a gallons-per-day figure carried in the head, the one-third rule worked on paper, a spreadsheet built years ago, the fuel page on the chartplotter, or a dedicated planning app. All five are real methods used by real skippers, and each has an honest case for it.
The useful comparison isn't which one is best. It's which failure modes each one catches, because each step up the ladder mostly exists to catch the mistake the one below it lets through. So ask the same questions of each: does it know your actual route? Does it see the day's conditions? Does it treat the return leg as its own, usually harder, problem? Does it enforce a reserve? Does it learn from the fuel you actually burn? And does it work before you're on the boat, when the go/no-go decision actually gets made?
Method 1: “She does about 25 gallons a day”
The baseline method deserves more respect than it gets. A gallons-per-day figure costs nothing, takes no time, and was calibrated the honest way — years of watching the fuel dock's meter. For a boater who runs the same water the same way most weekends, it's usually about right. That's why it survives.
What it catches is gross mismatch: nobody who thinks of their boat as a 25-gallon-a-day boat plans a 300-gallon weekend. What it misses is everything specific to the trip in front of you, because an average is built from typical days, and the day that gets a boater in trouble is not typical. Run one: a typical day of 30 nm at a measured 1.9 nmpg is about 16 gallons of cruising, call it 18 with idling and maneuvering — so “25 a day” feels padded. Now the untypical day: the fish are 48 nm out, and the ride home is into a 20-knot sea breeze that knocks the boat down to 1.5 nmpg. That's about 25 gallons out, 32 back, plus 4 for maneuvering — call it 61 gallons, nearly two and a half “days” of fuel in one day.
The average never saw it coming. It knows nothing about routes, conditions, or return legs, holds no explicit reserve, and quietly plans against total tank capacity — nobody's head-math discounts to usable fuel. The best upgrade here isn't a tool; it's replacing the folklore number with a measured one, which our guide to how much fuel a boat uses walks through.
Method 2: Pencil, paper, and the one-third rule
The classic. Measure the route distance you'll actually run, divide each leg by your measured nmpg, add an allowance for idling and maneuvering, then apply the one-third rule: a third of your fuel out, a third back, a third in reserve. We walk the full method, worked example included, in how to plan fuel for a boat trip.
Done rigorously, this is genuinely sound — it's the standard everything else here is judged against. It's about this trip, not the average one. The reserve becomes explicit gallons instead of a feeling. Plan the return leg on a pessimistic nmpg and it even sees the way home. It costs about ten minutes and one prerequisite: a burn number you measured, not remembered.
Its failure modes are all human. The burn number is static — it doesn't move with speed or sea state unless you move it, and most people don't. The pessimistic return leg happens only if your discipline holds. The exercise gets skipped on exactly the mornings it matters. And because you control every input, the numbers drift optimistic until the trip you want fits the fuel you have. Paper is exactly as good as your worst morning.
Method 3: The spreadsheet
The spreadsheet is the paper method encoded once, so you can't fumble the arithmetic at the ramp: columns for the legs, a cell for nmpg, a row that turns red when trip fuel crosses two-thirds of usable. It costs an evening. Over paper it catches arithmetic slips and inconsistency, and — if you log fill-ups in it — builds a real measured average burn.
What it misses is that it computes without questioning. Type 2.4 nmpg into the cell when your honest bad-day number is 1.6 and it blesses the plan in green. It doesn't know your route — you feed it a distance, usually the optimistic straight-line one. It can't see the forecast, and it treats the return like the outbound unless you built per-leg logic. And it lives at home: when the plan changes at the fuel dock, the recalculation happens by eyeball, which is method one again. A spreadsheet is a filing cabinet with arithmetic. The judgment stays manual.
Method 4: The chartplotter fuel page
If the boat has a fuel-flow sensor on the network, the plotter or engine display shows the one number every other method only estimates: your actual burn, right now, in gph. That reading is the closest thing to ground truth aboard — it includes the load, the bottom growth, the chop, everything a model has to guess at. Alongside it you get fuel used, fuel remaining, and some form of distance-to-empty.
The catch is timing and framing. It's an instrument, not a planner: it answers only once you're underway, and the go/no-go decision needed to happen at the dock. Distance-to-empty stretches this instant across the whole tank — a flat-water morning reading extended into an afternoon chop it can't see — and it counts to bone dry, not to a reserve left intact, trusting a sender or a fuel-used total that drifts unless it's reset at every fill. Most fundamentally, it doesn't know you have to come back. As our guide to cruising range works through, the distance you can cover and still get home with a reserve is roughly a third of what a to-empty number implies.
None of that makes the fuel page bad. It's the best verification instrument on the boat — the live number you check the plan against mid-trip, worth cross-checking against a fill-to-fill average now and then, because flow meters drift too. It just can't be the plan.
Method 5: A dedicated planning app
What a planning app adds is the whole method in one place, before you leave. Far Enough is ours, so take this section as the worked example it is. It plans the route on a chart, so the distance is the path you'll run, not the straight line. It works the outbound and return as separate legs, flipping wind and current for the way home. It reads burn off a speed-aware curve — faster costs more fuel, and it deliberately refuses to credit a planing hull with better per-mile economy for slowing off plane, the assumption that talks boaters into a false go. It pulls the day's wind and sea state into the estimate, and it holds the reserve off the top: 20 percent of the tank by default, adjustable in either direction — the one-third rule remains the honest benchmark, and if you set the reserve under 20 percent the app flags it as thin rather than obliging quietly.
The two pieces that are hard to do by hand are uncertainty and calibration. Every estimate carries a confidence band, and the go/no-go verdict keys off the pessimistic end of it, so an uncalibrated boat gets a cautious answer, not a confident one. As you log fill-ups — topped to full, with engine hours — the estimate converges on your measured burn, like a skeptical bookkeeper: a fill implying an impossibly low burn is treated as a typo and dropped, a burn higher than your rated number is believed immediately, a lower one only gradually. If the boat has NMEA 2000 or a SignalK Wi-Fi gateway, it can also read live fuel flow — read-only and entirely optional; everything works with manual entry — and a live reading hotter than plan raises a warning instead of being averaged away.
What it still misses
Whatever you feed it dishonestly. An app used carelessly is more dangerous than paper used carelessly, because it's more convincing — give it a flattering fuel-aboard figure or an optimistic cruise speed and it will confidently work the wrong plan. Until it's calibrated, it's a model. And it is not a gauge, a navigation system, or an emergency device. It's the planning step, done thoroughly.
The five methods at a glance
Same criteria, one line each:
- Rule of thumb. Costs nothing. Catches gross mismatch. Misses the route, the conditions, the return leg, and the reserve — and fails precisely on the untypical day, which is the day that matters.
- Paper and the one-third rule. Costs ten disciplined minutes and a measured burn number. Catches whether this specific trip fits, with an explicit reserve. Misses whatever your discipline misses: a static burn figure, optimistic nudges, skipped mornings.
- Spreadsheet. Costs an evening to build. Catches arithmetic slips and accumulates a fill-up history. Misses bad inputs it can't question, routes and forecasts it can't see, and every replan made away from the desk.
- Chartplotter fuel page. Costs a flow sensor, often already installed. Catches your true live burn — the only method that measures instead of estimates. Misses the plan itself: it works only underway, extrapolates the present, counts to empty rather than to reserve, and doesn't know you have to come back.
- Planning app. Costs a purchase — Far Enough is $24.99 once, no subscription. Catches the route, the conditions, the return leg, and the reserve before you leave, and calibrates toward your measured burn. Misses dishonest inputs — and it is not a gauge.
The honest ending
The ladder is real — each method catches a failure mode the one below lets through — but it isn't a ranking of skippers. Paper math done rigorously, with a measured bad-day nmpg, a separate pessimistic return leg, usable fuel instead of total capacity, and a reserve nobody touches, beats any app used carelessly, including ours. The five methods share almost all of their failure modes in one place: the honesty of the inputs. Tools lower the cost of rigor. They can't supply the honesty.
A good fuel plan needs the same four things regardless of tooling: a burn number you measured, a plan made before you leave the dock, a live check underway, and a reserve that isn't up for discussion. If your method covers all four, you're doing fine. If there's a gap, it's usually the plan-before-you-leave part — and that's what the next step up the ladder is for.
