Every boat fuel calculator is three formulas
Search for a boat fuel calculator and you'll find no shortage of them: type in some numbers, get a gallons figure back. Under the hood, every one of them is running the same three pieces of arithmetic on the same small set of inputs, and none of it is harder than dividing two numbers.
That's worth knowing, because a calculator is only as good as what you feed it. The difference between a fuel answer you can trust and one that leaves you drifting isn't the calculator — it's the inputs. So this guide is the whole calculation, opened up: the numbers to gather first, the three formulas with a worked example for each, and the reserve rule that turns a raw answer into a plan you can actually leave the dock on.
The numbers to gather first
Everything below is built from some combination of four inputs, plus a fuel price if you want the cost. Get these right and the arithmetic takes care of itself:
- Burn rate, in gallons per hour (GPH) — what your engine drinks at your normal cruise RPM. A number measured on your own boat is worth far more than a spec sheet; if you don't have one yet, there's a horsepower rule of thumb below.
- Cruise speed, in knots — the speed you actually hold at that RPM, not the brochure top end.
- Trip distance, in nautical miles — the real route around channels, shoals, and doglegs, not the straight line. [Planning fuel for a specific trip](/blog/how-to-plan-fuel-for-a-boat-trip) starts with an honest distance.
- Usable fuel aboard, in gallons — not tank capacity. You can't safely draw a tank to the bottom, so plan on no more than about 90 percent of the stated number, less on an older or oddly shaped tank.
- Fuel price, per gallon — use your marina's posted price, which usually runs well above the road price for the same fuel.
Formula 1: fuel needed for a trip
First, turn burn rate and speed into the number that does all the real work: nautical miles per gallon. nmpg = speed in knots ÷ GPH. A boat cruising at 24 knots while burning 12 gph is getting 2.0 nmpg — every gallon buys two miles.
The trip fuel is then one division: fuel needed = distance ÷ nmpg. A 50 nm run at 2.0 nmpg needs 25 gallons. (The time-based route gives the same answer: 50 nm at 24 knots is about 2.1 hours, times 12 gph — 25 gallons.)
Add a small allowance on top for everything that isn't cruising: idling out of the marina, trolling or working a spot, maneuvering at both ends. That low-speed time burns fuel while earning almost no distance, so a distance-only answer always comes in a little light. A few gallons covers it on most day trips.
Formula 2: what the fuel will cost
Cost = gallons × price per gallon. The 25-gallon leg above at $5.25 a gallon is about $131 — call it around $260 out and back.
Two things keep a cost estimate honest. Price it with the marina number, not the highway one you drove past. And if the total stings, remember speed is the lever: most hulls have a cruise sweet spot where each mile costs the least, and our full cost guide breaks the spending down per hour, per mile, and per trip.
Formula 3: how far you can go
Range = usable fuel × nmpg. The 2.0-nmpg boat with 90 usable gallons has a theoretical range of 180 nautical miles — the distance to a bone-dry tank in ideal conditions, which is a useful ceiling and a terrible plan.
The range you can actually use holds back a reserve and prices the trip home honestly. Both are covered in depth in our guide to calculating your boat's cruising range; the short version is that two-thirds of usable fuel is for travel, so this boat can cover about 120 nm on the water and still tie up with 30 gallons untouched.
No burn number yet? Start from horsepower
If you've never measured your boat's burn, you can bracket it from horsepower. Near wide-open throttle a gasoline engine burns very roughly a gallon per hour for every 10 to 12 horsepower; a diesel, about one for every 18. Treat the results strictly as ceilings for sanity-checking — at an efficient cruise you'll burn well under them, and age, prop, and tune all move the figure. Our boat fuel consumption guide covers where these rules come from and how to replace them with measured numbers, which is the real fix. The ceilings look like this:
- 150 hp gas outboard — very roughly 12 to 15 gph flat out.
- 250 hp gas — roughly 21 to 25 gph flat out.
- 300 hp gas — roughly 25 to 30 gph flat out.
- 300 hp diesel — roughly 15 to 17 gph flat out; diesels do more work per gallon.
The one-third rule turns the answer into a plan
A raw fuel-needed number is not a go / no-go. The day you lean on the plan hardest — building seas, a headwind home, a detour around weather — is the day the real burn beats the estimate. The time-tested margin is the one-third rule: one-third of your fuel to get out, one-third to get back, one-third never touched.
The working check: multiply your total trip fuel — both legs plus the low-speed allowance — by 1.5, and that's the minimum usable fuel you want aboard before you cast off. If your tank doesn't clear the bar, the answer isn't to shave the estimate; shorten the route, plan a fuel stop, or slow to your efficient cruise. And don't let the needle talk you into optimism near the bottom of the tank — fuel gauges are least accurate exactly where it matters.
The whole calculation, worked once
Put all three formulas together on one trip. The boat: 12 gph at a 24-knot cruise (2.0 nmpg), a 100-gallon tank planned at 90 usable. The trip: a spot 36 nm away by the real route, out and back, marina fuel at $5.25:
- Round-trip distance: 72 nm. Fuel to cruise it: 72 ÷ 2.0 = 36 gallons.
- Low-speed allowance for the marina, the idle time, and working the spot: 4 gallons. Trip total: 40 gallons.
- Reserve check: 40 × 1.5 = 60 gallons of usable fuel wanted aboard. Available: 90. Clears the bar with room to spare — a comfortable go.
- Cost: 40 gallons × $5.25 ≈ $210 for the day.
- Sanity check at the dock: you'd expect roughly 50 of the 90 usable gallons still aboard — well above the 30-gallon third you swore not to touch.
What a hand calculation can't see
Run by hand, the calculator above treats the whole trip as one number: flat water, one throttle setting, and a return leg that costs the same as the leg out. Real trips aren't like that. Wind and sea state move the burn, the afternoon chop on the nose is the classic way an on-paper plan comes up short, and the inputs themselves drift all season as the bottom fouls and the boat loads up.
None of that makes the math wrong — it makes the inputs a moving target. The fixes are the honest ones: measure your real burn and keep re-measuring it, plan the return leg pessimistically, and treat the reserve as if it were already spent.
