The one-third fuel rule and boating reserves
How to plan boat fuel with the third tank rule, why gph beats miles per gallon on the water, and when to add extra reserve.
Why boats plan fuel by the hour
Cars think in miles per gallon, but a boat is better measured in gallons per hour. A hull pushing through water burns fuel at a rate that depends mostly on engine load and speed, and small speed changes swing consumption dramatically once a planing boat climbs onto or falls off plane. Because a knot is one nautical mile per hour, planning by time is natural: distance over speed gives hours, and hours times your gph gives fuel. That is exactly the chain this calculator follows.
The one-third rule explained
The classic third tank rule splits a full tank into three parts: one third to reach the destination, one third to return, and one final third kept untouched as reserve. It is a simple mental check that keeps a boater from committing more than a third of their fuel to the outbound leg. In reserve terms that final third is roughly a 50 percent margin over the fuel the round trip actually needs, which is deliberately generous because help is far away at sea and options are limited if a tank runs dry.
Choosing a reserve for the conditions
The right reserve depends on how exposed the trip is. A short run in sheltered water on a calm day might be fine with 10 to 20 percent, the range this tool sits in by default. Longer offshore passages, unfamiliar waters or a forecast that could turn all argue for the fuller third tank cushion. Factors that quietly raise burn, a headwind, an adverse current, a heavy load of passengers and gear, or a hull fouled with growth, should nudge the reserve up because the trip fuel figure assumes none of them.
Sanity-check against your real tank
A fuel estimate is only useful next to the tank you actually carry. Compare the total fuel here with your usable capacity, remembering that senders and pickups rarely draw a tank completely dry, so usable volume is a little less than the rated size. If the total plus reserve approaches your usable capacity, shorten the trip, slow down to cut gph, or plan a fuel stop. Topping off before departure and logging fuel against engine hours over time also sharpens the gph number you feed back into the calculator.