Three numbers, and only one of them is stable
Range is usable fuel divided by gallons per hour, multiplied by speed. Tank capacity is close to fixed. Burn rate and speed are not, and they move together in a way that punishes the optimistic. A boat that burns nine gallons an hour at twenty-four knots in flat water might burn twelve at eighteen knots pushing into a two-foot chop, which is not a small change to a range figure — it is a forty percent cut, arrived at without anything going wrong.
That is the reason a reserve exists, and the reason it is expressed as a fraction of fuel rather than a distance. A reserve in miles assumes you get the speed back. A reserve in gallons survives the assumption failing.
The rule of thirds, and what it is for
A third of the fuel out, a third back, a third untouched, is a planning rule taught widely enough that most people who run boats offshore have heard it. It is not a law and it is not a specification. What it buys is a whole return trip worth of margin, which is the right shape of margin for the failure that actually happens: the wind gets up, the sea builds against you on the way home, and the leg that took two hours out takes three and a half back at a burn rate you did not plan for.
A percentage reserve is the same idea with a dial on it. Twenty-five percent held back is looser than thirds and is defensible on a protected bay with fuel at both ends. Anything under about fifteen percent stops being a reserve and starts being rounding error. Set it for the water you are on, not for the number that makes the trip work.
Where the tank figure lies to you
Nominal tank capacity is what the tank holds if you could fill it to the top of the fill neck and draw it to the last drop. Neither end is available. Fill hoses trap air, vents burp fuel back, and the pickup tube stops short of the tank floor deliberately so it does not lift sediment and water. That gap is the reason the usable field here defaults below a hundred percent.
Trim makes it worse in a way that is easy to overlook. A pickup that is comfortably submerged with the boat at rest can be uncovered with the bow up on plane and a quarter tank aboard, which is exactly the moment you find out, at speed, a long way from the ramp. If your gauge and your refills disagree by more than a couple of gallons, believe the refills.
Fuel is weight, and it leaves
Gasoline runs about 6.1 pounds per gallon and diesel about 7.1. A hundred-gallon tank is therefore six hundred pounds or so of payload that burns off during the day, and on a boat small enough for that to be a meaningful fraction of displacement, the trim at the end of the run is not the trim you left with. That is usually benign and occasionally not: a boat that was stern-heavy and settled becomes lighter aft and rides differently, and a boat that was balanced on the way out can be bow-light coming home into a sea. Weight, where it sits, and the total against the plate are worked out on the boat weight and capacity planner.
Gasoline vapor is heavier than air and it collects in a bilge, which is why powered blowers exist on gasoline inboards and why a fuel smell is a stop-everything event rather than a nuisance. This page does not give a fueling or ventilation procedure and should not be read as one. Get that from the engine and boat manuals for the specific vessel.
Getting a real burn rate
A flow meter gives you the number directly and is the least argumentative way to get it. Without one, the method is full-to-full: top the tank at the same pump with the boat sitting the same way, run a known distance at a held speed, top it again, and divide. Do it twice. One data point on a calm day with a clean bottom is a number you will regret trusting in October. If you are also tracking what the running costs to operate rather than just what it consumes, the fuel cost calculator handles the money side of the same trip.
Questions people ask
What is the rule of thirds for boat fuel?
A third of your usable fuel for the outbound leg, a third for the return, and a third untouched as reserve. It is a widely taught planning convention rather than a requirement, and its value is that the reserve is a full trip in size, which is what you need when the return leg costs more fuel than the outbound one did. That is the normal case rather than the unlucky one, because you usually leave on the good part of the day and come back on the worse part of it. You can set a different reserve on this page, and on protected water with fuel at both ends people commonly do. What you should not do is set the reserve after working out that the trip does not otherwise fit.
Why is my actual range so much less than the calculated range?
Because the calculation uses one burn rate and one speed, and the day supplies neither. Sea state is the big one — pushing into a chop can cost a third of your speed at an unchanged or higher fuel burn, which hits range from both directions at once. After that comes bottom growth, which is gradual enough that people do not notice the cost until the boat is hauled; load, which on a small boat is a large fraction of displacement; and propping, if the engine can no longer reach its rated speed and is being run hard to hold a cruise it used to make easily. Measure burn on a bad day as well as a good one and plan on the bad number.
How much does a gallon of fuel weigh, and does it matter?
Gasoline is roughly 6.1 pounds per gallon and diesel roughly 7.1, both varying a little with temperature and blend. On a large boat this is trivia. On a small one it is a serious fraction of what the boat is allowed to carry: sixty gallons of gasoline is around three hundred and seventy pounds, which is two adults you can no longer bring. It also changes during the day as it burns, which shifts trim. Anyone planning a load against a capacity plate needs the fuel and water in the total, not treated as part of the boat.
Should I plan in knots or miles per hour?
Use whichever your instruments and charts use, and above all do not mix them. A nautical mile is about 1.15 statute miles, so a route measured off a chart in nautical miles and divided by a speed read in miles per hour will overstate your time and understate your fuel by about fifteen percent. That is the wrong direction for an error in a fuel plan. Chart distances and marine electronics are usually in nautical miles and knots; a car-style GPS or a phone app may not be. Check which one you are looking at before you do arithmetic with it.
Does this account for a headwind or current?
Only in the sense that you can enter the speed you will actually make good rather than the speed the boat does in flat water, which is the honest way to use it. Current is the cleaner of the two to handle: a two-knot foul current against a twenty-knot boat is a ten percent penalty and you can enter eighteen knots. Wind and sea are messier because they cost you speed and add burn at the same time, so lowering the speed alone understates the damage. If you know the leg will be against it, lower the speed and raise the gallons per hour, then look at what that does to the turn-around point.