Boat systems
Fuel burn and range calculator
Quick answer
A 150 hp outboard at cruise burns roughly 6.8 gallons per hour, so a 40 gallon tank gives about 5.9 hours and 165 miles at 28 mph. Under the thirds rule that is 55 miles out and back, because wind and sea state on the way home are not the ones you had going out.
Everything on this page is a planning figure rather than a specification, and it is worth being explicit about that before the numbers. Your own engine published fuel curve, or a fuel flow sensor on the network, beats this by a wide margin, and either is a genuinely better source than any calculator.
What this is good for is the question people actually have at the ramp: roughly how far can I go and still get back. The answer to that is governed less by the burn rate than by the oldest rule in small boat seamanship.
Run your own numbers
About 6.8 gph at cruise. Under the thirds rule that is 55 miles out, with 13 gallons still in the tank when you get home.
| Figure | Value |
|---|---|
| Estimated burn at cruise | 6.8 gph |
| Hours on usable fuel | 5.9 h |
| Theoretical total range | 166 mi |
| Thirds rule, distance out | 55 mi |
| Thirds rule, out and back | 111 mi |
| Fuel held in reserve | 13 gal |
| Miles per gallon | 4.1 mpg |
Shown with the example figures above. Change any field to run your own numbers.
The working
Rule of thumbburn at wide open throttle = horsepower x 0.1 gph per hp burn at cruise = that x 0.45 hours = usable fuel / burn range = hours x speed thirds rule: distance out = range x 0.3
The tenth of a gallon per horsepower figure and the thirds rule are both convention rather than published standards. Your own engine published fuel curve beats both. Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
Navigation and the radio
Fuel planning is only half of getting home. A fixed VHF into a proper antenna reaches far further than any handheld, and DSC sends a position with one press if the radio has GPS or is fed one.
Lowrance Point-1 GPS antenna with built-in compass
$321.00The Lowrance equivalent, and the one that pairs with an Elite FS without an adapter.
Check price on Amazon
Simrad GS25 waterproof GPS antenna with heading sensor
$321.00The Simrad version of the same hardware, for a boat already on a Simrad network.
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Garmin GPS 24xd with heading sensor, NMEA 2000
$279.94Adds a heading sensor, which is what makes the chart hold its orientation when the boat is barely moving.
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Garmin GPS 24xd antenna and receiver, NMEA 2000
$269.99An external antenna gets a far better fix than a unit buried under a console top, and puts it on the network for everything else.
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Humminbird NMEA 2000 GPS heading sensor
$248.28The Humminbird heading sensor, which is what a Helix or XPLORE needs for a stable chart heading.
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Better Boat 13 lb anchor kit with 100 ft rope and 8 ft chain
$125.99 Published specsAnchor, chain and rode in one purchase, with enough chain to actually set the fluke rather than lift it.
Check price on AmazonEvery figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.
The thirds rule
A third out, a third back, a third still in the tank. That turns a 165 mile theoretical range into 55 miles out, and it exists because the trip home is not the trip out.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Basis | Fuel used | Distance | What it assumes |
|---|---|---|---|
| Theoretical total range | 40 gal | 166 mi | Running the tank dry, which nobody should plan for |
| Half out, half back | 20 gal each way | 83 mi out | No reserve at all for weather, current or a longer route home |
| Thirds rule | 13 gal each way | 55 mi out | A full third left for headwind, chop, a detour or a tow |
The thirds rule is seamanship convention rather than a regulation. It exists because a headwind, a building sea or a following current that turns against you can easily double the fuel a return leg takes.
Where the burn figure comes from
The rule of thumb is that a four stroke gasoline outboard burns roughly a tenth of a gallon per hour for each horsepower at wide open throttle. It falls out of a typical brake specific fuel consumption near half a pound per horsepower hour against about 6.1 pounds per gallon of gasoline.
That is a planning figure for an engine at full power, and cruise is well below it. This calculator uses about 45 percent of the wide open figure for cruise, which is a reasonable general assumption and not a specification for any particular engine. A boat that is overloaded, has a fouled hull or is running the wrong prop will burn considerably more than this predicts.
What this calculator cannot know
Quite a lot, and the honest thing is to list it rather than imply precision.
- Hull efficiency. Two boats with the same engine and the same speed can differ substantially in burn.
- Load. A loaded boat pushes more water, and the effect is largest just below planing speed.
- Sea state and current. Both change the fuel a leg takes and neither is symmetric between out and back.
- Whether the tank figure is real. A stated tank capacity is rarely all usable fuel, because pickups sit above the very bottom.
Those are the reasons the thirds rule exists. It is a margin that absorbs things a calculation cannot see.
Frequently asked questions
How much fuel does a boat use per hour?
A common planning figure for a four stroke gasoline outboard is a tenth of a gallon per hour for each horsepower at wide open throttle, so a 150 hp engine burns about 15 gallons per hour flat out and considerably less at cruise. It is a rule of thumb rather than a specification, and your engine own published fuel curve is a far better source.
What is the thirds rule?
A third of the fuel out, a third back, and a third still in the tank when you get home. It is the oldest rule in small boat seamanship and it exists because the conditions on the way home are not the ones you had on the way out. A headwind, a building sea or a current that has turned can easily double what a return leg costs.
Is half out and half back good enough?
It leaves you with nothing. Half and half assumes the return leg is identical to the outward one, which is exactly the assumption that gets boats towed in. A following sea on the way out is a headwind on the way back, and the fuel cost of that difference is far larger than most people expect.
How accurate is this calculator?
It is a planning figure, not a specification. It cannot know your hull efficiency, your load, the sea state, or how much of your stated tank capacity is actually usable fuel, since pickups sit above the very bottom. A fuel flow sensor on the network or the manufacturer published fuel curve for your engine beats it by a wide margin.
Does a bigger engine always use more fuel?
Not at the same speed. A larger engine loafing at a moderate throttle setting frequently burns less than a smaller one working hard to push the same hull, because it is operating at a more efficient point on its own curve. The economical answer is usually the engine that can reach your cruise speed without being near full throttle.
How much of my tank is actually usable?
Less than the stated capacity, and the difference is not published as a standard figure. Fuel pickups sit above the very bottom of the tank so they do not draw sediment and water, and a boat pitching in a seaway can uncover a pickup with fuel still aboard. Treat the last portion of any tank as unavailable rather than as reserve.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.