What This Boat Fuel Calculator Does
Enter your engine horsepower, fuel type, throttle setting, and trip distance, and this tool estimates how much fuel you will burn, what it will cost, how long the run takes, and your range. The engine underneath is the burn-rate rule most boaters carry in their head: a gas engine uses roughly 0.10 gallons per hour for every horsepower at wide-open throttle, and a diesel about 0.055. From there, trip fuel is simply gallons per hour multiplied by the hours you spend underway — distance divided by speed.
A Worked Example
Say you run a 200 HP outboard. Wide open it can gulp near 20 GPH, but at a normal cruise it burns about 10 GPH. Plan a 4-hour run — out to a reef and back — and that is roughly 10 × 4 ≈ 40 gallons of actual travel fuel. Now apply the one-third rule: that 40 gallons should be only two-thirds of what you carry, so you want a tank holding about 60 gallons for this trip. Twenty gallons out, twenty back, and twenty in reserve that you never plan to touch. Dial the calculator to your real numbers and it does this arithmetic for you, including cost at today's dock price.
Plan to the One-Third Rule, Not the Bare Minimum
The single most useful habit in fuel planning is the rule of thirds: budget one-third of your fuel to get where you are going, one-third to get home, and keep the last third as untouched reserve. Wind, current, a building chop, or a longer route than planned all push real burn above any estimate, and a boat has no shoulder to coast onto. This calculator shows a 10% reserve as a floor, but treat the one-third rule as your working target any time you are offshore, in unfamiliar water, or a long way from the next fuel dock.
Boat Fuel Calculator
Boat Fuel Consumption by Engine HP (Gasoline, 75% Throttle)
Estimated GPH and range at cruising speed.
| Engine HP | GPH | At 20 kts (NMPG) | 30-gal Range | 50-gal Range |
|---|---|---|---|---|
| 40 HP | 3.0 | 6.7 | 200 nm | 333 nm |
| 75 HP | 5.6 | 3.6 | 107 nm | 179 nm |
| 115 HP | 8.6 | 2.3 | 70 nm | 116 nm |
| 150 HP | 11.3 | 1.8 | 53 nm | 89 nm |
| 200 HP | 15.0 | 1.3 | 40 nm | 67 nm |
| 250 HP | 18.8 | 1.1 | 32 nm | 53 nm |
| 300 HP | 22.5 | 0.9 | 27 nm | 44 nm |
| 350 HP | 26.3 | 0.8 | 23 nm | 38 nm |
| Twin 150 HP | 22.5 | 0.9 | 27 nm | 44 nm |
| Twin 250 HP | 37.5 | 0.5 | 16 nm | 27 nm |
How to Use This Calculator
- Enter Your Engine Horsepower: Type in the horsepower of your engine — this drives the whole estimate. For twin or triple setups, set the number of engines so the burn rate is totaled correctly.
- Set Fuel Type and Throttle: Pick gasoline or diesel, then choose the throttle setting you actually run — most cruising happens at 65–75%, not wide open. This is where estimates go wrong most often, so be honest about your real cruise.
- Add Trip Distance, Speed, and Fuel Price: Enter trip distance in nautical miles, your cruising speed in knots, and the current price at the pump so the tool can total both fuel and cost.
- Calculate and Plan to Thirds: Hit Calculate to see gallons needed, trip time, cost, GPH, and nautical MPG. Check that your usable tank covers the trip with a full one-third left over before you head out.
How It Works
Boat fuel burn tracks almost directly with how much horsepower the engine is actually making, which is why a simple HP-based rate gets you close before you ever look at a fuel-flow gauge. Gasoline engines burn roughly 0.10 gallons per hour for every horsepower at wide-open throttle; at a normal cruise you are only using part of that power, so the real number is lower.
The basic rule:
- Gasoline engines: GPH ≈ HP × 0.10 at wide-open throttle; a typical cruise pulls that down to about half a gallon per hour per 10 HP
- Diesel engines: GPH ≈ HP × 0.055 at full load — diesels sip roughly 40% less than gas for the same power
- Trip fuel = GPH × hours underway, where hours = distance ÷ speed. Then plan the tank around the one-third rule, not a bare 10% pad
Treat every number here as a planning estimate, not a fuel gauge. Hull type, load, chop, current, a dirty bottom, and a slipping prop all move real burn up or down. On the water the honest test is a full-tank-to-full-tank measurement over a known distance — until you have that, always leave the dock with more fuel than the math says you need.
Tips & Considerations
- The listed HP × 0.10 rate is a wide-open figure — at a realistic 65–75% cruise your gas engine burns closer to half of that, so match the throttle setting to how you actually run.
- Plan every longer or offshore run to the one-third rule: one-third out, one-third back, one-third untouched reserve. The 10% pad is a minimum, not a plan.
- Nautical MPG (speed ÷ GPH) usually peaks just above planing speed. Backing off a few hundred RPM often stretches your range noticeably for only a small speed loss.
- Twin engines roughly double the burn rate — a twin-250 rig can push 35–40 GPH, so total both engines when you size your tank or spare cans.
- Confirm your true burn with a full-tank-to-full-tank measurement over a known distance; hull load, a dirty bottom, and current all move real consumption off the estimate.
Frequently Asked Questions
How much fuel does a boat burn per hour?
It scales with engine size and throttle. A rough rule for gas engines is 0.10 gallons per hour per horsepower at wide-open throttle, so a 200 HP outboard maxes out near 20 GPH but settles around 10 GPH at a normal cruise. Small kickers (9.9–25 HP) burn 1–3 GPH, mid-size single outboards (115–150 HP) burn 8–12 GPH at cruise, and twin-engine offshore rigs can easily push 25–40 GPH. Diesels burn noticeably less for the same power.
How do I estimate gallons per hour from horsepower?
Multiply horsepower by a burn rate: about 0.10 GPH per HP for gasoline and about 0.055 for diesel, both at full throttle. So a 150 HP gas engine tops out near 15 GPH. Since you rarely run wide open, scale by throttle — at roughly 75% you might see 11 GPH, and at trolling speed closer to 5–7 GPH. This is a starting point; the fuel-flow readout on modern engines is more accurate once you have one.
What is the one-third rule for boat fuel?
The rule of thirds is the standard fuel-planning safety margin: use one-third of your fuel getting out, one-third getting back, and keep the final third untouched as reserve. It cushions you against headwinds, a rising chop, current, or an unplanned detour — all of which raise burn beyond your estimate. It is more conservative than a simple 10% reserve and is the sensible default any time you are offshore, in unfamiliar water, or far from a fuel dock.
How much fuel does a 200 HP outboard use on a trip?
Take the worked case: a 200 HP gas outboard burning about 10 GPH at cruise, run for 4 hours, uses roughly 40 gallons of actual travel fuel. Under the one-third rule that 40 gallons should represent only two-thirds of what you carry, so you want a tank holding about 60 gallons for that run — 20 out, 20 back, and 20 in reserve you never plan to touch.
How far can a boat go on a tank of fuel?
Range = usable fuel ÷ GPH × speed. A boat with a 100-gallon tank burning 11 GPH at 20 knots covers about 180 nautical miles on a full tank — but that is the theoretical maximum, not your safe range. Apply the one-third rule and your planning range drops to roughly 120 nautical miles round trip, leaving a full third in the tank. Nautical MPG (speed ÷ GPH) is the handy shorthand: at 20 knots and 11 GPH that boat gets about 1.8 NMPG.
Why does speed change fuel economy so much?
Drag climbs sharply with speed, especially as a planing hull fights to get up and over its bow wave. Most boats have an efficiency sweet spot just above planing speed, often near 60–75% throttle, where nautical MPG peaks. Push to wide-open throttle and burn can jump 50–100% while boat speed only gains 20–30% — you pay a lot of extra fuel for a little extra speed. Backing off a few hundred RPM is the easiest way to stretch range.
Is diesel really more fuel-efficient than gas on the water?
Yes. A marine diesel burns roughly 0.055 gallons per hour per horsepower versus about 0.10 for gasoline, and diesel fuel carries more energy per gallon, so the range advantage compounds. Diesels also make peak torque low in the rev range, which suits displacement and heavier hulls. The trade-off is higher upfront engine cost, so diesel pays off most on boats run often or over long distances.