Boat Trip Carbon Calculator
Work out the carbon dioxide from a day on the water, from the fuel you bought or from your engine, horsepower and hours, and see how much slowing down and keeping the hull clean would save.
Your trip
Twin engines: add their horsepower together. The fuel dock receipt is always more accurate than an estimate.
Carbon dioxide from the trip
How it's calculated
Burning a gallon of gasoline releases 8.887 kg (19.6 lb) of carbon dioxide, and a gallon of diesel 10.180 kg (22.4 lb). These are the EPA's figures, the same ones behind its Greenhouse Gas Equivalencies Calculator. The EPA's corporate emissions tables use 8.78 and 10.21, within about 1%. Liters are converted at 3.785 to the gallon. The total counts only the CO2 from the engine, not from producing and shipping the fuel.
Estimating fuel from horsepower
When you don't know the fuel used, the tool estimates it from the engine's rated horsepower, which it can only reach at full throttle. At full throttle a gasoline engine burns about 0.08 to 0.11 gallons an hour per horsepower. The low end comes from Boating Magazine's formula (0.50 lb of fuel per horsepower-hour ÷ 6.1 lb per gallon). North Carolina Sea Grant gives "one gallon of fuel per hour for every 10 horsepower", and two Yamaha tests of a 150 hp outboard measured 16.8 and 16.9 gallons an hour. For diesel the same formula gives about 0.056 gallons an hour per horsepower (0.40 lb ÷ 7.2 lb per gallon), roughly horsepower ÷ 18.
Most of the time you are not at full throttle. The speed setting scales the estimate using the two Yamaha performance bulletins below, for a 150 hp four-stroke on 20-foot center consoles. Your boat, load and sea state will differ, which is why the answer is a range.
| RPM | Key West 203 FS, gph | mpg | Sea Hunt 207 LE, gph | mpg |
|---|---|---|---|---|
| 1,000 | 0.7 | 7.0 | 0.8 | 6.4 |
| 2,500 | 3.7 | 2.7 | 3.2 | 3.8 |
| 3,000 | 4.3 | 4.7 | 4.0 | 5.3 |
| 3,500 | 5.8 | 4.6 | 5.0 | 5.5 |
| 4,000 | 7.3 | 4.3 | 6.4 | 5.1 |
| 4,500 | 9.5 | 3.8 | 8.4 | 4.5 |
| 5,000 | 11.9 | 3.5 | 9.7 | 4.3 |
| 5,500 | 15.2 | 3.0 | 13.7 | 3.4 |
| 5,900 (full) | 16.8 | 2.9 | 16.9 | 2.9 |
Why slowing down saves the most
Fuel burn climbs much faster than speed. In both tests the best mileage on plane came at 3,000 to 3,500 rpm, 4.6 to 5.5 miles per gallon, against about 2.9 at full throttle: 39 to 46% less fuel for the same distance. The worst place to sit is just below plane, where the hull plows and mileage fell to 2.7 to 3.8. North Carolina Sea Grant's example is a 250 hp outboard that burns 25 gallons an hour flat out and half that at 77.5% of full rpm. For slower displacement boats, the University of Alaska's Marine Advisory Program says easing off by one or two knots can cut fuel use by 30 to 50%.
A clean bottom and good trim help too. Practical Boat Owner puts the saving from a pre-season scrub and fresh antifouling at more than 8% of a season's fuel. For ships, the International Maritime Organization says slime just 0.5 mm thick over half the hull can raise emissions 20 to 25%. For the wider picture of fuel at sea, see how cruise lines are cutting their carbon.
Sources
- US EPA, Greenhouse Gas Emissions from a Typical Passenger Vehicle: 8,887 g CO2 per gallon of gasoline, 10,180 g per gallon of diesel. www.epa.gov
- US EPA, Greenhouse Gas Equivalencies Calculator: calculations and references (updated Aug. 2026): 3.93 × 10⁻⁴ t CO2e per mile driven, 1.24 × 10⁻⁵ t per smartphone charge, 9.00 × 10⁻⁴ t per pound of coal. www.epa.gov
- US EPA, GHG Emission Factors Hub (2025): motor gasoline 8.78, diesel 10.21 kg CO2 per gallon. www.epa.gov
- Boating Magazine, "Calculating Boat Fuel Consumption" (2019). boatingmag.com
- North Carolina Sea Grant, "Running Your Boat by the Numbers" (2014). ncseagrant.ncsu.edu
- Yamaha Performance Bulletin, Key West 203 FS with F150 (2011). keywestboats.com
- Yamaha Performance Bulletin, Sea Hunt Escape 207 LE with F150 (2009). www.njoutboards.com
- University of Alaska Marine Advisory Program, fuel-saving advice for boats (2008). www.uaf.edu
- Practical Boat Owner, "Boat fuel economy" (2025). www.pbo.co.uk
- International Maritime Organization, GloFouling preliminary results (2021). www.imo.org
- Schultz, "Effects of coating roughness and biofouling on ship resistance and powering", Biofouling (2007): heavy barnacle fouling adds up to 86% to a frigate's power needs. www.usna.edu
Questions
How much CO2 does a gallon of gasoline make?
8.887 kg, or about 19.6 lb, according to the EPA. A gallon only weighs about 6 lb, but when it burns, the carbon in it combines with oxygen from the air. A gallon of diesel makes 10.180 kg (22.4 lb).
How much fuel does a 150 hp outboard burn?
About 15 to 17 gallons an hour at full throttle, and 4 to 6 gallons an hour at an economical cruise of 3,000 to 3,500 rpm, going by two Yamaha tests on 20-foot boats. Hull, load and conditions change this a lot.
Is diesel better than gasoline for the climate?
Per gallon, diesel makes about 15% more CO2. But a diesel burns roughly half to two-thirds as many gallons for the same work, so it usually comes out ahead.
Does this include methane and other gases?
No. It counts the carbon dioxide from burning the fuel. Methane and nitrous oxide from the exhaust, and the emissions from producing and shipping the fuel, come on top. The car-miles comparison uses the EPA's per-mile figure, which includes a little methane and nitrous oxide.
What if my engine is an old two-stroke?
Older carbureted two-strokes can burn more fuel than the modern four-stroke engines behind these figures, so the estimate may come out low. Enter the fuel you actually bought instead.
Is my data stored?
Your engine and settings are saved only in your own browser. Nothing is sent to Lazy Seas.