Keeping emergency gasoline reserves on the boat is a very good safety procedure. Running out of gas is a very common occurrence in boating, so making sure you have enough to get to a harbor is a very smart idea.

You need to know much weight you are adding to your boat, so you can determine how much reserves to keep on board. Let’s break down the different weight calculations for ethanol and ethanol-free gas.

To help you figure out the maximum amount of weight that your boat can safely handle, it’s important to start with the basics.

**How Much Does Gas Weigh?**

Whether you’re a seasoned boater or an absolute beginner – this is a question that everyone needs to answer.

According to the *Science and Technology Desk Reference*, **a U.S. gallon of Gas weighs 6.1 pounds and an Imperial Gallon weighs 7.2 pounds.**

To give you a clearer idea of just how much that is, here are a few things to keep in mind:

**1. A gallon of gas is lighter than a gallon of water.**

In comparison to gas, water weighs 8.4 pounds. That’s more than a 2 pound difference – which explains why gas floats on water. This is why pouring water on a gasoline fire won’t do very much good.

**2. Diesel is a lot heaver than gasoline.**

A gallon of diesel weighs approximately 7.1 pounds. That 1 pound difference is because of its larger molecules and higher density. Its flash point and autoignition temperatures are significantly higher as well.

Most boats over 45 feet usually run purely on diesel – which is why the engines tend to be bigger and heavier.

To make it even easier for you to measure, picture these few easy illustrations:

- A gallon of gas is a heavier than a 2-liter bottle of soda
- A gallon of gas is lighter than two average sized textbooks
- A gallon of gas is twice the weight of a two-slice toaster
- A gallon of gas is equal to the weight of a small bowling ball

**UK Imperial vs. US Gallon Weight: What’s the Difference?**

The simple answer is – **the Imperial gallon is heavier in weight than the US gallon.**

- 1 Imperial gallon of gas is equivalent to 7.2 pounds (3.2659 liters)

- 1 US gallon of gas is equivalent to 6.1 pounds (2.7669 liters)

Based on these numbers, the Imperial gallon is about 1/5 or 20% greater in volume than the US gallon.

**Does the Weight of Gas Change Depending on the Weather?**

Yes, but only up to a VERY small amount – less than a pound per gallon at most.

The weight of gas is contingent on a lot of factors, particularly: humidity, temperature, and weather change.

The general rule is that on hotter days, gasoline tends to weigh more because the volume expands – otherwise known as thermal expansion. Marine diesel, in particular, is quite sensitive to temperature change because of its higher density.

Since the fluctuations in weight are very minimal, it really won’t make a huge dent on your boat’s overall capacity. But if you want to load up on a lot of extra gas, it’s best to pack light during hot summer days.

**Why Is This Number Important?**

Determining the weight of gas per gallon is one of the easiest ways to estimate fuel burn.

“Fuel burn” is your boat’s fuel consumption – the measure of how much fuel your boat uses per nautical mile. This in turn will determine how much spare gallons you need for a single voyage.

*Boating Magazin*e has come up with an easy-to-compute formula that measures fuel efficiency in pounds of fuel used per horsepower developed per hour.

Taking this into consideration, pontoon and bass boats generally consume around 4 – 5 gallons per hour. This means that for a 5 hour voyage, you’ll need around 25 gallons of gas. For boats with a hull length of over 20 feet, like center console boats, the average fuel consumption is 5 – 6 gallons per hour.

These numbers are based on cruising speed. For an average-sized recreational boat, this speed is often slower than 20 knots. Fuel consumption differs according to a lot of factors, like steer control and rough weather conditions.

**Should You Bring Extra Gas on Board During Every Boating Trip?**

Here’s the deal – you don’t need to carry extra gas on board, unless it’s absolute necessary.

Most boats have a tank that stores ample fuel to last a day on the water. And it’s important to remember that gas is extremely flammable, so carrying too much on board will be increase the risk of boat fires.

But if you plan on spending more than a few days on the water, you’ll need the extra fuel to complete the journey.

In cases like these, here are a few things to remember:

**1. The safest amount to bring on board is a single 6-gallon gas can.**

It’s easy to move around and won’t change the weight of your boat too much. This is enough to run for more than an hour on cruising speed.

**2. Use proper cans specifically designed for fuel storage.**

This includes USCG-approved tanks and portable/auxiliary fuel tanks. Using plastic and other containers could allow the fuel to drip and escape.

**3. Store in a ventilated area but away from direct sunlight.**

This is because gas tends to expand and build up pressure when exposed to heat.

**4. Extra cans should be securely stowed.**

This can be done by using straps and long fittings to prevent them falling over and leaking all over the boat.

**5. If you need to re-fill, do it in an open area and away from any sources of ignition.**

**Boat Capacity and the Dangers of Overloading**

Before deciding whether or not to bring an extra can of gas on board, it’s important to determine your boat’s maximum capacity.

Most state boating laws require that a capacity plate be placed on recreational watercrafts, particularly on boats smaller than 20 feet.

This plate lists the boat’s capacity in terms of number of persons and pounds that the boat can safely hold. But for purposes of extra fuel storage, the most important number is the amount of pounds.

Overloading your boat with too many extra cans of gas would make your boat susceptible to swamping. And even if you are within the allowable weight limit, the cans have to be stored and distributed evenly to keep the boat steady.

It’s an important factor to take in especially in bad or choppy weather, when an overloaded boat is more likely to being swamped or even capsized.

**The Bottomline**

All successful boating trips start with proper planning and preparedness.

Before getting on a boat, it’s important to understand and properly estimate your boat’s fuel burn, gas mileage, and maximum capacity – all of which can be easily determined once you know how much a gallon of gas weighs.