Quick Answer
No, a gallon is not 5 pounds. A gallon is a unit of volume, while pounds are a unit of weight. Specifically, one US gallon equals approximately 3.78 liters or 0.1337 cubic feet. A gallon does not have a set weight, as it depends on the density of the liquid being measured. For reference, one gallon of water weighs about 8.34 pounds.
What is a Gallon?
A gallon is a unit of volume typically used to measure liquids. There are several definitions for gallons depending on context:
- US liquid gallon – Equals approximately 3.78 liters or 0.1337 cubic feet. This is the common gallon used in the US to measure volumes of liquids like milk, gasoline, etc.
- US dry gallon – Equals approximately 4.40 liters or 0.1579 cubic feet. This is used to measure volumes of dry goods in the US.
- Imperial gallon – Equals approximately 4.55 liters or 0.1605 cubic feet. This gallon definition is used in the United Kingdom and some other countries in the Commonwealth.
The US liquid gallon is the most common definition used in the US and will be the primary focus of this article.
What are Pounds?
Pounds are a unit of weight or mass in both the US customary system and the imperial system. Specifically:
- 1 US pound (lb) equals 16 ounces or about 0.45 kilograms.
- 1 imperial pound equals 16 ounces or about 0.45 kilograms.
Pounds are commonly used to measure the weight or mass of various items from produce to packages to people.
The key difference between pounds as a unit of weight and gallons as a unit of volume is that weight is a measure of the force of gravity on an object, while volume measures the three-dimensional space an object occupies. Weight varies based on gravity and mass, while volume is fixed for a given amount of liquid.
Does a Gallon Have a Fixed Weight?
Because a gallon is a unit of volume, it does not have a fixed weight in pounds. However, you can calculate the approximate weight of a gallon of any liquid if you know its density.
Density measures how much mass occupies a certain volume. Substances with a higher density weigh more per gallon than substances with low density.
For example, one gallon of water weighs about 8.34 pounds at room temperature because water has a density of about 1 g/mL. But one gallon of milk weighs around 8.6 pounds because milk has a higher density than water.
Some common approximate densities and gallon weights:
| Liquid | Density (g/mL) | Gallon Weight (lbs) |
|---|---|---|
| Water | 1.00 | 8.34 |
| Milk | 1.03 | 8.60 |
| Gasoline | 0.72 | 6.01 |
| Vegetable oil | 0.92 | 7.67 |
| Honey | 1.42 | 11.83 |
As you can see, density varies widely between liquids, so there is no fixed conversion between gallons and pounds. You need to know the specific liquid in question to calculate an approximate weight.
Does a Gallon of Water Weigh 5 Pounds?
Based on the density of water, a gallon of water actually weighs about 8.34 pounds at room temperature. Water has a density of 1.00 g/mL, which converts to 8.34 pounds per gallon.
So a gallon of water definitely does not weigh 5 pounds – it weighs significantly more than that. Five pounds equals approximately 0.60 gallons of water rather than a full gallon.
This 8.34 pound per gallon conversion for water is easy to remember and can serve as a good point of reference when estimating weights. It’s over 50% more than 5 pounds per gallon.
Why a Gallon Doesn’t Equal 5 Pounds
There are a few key reasons why a gallon does not equal 5 pounds, especially for water:
- Density of water: As previously discussed, water has a density of 1.00 g/mL or 8.34 lbs/gallon.
- Definition of weight vs. volume: Weight and volume are different physical properties. There is no fixed conversion between any given volume like a gallon and a given weight like 5 pounds without knowing density.
- SI definitions: In the metric system, a liter of water weighs 1 kg which equals 2.20 lbs. A gallon is 3.78 liters, so a gallon should weigh around 8.34 lbs.
- Impractical conversion: Using 5 lbs/gallon for consumer products and applications would be highly impractical given the real density of water. This inaccurate conversion could lead to major errors.
So in summary, the density of water in both imperial and metric units shows there is no direct link between a gallon volume and 5 pounds of weight. Any claim that a gallon of water weighs 5 pounds is physically incorrect.
Does Any Liquid Weigh 5 Pounds per Gallon?
While water and many common liquids weigh significantly more than 5 pounds per gallon, there are some liquids with lower densities that convert roughly to 5 lbs/gallon.
For a liquid to weigh close to 5 lbs per gallon, it needs to have a density of around 0.6 g/mL based on the volume of a US liquid gallon.
Some examples of liquids in the 0.6 g/mL density range include:
- Gasoline – Weighs 4.5-5 lbs/gallon
- Diesel fuel – Weighs 5-5.5 lbs/gallon
- Kerosene – Weighs 4.8-5.2 lbs/gallon
- Some light fuel oils – Weigh 5-6 lbs/gallon
So certain light petroleum products used as fuels can convert close to 5 pounds per gallon. However, water and most other common liquids are significantly denser than this. And gasoline and other fuels are typically measured in gallons rather than pounds. So the 5 pound per gallon density is more of an approximation, not a universally applicable conversion.
Effects of Temperature on Gallon Weight
The density and thus weight of most liquids changes slightly based on temperature. As temperature increases, molecular motion increases, causing liquids to expand somewhat and become less dense.
For example, the density of water at its freezing point of 32°F (0°C) is 0.9998 g/mL. At room temperature of 68°F (20°C) it is 0.9982 g/mL. And at boiling 212°F (100°C) it is 0.9584 g/mL.
This means a gallon of water would weigh:
- 8.34 lbs at 68°F (20°C)
- 8.35 lbs at 32°F (0°C)
- 8.01 lbs at 212°F (100°C)
The differences are small, but measurable. Most other liquids exhibit slight variations in density and weight per gallon based on temperature as well.
However, the density differences due to reasonable temperature changes are not nearly enough to make a gallon of water or most other liquids weigh only 5 pounds. The 8-9 pound per gallon weight holds across a wide temperature range.
Does Saltwater Weigh 5 Pounds per Gallon?
Saltwater or seawater is water that contains relatively high dissolved salt content, usually about 3.5% salt. However, the presence of salt causes only a minor increase in the density and weight of water.
Specifically, saltwater density increases to around 1.025 g/mL compared to 1.00 g/mL for freshwater. This means that one gallon of saltwater at 68°F (20°C) weighs about 8.56 pounds.
So again, saltwater weighs significantly more than 5 pounds per gallon due to its density. Salt increases the weight slightly but not dramatically. Even with concentrated brine solutions, the density is still over 1 g/mL and far too high for a 5 pound per gallon conversion.
Can Any Substances Weigh 5 lbs per Gallon?
While liquids like water, milk, oil and saltwater all weigh significantly more than 5 pounds per gallon, there are some solid substances that can weigh close to 5 pounds per gallon when compacted.
For example, one gallon of the following compacted items weighs about 5 pounds:
- White sugar (granulated sucrose)
- All-purpose flour
- Table salt (sodium chloride)
However, these are not liquids but compacted granular solids. When not compacted, their density decreases drastically. One gallon of loosely packed flour or sugar weighs around 3-4 pounds.
So while no common liquids weigh close to 5 pounds per gallon, some densely packed dry goods can convert roughly into a 5 pound gallon weight when compacted. But again, the 5 lbs/gallon density is not applicable for liquids or unpacked solids.
Key Takeaways
- A gallon is a unit of volume, not weight. Liquids have varying densities so there is no fixed gallon-to-pound conversion.
- Water weighs over 8 pounds per gallon due to its density of 1 g/mL.
- Gasoline, diesel, and kerosene weigh around 5 lbs/gallon but are measured in gallons, not pounds.
- Temperature affects density but not significantly enough to change the 5 lbs/gallon relationship.
- Saltwater is slightly denser than freshwater but still weighs over 8 pounds per gallon.
- Some compacted solid goods like flour and sugar can weigh close to 5 lbs/gallon but liquids do not.
Conclusion
In summary, a gallon is a unit of liquid volume and does not weigh an exact number of pounds without considering the density of the specific liquid. Water and most common liquids weigh significantly more than 5 pounds per gallon, typically 8-9 pounds. Certain fuels like gasoline weigh close to 5 pounds per gallon but would still be measured and sold by volume. Temperature and salt content have minor effects on density but do not change the weight enough for a 5 pound gallon conversion. While no liquids truly weigh 5 lbs/gallon, some packed dry goods can when compacted. So the claim that a gallon equals 5 pounds is not accurate in most cases. Density, not predefined conversions, must be considered when weighing liquid gallons.
