Gallon Calculator
Calculate gallons, convert between volume units, and find container capacity in gallons. Convert gallons to liters, cups, quarts, pints, and fluid ounces.
Gallon Calculator
Calculator
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Formula: 1 US gallon = 231 in3 = 3.78541 L = 4 qt = 8 pt = 16 cups = 128 fl oz
Worked example — 44.88 US gallons = 169.9 liters
Formula
1 US gallon = 231 in3 = 3.78541 L = 4 qt = 8 pt = 16 cups = 128 fl oz
The US gallon is defined as exactly 231 cubic inches. For container capacity, calculate the volume in cubic inches (length x width x height for rectangular containers) and divide by 231. All sub-units follow a doubling pattern: 1 gallon = 4 quarts, 1 quart = 2 pints, 1 pint = 2 cups, 1 cup = 8 fluid ounces.
Worked Examples
Example 1: Aquarium Volume
Problem:Calculate the gallon capacity of a fish tank measuring 36 x 18 x 16 inches.
Solution:Volume = L x W x H Volume = 36 x 18 x 16 = 10,368 cubic inches Gallons = 10,368 / 231 = 44.88 US gallons Liters = 44.88 x 3.78541 = 169.9 L Weight of water = 44.88 x 8.345 = 374.5 lbs
Result:44.88 US gallons = 169.9 liters
Example 2: Recipe Scaling
Problem:Convert 2.5 gallons to cups and fluid ounces.
Solution:Cups = 2.5 x 16 = 40 cups Fluid ounces = 2.5 x 128 = 320 fl oz Quarts = 2.5 x 4 = 10 quarts Liters = 2.5 x 3.78541 = 9.464 L
Result:2.5 gallons = 40 cups = 320 fl oz
Frequently Asked Questions
How many liters are in a gallon?
One US gallon equals exactly 3.78541 liters. One US gallon is also defined as 231 cubic inches or 128 US fluid ounces. The imperial (UK) gallon is larger at 4.54609 liters. The US gallon originated from the English wine gallon of 231 cubic inches, while the imperial gallon was defined in 1824 as the volume occupied by 10 pounds of water at a specific temperature. For everyday use, remember that 1 gallon is approximately 3.8 liters.
How do I calculate how many gallons a container holds?
For a rectangular container, multiply length by width by height (all in inches) to get cubic inches, then divide by 231 to get US gallons. For example, a container measuring 24 x 12 x 12 inches has a volume of 3,456 cubic inches, which is 3,456 / 231 = 14.96 gallons. For cylindrical containers, calculate cubic inches using pi x radius-squared x height, then divide by 231. Always use interior dimensions for accurate capacity calculations.
What is the relationship between gallons, quarts, pints, and cups?
The US liquid volume system follows a clear hierarchy: 1 gallon = 4 quarts = 8 pints = 16 cups = 128 fluid ounces. Each step doubles: 1 quart = 2 pints, 1 pint = 2 cups, 1 cup = 8 fluid ounces. One fluid ounce equals 2 tablespoons, and 1 tablespoon equals 3 teaspoons. This system makes it easy to scale recipes up or down. Memorize the key conversion: 1 gallon = 16 cups, and you can derive all other relationships.
How much does a gallon of water weigh?
One US gallon of water weighs approximately 8.345 pounds (3.785 kilograms) at room temperature (about 62 degrees Fahrenheit or 17 degrees Celsius). The exact weight varies slightly with temperature because water density changes. At its densest point (39.2 degrees F or 4 degrees C), a gallon weighs about 8.35 pounds. The old saying goes, a pint is a pound the world around, which is a reasonable approximation since a pint of water weighs about 1.04 pounds.
How do you work out the gallon capacity of a round swimming pool?
Work in feet and use the fact that one cubic foot holds 7.48052 US gallons, since 1,728 cubic inches divided by 231 gives that figure. For a circular pool, volume equals pi x radius squared x average depth. An 18 foot diameter pool with an average depth of 4 feet holds pi x 9 squared x 4 = 1,017.9 cubic feet, which is about 7,614 gallons. Use average depth for sloped floors rather than the deep end alone.
What is a US dry gallon and when is it used?
The dry gallon is a separate measure of 268.8025 cubic inches, about 4.405 litres, compared with the liquid gallon's 231 cubic inches. It comes from the Winchester bushel, which is eight dry gallons, and survives mainly in United States agricultural trade for grains, berries, and other produce sold by volume. Gallon Calculator works entirely in liquid gallons, so do not apply its figures to produce quoted in dry measure.
Why is one US gallon equal to 0.832674 imperial gallons?
Because the two gallons are defined from different historical standards: 3.78541 litres for the US gallon against 4.54609 litres for the imperial gallon. Dividing gives the 0.832674 ratio shown in the results, and the inverse is about 1.2009. This is the reason UK fuel economy figures look flattering next to US ones for the same car, since miles per imperial gallon are roughly 20 percent higher than miles per US gallon.
Is a UK fluid ounce the same size as a US fluid ounce?
No, and the relationship is counter-intuitive. A US gallon divides into 128 fluid ounces of about 29.57 mL each, while the larger imperial gallon divides into 160 fluid ounces of about 28.41 mL each. So the imperial gallon is bigger but the imperial ounce is smaller. Recipes and drinks measures crossing the Atlantic therefore need converting ounce by ounce, not just by scaling the gallon.
How do you convert gallons into cubic feet for tank and delivery specifications?
Divide gallons by 7.48052, or multiply cubic feet by that same number to go the other way. A standard 55 gallon drum occupies 55 / 7.48052 = 7.35 cubic feet of liquid volume. Freight quotes, storage footprints, and gas or vapour space calculations are usually stated in cubic feet, so this step comes up whenever you move from what a vessel holds to how much room it needs.
Why should you use interior dimensions and leave headspace when sizing a container?
The length, width, and height entered here give the geometric volume of the box, so measuring the outside adds the wall thickness on all six faces and overstates capacity. Anything placed inside also displaces liquid: substrate, rock, and equipment in an aquarium, or a pump and fittings in a sump. Practical fill level is lower still, because tanks need freeboard for thermal expansion, sloshing, and a working margin below the rim.
References
Reviewed for accuracy by Manoj Kumar, Mathematics Educator · Editorial policy
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