One must be careful and precise here, because a gallon is not necessarily a gallon. There are two primarily used gallons defined—the British imperial gallon and the US customary gallon. The latter is used only for fluids; historically there has been some reference to a dry US gallon (which is of yet a different size), but it is not mentioned in NIST Handbook 44 Appendix C (2017) and seems to have gone by the wayside, so I will not include that one.
The British imperial gallon is defined as: 1 [imp] gal = 4.54 609 L.
The US customary (liquid) gallon is defined as 1 [US] gal = 231 in³. Since
1 in = (1/36) yd and 1 yd = 0.9144 m, we have 1 in = 0.0254 m = 0.254 dm.
Thus, 1 [US] gal = 231 × (0.254 dm)³ = 3.785 411 784 dm³ = 3.785 411 784 L since 1 L = 1 dm³.
All definitions are, by definition, exact, and all calculations have been done exactly. Therefore, the bolded conversions are exact values—no rounding. You might need to know the exact value to avoid calculation conflicts, or you may be handling measurements with nowhere near that level of precision (or accuracy) so you may wish to round for your practical use the values I have provided. I provide all of the digits so you can make the choice instead of me making it for you how many digits you need.
The gallon (gal) is a unit of measuring for measuring liquid capacity. It is used in both US customary and British imperial systems of measurements. There are three different sizes of gallons used today. The imperial gallon is 4.54 liters and common in the Commonwealth states and some Caribbean nations. The US gallon is about 3.785 liters and common in the United States and Latin America. The U.S dry gallon is about 4.405 Litres or 1⁄8 US bushel.
The Imperial Gallon
The imperial or UK gallon is a unit of measurement exactly 4.54609 liters or 277.42 cubic inches. It is common in the Commonwealth countries and some Caribbean states. The imperial gallon was initially based on 10 pounds which translates to 4.54kg of water at 17 °C. The imperial fluid ounce weighs 1⁄160 of the imperial gallon. An imperial gallon is divided into four quarts, each quart is consisting of two pints, and each pint consists of 20 imperial fluid ounces.
US Liquid Gallon
One US gallon is defined as 3.7854 liters or 231 cubic inches. At 62°F (17°C), a US liquid gallon of water is equal to 3.78 kgs or 8.34 pounds. It is 16.6% lighter compared to the imperial gallon. However, just like the imperial gallon, a US gallon is divided into four quarts, each quart is divided into two pints, and each pint contains 16 US fluid ounces. Therefore, it takes 128 US fluid ounces to fill a US liquid gallon. It is common to specify the temperature at which the material will weigh or occupy a particular volume. This is done to overcome the change in volume or mass that results from change in temperature. In the US, the weight of alcoholic and petroleum products are defined as 60°F (16°C).
US Dry Gallon
The US dry gallon is equal to an eight of the Winchester bushel, 268.8025 cubic inches, or 4.4.5 liters.
- 1 Gallon [Fluid, US] = 3.7854118 Liters
- 1 Gallon [Dry, US] = 4.4048838 Liters