Qu'est-ce qu'un/une US Gallon (gal) ?
The US gallon (symbol: gal) is a unit of volume in the United States customary system of measurement. One US gallon is defined as exactly 231 cubic inches, which equals exactly 3.785411784 liters. The US gallon is divided into 4 US quarts, 8 US pints, 16 US cups, or 128 US fluid ounces.
Comparison with the Imperial Gallon
The US gallon is distinct from the imperial (UK) gallon, which is defined as exactly 4.54609 liters. The US gallon is approximately 83.27% of the size of an imperial gallon, a difference of roughly one-fifth. This difference is a frequent source of confusion in international contexts and is critically important in trade, fuel economy comparisons, and any situation where volumes are exchanged between American and British or Commonwealth systems.
The US gallon is used exclusively in the United States for measuring liquid volumes in commerce, fuel sales, and everyday use. It should not be confused with the US dry gallon (approximately 4.405 liters), which is a separate unit historically used for measuring grain and other dry commodities, though it is rarely encountered today. In practice, when Americans refer to a "gallon" without qualification, they always mean the US liquid gallon of 231 cubic inches.
The Geometric Origin of 231 Cubic Inches
The gallon's position as 231 cubic inches — seemingly an arbitrary number — actually derives from the dimensions of a specific cylindrical measure: a cylinder 7 inches in diameter and 6 inches deep holds approximately 231 cubic inches (7² × π/4 × 6 ≈ 230.9 cubic inches, rounded to 231). This geometric origin gives the gallon an elegant physical definition that was practical for manufacturing standard measuring vessels.
Etymology
French and Latin Roots
The word "gallon" has a complex etymological history with roots in medieval French and Latin. The most widely accepted origin traces it to Old Northern French "galon" or "jalon," meaning a measure of liquid. Some scholars connect it to Medieval Latin "galona" or "galleta," a term for a pail or bucket used as a liquid measure. Another proposed etymology links it to the Gaulish (Celtic) root "galla," though this connection is less certain.
Entry into English
The word entered Middle English as "galoun" or "galun" in the 13th century, appearing in trade records and legal documents related to the wine and ale trades. By the 14th century, "gallon" had become the standard English spelling. The term was used specifically in the context of liquid measurement for commercial transactions, particularly in the wine trade between England and France.
A Merchant's Unit
Interestingly, the word "gallon" gave rise to no widely used derivatives or compound words in English beyond the unit itself and its multiples (half-gallon, gallon jug). However, the concept it represents — a standard vessel for liquid trade — reflects the medieval European practice of defining volume units by the capacity of common containers. The gallon was always a practical merchant's unit rather than a scientific one, and its etymology reflects this commercial heritage.
Precise Definition
Exact Metric Equivalent
The US liquid gallon is defined as exactly 231 cubic inches. Using the international inch of exactly 25.4 millimeters, this yields a precise metric equivalent: 1 US gallon = 231 × (25.4 mm)³ = 231 × 16,387.064 mm³ = 3,785,411.784 mm³ = 3.785411784 liters exactly. This value is exact, not an approximation, because it derives from the exact definition of the inch.
Subdivisions
The US gallon is subdivided into smaller units in a consistent system: 1 US gallon = 4 US quarts = 8 US pints = 16 US cups = 32 US gills = 128 US fluid ounces. Each US fluid ounce is exactly 1.8046875 cubic inches or approximately 29.5735295625 milliliters. These subdivisions are all exact fractions of the 231-cubic-inch gallon.
US Gallon vs. Other Gallons
The US liquid gallon must be distinguished from two other gallon measurements. The US dry gallon, defined as 1/8 of a US bushel or approximately 4.40488 liters, was historically used for measuring grain and dry commodities but is rarely used today. The imperial (UK) gallon is defined as exactly 4.54609 liters, making it approximately 20.1% larger than the US liquid gallon. The US gallon is approximately 83.27% of the volume of an imperial gallon. This difference is significant and has practical consequences in international trade, fuel economy comparisons, and recipe conversions.
Histoire
Medieval English Gallons
The US gallon descends from the English wine gallon, one of several gallon measures that coexisted in England for centuries. Medieval England used multiple gallon standards: the wine gallon for wine and spirits, the ale gallon for beer, and the corn gallon for grain. These different gallons reflected the commercial importance of distinguishing between different commodities in trade.
The wine gallon's size varied over the centuries, but it was formalized at 231 cubic inches by an Act of Parliament in 1706 during the reign of Queen Anne. The 231-cubic-inch definition was based on a cylindrical container approximately 7 inches in diameter and 6 inches deep. This standardization was part of a broader effort to regularize weights and measures in English commerce.
American Independence and Retention
When the American colonies gained independence, they retained the English wine gallon of 231 cubic inches as their standard liquid gallon. This was natural since American commerce, infrastructure, and daily life had been built around English measurement standards for over 150 years. The choice to keep the wine gallon rather than the ale gallon or corn gallon reflected the wine gallon's dominant role in general liquid commerce.
The 1824 UK Reform and the US-UK Divergence
Meanwhile, in 1824, the United Kingdom undertook a major reform of its measurement system. The Weights and Measures Act of 1824 swept away the multiple competing gallon standards and replaced them all with a single new imperial gallon, defined as the volume of 10 pounds of distilled water weighed in air at 62 degrees Fahrenheit with the barometer at 30 inches of mercury. This yielded a gallon of approximately 277.42 cubic inches or 4.546 liters — substantially larger than the 231-cubic-inch wine gallon retained by the United States.
This 1824 reform created the lasting divergence between US and British gallons. Before 1824, Americans and Britons had used the same wine gallon. After 1824, the UK's imperial gallon was about 20% larger than the US gallon. The divergence caused immediate problems in transatlantic trade and has been a source of confusion ever since.
Metrication and Standardization
The US formally standardized its gallon through the work of the Office of Standard Weights and Measures (later the National Bureau of Standards, now NIST). The Mendenhall Order of 1893 tied US customary units to metric standards by defining the meter and kilogram as the fundamental standards from which all US customary units would be derived. This established the exact metric equivalent of the US gallon through the chain: 1 gallon = 231 cubic inches, 1 inch = 25.4 mm (after 1959), therefore 1 gallon = 3.785411784 liters.
The Petroleum Industry and Modern Persistence
The petroleum industry has been a major driver of gallon usage throughout American history. When the modern oil industry began in Pennsylvania in the 1860s, oil was initially measured in various barrel sizes. The 42-gallon barrel (approximately 159 liters) was adopted as the standard petroleum barrel, a convention that persists worldwide today. Gasoline retail sales in gallons became standard as the automobile transformed American life in the early 20th century.
Despite periodic metrication efforts — including the Metric Conversion Act of 1975 and the Omnibus Trade and Competitiveness Act of 1988 — the US gallon remains firmly entrenched in American commerce. Unlike many measurement units that were displaced by metrication in other English-speaking countries, the gallon's role in fuel sales, beverage containers, and household measurement ensures its continued dominance in American daily life.
Utilisation actuelle
Fuel and Automotive
The US gallon is the standard unit of liquid volume measurement in the United States, touching nearly every aspect of daily commerce and household life. Gasoline is sold by the gallon at approximately 150,000 filling stations across the country, with prices posted per gallon on signs visible from the road. The Environmental Protection Agency (EPA) rates vehicle fuel economy in miles per US gallon (mpg), a figure that appears on every new car's window sticker and is a key factor in consumer purchasing decisions.
Beverages and Water
Milk, water, juice, and other beverages are commonly sold in gallon and half-gallon containers in American grocery stores. The iconic plastic milk jug holding one US gallon is a fixture of American refrigerators. Water consumption is measured and billed in gallons by municipal water utilities — the average American household uses approximately 300 gallons (1,136 liters) of water per day. Swimming pools are measured in gallons (a typical residential pool holds 15,000-30,000 gallons), and water heater capacity is specified in gallons (40-gallon and 50-gallon models are most common).
Household and Commercial Use
In household use, the gallon serves as the standard unit for paint (sold in gallon and quart cans, with one gallon covering approximately 350-400 square feet of wall area), cleaning solutions, and gardening supplies. Aquarium sizes are specified in gallons (10-gallon, 20-gallon, 55-gallon tanks are standard sizes). Cooking and food service use cups, pints, quarts, and gallons as the standard volume progression.
International Context
The US gallon is not used outside the United States in any official capacity. Canada, which once used the imperial gallon, adopted the liter for fuel sales and general liquid measurement during metrication in the 1970s and 1980s. The distinction between the US gallon and the imperial gallon is critically important in the petroleum industry, where pricing, volume calculations, and trade agreements must clearly specify which gallon is meant. A barrel of crude oil is 42 US gallons (approximately 159 liters or 35 imperial gallons), and this standard is used in global oil trading regardless of the buyer's or seller's country.
Everyday Use
Gasoline and the Economy
The gallon is deeply woven into American daily life in ways that extend far beyond simple measurement. Gasoline purchases are perhaps the most visible gallon-based transaction: the average American driver buys approximately 500 gallons of gasoline per year, and gas prices — expressed as dollars per gallon — are a closely watched economic indicator. The phrase "gas is $3.50 a gallon" is universally understood in the US, and gas station price signs displaying per-gallon prices are an iconic element of the American roadside landscape.
Kitchen and Grocery
In the kitchen and grocery store, the gallon and its subdivisions define how Americans think about liquid volumes. Recipes use cups (1/16 gallon), pints (1/8 gallon), and quarts (1/4 gallon) as standard measures. A gallon of milk is the standard large-format purchase for families, while single people might buy a half-gallon. Orange juice, water, iced tea, and other beverages come in gallon jugs. The informal "gallon challenge" — attempting to drink a gallon of milk in one hour — is a well-known (and usually unsuccessful) dare, since the human stomach typically holds only about 1/4 to 1/3 of a gallon.
Home Improvement
Home improvement projects frequently involve gallon calculations. A gallon of interior latex paint covers approximately 350-400 square feet, so painting a typical room (about 400-500 square feet of wall area) requires just over one gallon. Stain, varnish, and primer are similarly sold in gallon containers. Pool chemicals are dosed per thousand gallons of water, and lawn and garden products specify application rates in ounces per gallon of water.
Water Conservation
Water usage awareness has made the gallon a unit of environmental consciousness. Americans learn that a 5-minute shower uses approximately 10-25 gallons of water (depending on the showerhead), a toilet flush uses 1.6 gallons (modern low-flow) to 3.5 gallons (older models), and a running kitchen faucet flows at about 2 gallons per minute. These gallon-based figures are central to water conservation messaging in the United States.
In Science & Industry
Petroleum Industry
In the petroleum industry, the gallon and its derivative unit the barrel (42 US gallons) are fundamental to global commerce. Crude oil production is measured in barrels per day worldwide, with the 42-gallon barrel serving as the international standard regardless of a country's measurement system. The Organization of the Petroleum Exporting Countries (OPEC) sets production quotas in barrels, oil futures on the New York Mercantile Exchange (NYMEX) are priced per barrel, and refinery capacity is measured in barrels per day. A typical US refinery processes 100,000-500,000 barrels (4.2-21 million gallons) of crude oil per day.
Chemical Engineering and Water Treatment
In chemical engineering and industrial processing within the United States, volumes are commonly specified in gallons for tanks, vessels, and flow rates. Chemical storage tanks are rated in gallons, with common industrial sizes including 275-gallon totes (intermediate bulk containers), 330-gallon tanks, and multi-thousand-gallon storage vessels. Flow rates in US industry are often expressed in gallons per minute (GPM) or gallons per hour (GPH). Municipal water treatment plants process millions of gallons per day (MGD), with a large facility treating 100-500 MGD.
Environmental Regulation
Environmental science and regulation in the United States use gallons extensively. The EPA regulates underground storage tanks (USTs) for petroleum products based on gallon capacity. Spill reporting thresholds are stated in gallons — for example, any oil spill of 42 gallons or more that reaches navigable waters must be reported. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or "Superfund") uses gallons and pounds as its standard units for reportable quantities.
Brewing and Winemaking
In brewing and winemaking, the gallon serves as a traditional and legal unit. Federal regulations from the Alcohol and Tobacco Tax and Trade Bureau (TTB) define beer barrel as 31 US gallons, and excise taxes are calculated per barrel. Craft breweries measure production in barrels per year, with the legal threshold for "small brewery" tax benefits set at certain barrel-per-year limits. Home brewing recipes in the US are typically scaled to 5-gallon batches, the standard size for a home brewing carboy.
Interesting Facts
The US gallon (3.785 liters) is approximately 20% smaller than the imperial gallon (4.546 liters). This means a car rated at 30 miles per US gallon would achieve approximately 36 miles per imperial gallon — the exact same fuel consumption expressed with different gallons.
The 42-gallon petroleum barrel became the standard in the 1860s because it was the size of the old English wine tierce (a cask containing 42 gallons of wine). Oil producers in Pennsylvania adopted existing barrel sizes rather than creating new ones.
A US gallon of water weighs approximately 8.34 pounds (3.78 kg) at room temperature. The old saying 'a pint's a pound the world around' is only an approximation: a US pint of water actually weighs about 1.04 pounds.
The average American household uses about 300 gallons (1,136 liters) of water per day, with the largest uses being toilet flushing (approximately 24%), showering (20%), and faucet use (19%), according to the EPA WaterSense program.
It takes approximately 1,000 gallons (3,785 liters) of water to produce one gallon (3.785 liters) of milk, accounting for the water needed to grow feed crops, hydrate cows, and process the dairy product.
The US produces approximately 370 million gallons (1.4 billion liters) of gasoline per day, and the country's Strategic Petroleum Reserve can hold up to 714 million barrels (approximately 30 billion gallons) of crude oil in underground salt caverns along the Gulf Coast.
A standard Olympic swimming pool holds approximately 660,430 US gallons (2,500,000 liters) of water. The popular comparison 'Olympic swimming pools' has become an informal unit for communicating large volumes to the public.
The difference between US and imperial gallons has caused real-world problems: in 1983, Air Canada Flight 143 ran out of fuel mid-flight partly because of a confusion between pounds, kilograms, and the different gallon sizes used in fueling calculations during Canada's metric transition.
The human body contains approximately 10-12 gallons (38-45 liters) of water, accounting for about 60% of an adult's body weight.
US federal law requires gasoline pumps to be accurate to within 6 cubic inches per 5 gallons dispensed — a tolerance of approximately 0.05%, or about 3 tablespoons per 5 gallons.
Regional Variations
US Gallon vs. Imperial Gallon
The most significant regional variation in gallon measurements is the fundamental difference between the US gallon and the imperial gallon. The US gallon (231 cubic inches, 3.785 liters) is used only in the United States, while the imperial gallon (277.42 cubic inches, 4.546 liters) was historically used in the United Kingdom and throughout the British Commonwealth. The imperial gallon is approximately 20.1% larger than the US gallon — a difference large enough to cause serious confusion in trade, cooking, and fuel economy comparisons.
Canada's Transition
Canada presents an interesting transitional case. Before metrication in the 1970s and 1980s, Canada used the imperial gallon. Older Canadians remember buying gasoline by the imperial gallon, and some vintage Canadian cookbooks use imperial measurements. During the transition, Canada switched directly from imperial gallons to liters — it never used US gallons. Today, all Canadian fuel is sold in liters, and younger Canadians have no practical experience with gallons of any kind. However, proximity to the United States means that Canadians regularly encounter US gallon measurements in American media, products, and cross-border shopping.
Caribbean and Latin America
The Caribbean and Latin American countries near the United States show mixed gallon usage. Puerto Rico and the US Virgin Islands, as US territories, use the US gallon. Some Caribbean nations that were historically British colonies may still informally reference imperial gallons in older contexts, though most have adopted metric measures. Belize and some Caribbean islands still use imperial gallons for fuel sales. In Central and South America, the metric system prevails, but gallons appear in the petroleum industry due to American influence — Venezuelan oil production, for instance, is reported in US barrels (42 US gallons each) for international markets.