Gallons to Litres Calculator

Gallons to Litres Conversion Table (1–20 Gallons)

1 UK gallon = 4.54609 litres exactly (Weights and Measures Act 1985). 1 US gallon = 3.785411784 litres exactly.

UK Gallon vs US Gallon: Why the Difference?

The UK imperial gallon and the US customary gallon are both called "gallons" but measure very different volumes. The UK gallon contains 4.54609 litres, while the US gallon holds only 3.78541 litres — a difference of approximately 20%. This gap has caused confusion in fuel economy comparisons, recipe conversions, and international trade for over two centuries.

The divergence occurred after American independence. Britain standardised its weights and measures in the Weights and Measures Act 1824, defining the imperial gallon as the volume of exactly 10 pounds of distilled water at 62°F (16.67°C) under standard pressure. This yielded 277.419 cubic inches, or 4.54609 litres.

The United States, meanwhile, retained the older Queen Anne wine gallon of 231 cubic inches (3.785 litres) that had been in use since the early 18th century. As a result, the same word — "gallon" — refers to substantially different volumes depending on whether you are in London or New York.

UK Petrol and Fuel: From Gallons to Litres

Until 1 January 1994, petrol and diesel were sold in imperial gallons at UK forecourts. The switch to litres was mandated by EU Directive 80/181/EEC on units of measurement. Many British drivers who remember the transition recall prices like 44p per litre appearing suddenly cheaper than the £2 per gallon equivalent they were used to seeing.

Despite selling fuel in litres, the UK still widely uses miles per gallon (MPG) as the standard fuel economy metric. This creates an interesting anomaly: consumers buy in litres but measure efficiency in gallons. When comparing a UK car's MPG with a US car's MPG, remember:

• UK MPG uses imperial gallons (4.546 L) — so 40 UK MPG = 7.06 L/100km
• US MPG uses US gallons (3.785 L) — so 40 US MPG = 5.88 L/100km
• A car rated 40 UK MPG would achieve approximately 33 US MPG — they are not equivalent

This is why American cars often appear to have better fuel economy than their UK equivalents when comparing raw MPG figures. The UK imperial gallon gives a larger number because it measures a larger volume.

Car Fuel Tank Sizes in Gallons and Litres

UK car fuel tanks are now specified in litres, but many drivers still think in gallons. Here is a practical reference for common vehicle tank sizes:

Pints to Gallons: UK Brewery Measurements

The UK brewing industry historically used a complex hierarchy of vessel sizes, all built on the gallon and pint. Although commercial breweries now work predominantly in metric, traditional cask ale sizes are still described in these imperial terms:

• Pin: 4.5 gallons = 20.46 litres = 36 UK pints
• Firkin: 9 gallons = 40.91 litres = 72 UK pints
• Kilderkin: 18 gallons = 81.83 litres = 144 UK pints
• Barrel: 36 gallons = 163.66 litres = 288 UK pints
• Hogshead: 54 gallons = 245.49 litres = 432 UK pints
• Butt: 108 gallons = 490.98 litres = 864 UK pints

The CAMRA (Campaign for Real Ale) organisation still uses these traditional measures. The firkin of 9 gallons is the most common cask size for real ales delivered to UK pubs, yielding 72 pints — enough for a moderate evening's trade.

The UK Weights and Measures Act 1985

The Weights and Measures Act 1985 is the primary legislation governing UK measurements. It formally defined the UK gallon as exactly 4.54609 litres — a definition that remains in force today. The Act was updated following the 1985 European Communities Act requirements, but the imperial gallon definition itself remained unchanged from the 1963 Act.

Importantly, the 1985 Act mandated metric measurements for most retail sales. From 1995, goods sold loose (such as fruit, vegetables, and petrol) had to be priced by metric weight or volume. The gallon ceased to be a permitted unit for petrol sales from January 1994, one year before the broader metrication of retail goods.

However, imperial units remain legal as supplementary units alongside metric, and the gallon is still widely used informally for fuel economy discussions, boating and maritime, and agricultural measurements.

Swimming Pools, Water Tanks, and Rainfall in Gallons

Large volume measurements are one area where gallons remain practically useful for estimation. Here are common reference points:

• Standard swimming pool (25m × 10m × 1.4m average): approximately 350,000 litres = 77,000 UK gallons
• Garden water butt (typical 210 L): approximately 46 UK gallons
• Domestic hot water cylinder (180 L): approximately 40 UK gallons
• 1 inch of rainfall on 1 acre: approximately 22,650 UK gallons (102,790 litres)
• Olympic swimming pool (50m × 25m × 2m): 2.5 million litres = 549,800 UK gallons

In the UK, domestic water usage is sometimes quoted in gallons by older consumers and plumbers. Average UK household water consumption is approximately 330 litres per day, or about 73 UK gallons per day per household.

US vs UK Fuel Economy: The MPG Confusion

The MPG confusion extends to car advertising and reviews. When a British car magazine tests a vehicle and quotes 50 MPG, they mean 50 miles per UK imperial gallon. When a US magazine quotes the same model at 42 MPG, they mean 42 miles per US gallon. The actual fuel efficiency of the car is the same; only the gallon size differs.

To convert between UK and US MPG:

• UK MPG × 0.8327 = US MPG (UK figure is always higher)
• US MPG × 1.2009 = UK MPG (US figure is always lower)

This distinction matters enormously for anyone importing a US vehicle to the UK, reading American motoring reviews, or comparing fuel costs between the two countries. A US gallon costs less to fill than a UK gallon simply because it contains less fuel.

Gallons in Agriculture and Irrigation

UK agriculture has been slower to fully metricate than retail. Many farmers and agricultural suppliers still quote quantities in gallons, particularly for:

• Crop spraying: spray rates may be quoted in gallons per acre alongside litres per hectare
• Livestock water consumption: a dairy cow drinks approximately 30 UK gallons (136 litres) per day
• Irrigation rates: smaller operations may use gallons per hour for pump ratings
• Milk production: British dairy farmers occasionally reference gallons per cow per day (average 26 litres = 5.7 gallons daily per cow)

Agricultural chemical dilution rates are typically given in both metric and imperial by UK suppliers, as older farm equipment may have imperial-calibrated sprayers.

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