Liters to Gallons

To go from liters to US gallons, divide by 3.785: 10 L ÷ 3.785 ≈ 2.64 gal. For the reverse, multiply by 3.785. The US gallon is defined as exactly 231 cubic inches. Because the inch is itself exactly 2.54 cm, that works out to exactly 3.785411784 liters with no rounding anywhere in the chain. A liter, in turn, is exactly one cubic decimeter — a cube 10 cm on a side. Useful anchors: 1 L ≈ 0.264 gal; 4 L ≈ 1.06 gal (a 4-liter jug is a hair more than a gallon); 20 L ≈ 5.28 gal; 50 L ≈ 13.2 gal; 60 L ≈ 15.9 gal. And in reverse: 1 gal = 3.79 L; 5 gal = 18.9 L; 15 gal = 56.8 L.

The US gallon used here is 3.785411784 L. The imperial gallon used in the UK and some Commonwealth countries is 4.54609 L — about 20% larger. The split is historical rather than logical. The US kept the older English wine gallon of 231 cubic inches after independence; Britain redefined its gallon in 1824 around the volume of ten pounds of water and ended up with a bigger unit. Neither side changed course, so the same four-letter word now names two different volumes. The practical consequence is that any gallon figure crossing the Atlantic has to be checked. A UK car review claiming 40 MPG is only 33.3 US MPG for the identical car burning the identical fuel — the car did not change, the gallon did. Multiply imperial MPG by 0.8327 to get US MPG, or divide by that to go back.

These are not two scales of the same thing. MPG measures distance per volume; L/100 km measures volume per distance. They are reciprocals, so the conversion is a division, not a multiplication: L/100 km = 235.2 ÷ MPG (US), and the same formula run backwards. So 20 MPG is 11.8 L/100 km; 30 MPG is 7.8; 40 MPG is 5.9; 50 MPG is 4.7. If the figure is imperial, the constant is 282.5 instead of 235.2. The reciprocal relationship has a consequence most drivers get wrong: MPG gains are not linear. Going from 10 to 20 MPG saves 5 gallons per 100 miles. Going from 30 to 40 MPG — the same 10 MPG on paper — saves only 0.83 gallons per 100 miles, six times less fuel. Replacing an old truck with a merely average one saves far more fuel than upgrading an already efficient car, and MPG hides that. L/100 km does not, which is one reason regulators outside the US prefer it.

The gallon sits at the top of a whole family, and the US and imperial versions of every member disagree. On the US side: 1 gallon = 4 quarts = 8 pints = 16 cups = 128 fluid ounces. That makes a US quart 946 mL, a US pint 473 mL, a US cup 237 mL and a US fluid ounce 29.5735 mL. On the imperial side: 1 gallon = 4 quarts = 8 pints = 160 fluid ounces, because an imperial pint holds 20 fluid ounces rather than 16. An imperial pint is 568 mL against the US 473 mL — which is why a pint of beer in London is visibly larger than a pint in Boston, roughly 20% more for the same word. One more trap: the imperial fluid ounce (28.4131 mL) is *smaller* than the US one (29.5735 mL), even though every larger imperial unit is bigger. The ounce counts differ enough to flip the direction.

Car fuel tanks are the most common lookup: a compact car at 45 L holds 11.9 gal, a midsize sedan at 60 L holds 15.9 gal, and a full-size pickup at 100 L holds 26.4 gal. Filling a rental abroad, a 40-liter fill is about 10.6 gallons. Aquariums are sold by nominal gallons in the US and by liters almost everywhere else, and the numbers rarely line up: a 20-gallon tank is 75.7 L, a 55-gallon is 208 L, and a 75-gallon is 284 L. Bear in mind that the labeled size is the empty glass volume — substrate, rock and displacement typically leave 10 to 15% less actual water, which matters when dosing anything by volume. In the kitchen, the units that appear are the small ones: a US gallon of milk is 3.79 L, a half gallon is 1.89 L, and a quart is 946 mL, which is close enough to a liter that recipes often treat them as interchangeable — a 5.4% substitution error that bread and stock will forgive and candy will not.