Convert l to gal

A litre is a thousand cubic centimetres. It is not strictly an SI unit — the SI unit of volume is the cubic metre — but it is accepted for use with SI and is what fuel, drinks and engines are measured in.

A US gallon is defined as 231 cubic inches, exactly — a definition that comes from an English wine gallon of 1707. The imperial gallon is 4.55 litres, about 20% larger.

The factor is 0.264172, and almost nobody carries that around. Rounded to 0.26 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 l.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The US gallon is two different sizes wearing one name. This page uses the US one, which is 3.78541 l; the imperial one still used in the UK and Ireland is 4.54609 l — larger by 20.1 %.

That gap is the awkward size: far too big to ignore in a recipe, and far too small to spot from the answer alone. If your figure came from a British source, take the second number, and note that the fluid ounce is the one unit here where the imperial measure is the smaller of the two.

The two systems each settled on their own convenient containers, and the sizes landed close enough to be confused and far enough apart to matter. A 200-litre drum is 52.8 US gallons and a 55-gallon drum is 208.2 litres. A 20-litre jerrycan is 5.28 gallons where the American equivalent is a five-gallon pail of 18.9 litres. A 1,000-litre tote is 264 gallons and the American one is sold as 275.

None of those pairs is a rounding of the other, which is the thing to hold on to when a catalogue lists both. Where a specification quotes a capacity in gallons and a drawing quotes it in litres and the two do not agree to the litre, the usual explanation is that one figure describes a nominal size and the other describes what the vessel actually holds. Fill levels, not names, are where this becomes somebody’s problem.

Tank volumes in the hobby are gallon figures because the standard glass sizes came out of American manufacturing: 10, 20, 29, 40, 55, 75, 125. Those are 37.9, 75.7, 109.8, 151.4, 208.2, 283.9 and 473.2 litres, and none of them is a number a European retailer would print. A tank sold in Europe as 180 litres is 47.6 gallons and belongs to no American size class at all.

The conversion matters because almost every dosing instruction in the hobby is per gallon — water conditioner per 10 gallons, medication per 20. Getting the tank into gallons first and then dosing is more reliable than converting the dose, because the dose is usually the smaller and more rounded number. Subtract for substrate and rock before dosing, too: a 208-litre tank rarely contains 208 litres of water.

Domestic storage is where metric capacities meet gallon-based rules of thumb. A rain butt at 200 or 250 litres is 52.8 or 66 gallons; a domestic heating oil tank at 1,000 or 1,250 litres is 264 or 330; a hot water cylinder at 150 litres is 39.6. American guidance on sizing any of these is written per gallon, and converting the tank once is easier than converting every rule.

Watch for the imperial gallon in this corner specifically. British heating oil was sold by the imperial gallon for decades and older tank plates, delivery notes and boiler manuals still carry it, which makes a 1,000-litre tank 220 imperial gallons rather than 264. A capacity that comes out about 20% short of what a document claims is the signature of that mismatch rather than a mismeasured tank.

A litre is exactly one cubic decimetre — a cube 10 cm on a side, 1,000 cubic centimetres — and has been since 1964. It is not formally an SI unit, since SI measures volume in cubic metres, but it is accepted for use with SI and is the unit every metric container is labelled in. Its neatness is the point: a litre of water is a kilogram to within a fraction of a per cent, and a cubic metre is exactly 1,000 litres.

The US gallon has none of that. It is 231 cubic inches, a figure carried forward from the English wine gallon standardised in 1707 and kept when the United States declined to follow Britain into the imperial system of 1824. So the gallon subdivides beautifully into quarts, pints and fluid ounces and relates to nothing else, while the litre relates to everything and subdivides into nothing but decimal fractions of itself.

A converted capacity is only as good as the capacity it started from, and container ratings are quoted in at least three ways: nominal size, brimful volume and safe working volume. A "200-litre" drum brims at about 216 litres and is filled to well under its brim for anything that expands. Converting the nominal figure to 52.8 gallons and then filling to 52.8 gallons is how a drum ends up overfull.

The same applies to anything with a vent, a float or a thermal expansion allowance. Where the American document gives a fill limit in gallons and the vessel is labelled in litres, convert the limit rather than the capacity — it is the number with the consequence attached, and it is usually the one the manufacturer actually tested.

A litre is a bit over a quarter of a gallon, so divide by four and add about six per cent. 100 litres is 25 plus 1.5, so about 26.5 gallons against 26.4 exactly. 500 litres is 125 plus 7, about 132 against 132.1. The six per cent comes from 3.785 being that much less than 4, and it is the whole of the correction.

For a rough sanity check without any arithmetic, remember three anchors: 4 litres is a shade over a gallon, 20 litres is a shade over five, and 200 litres is a shade under 55. Anything between them can be interpolated by eye closely enough to tell whether a container is the size a listing claims. The field above is there for when the number has to be defensible rather than approximate.

Three cases justify carrying the full 0.264172 rather than a rounded quarter: anything charged by volume, anything mixed to a ratio, and anything with a regulatory limit. Water billed per thousand gallons, a chemical dosed at so many ounces per gallon, and a storage threshold that triggers a permit are all places where a four per cent slip in the conversion changes the answer to a question somebody else is asking.

Everywhere else, the container is far less precise than the conversion. A drum, a butt or a tank is a moulded object with a tolerance of a per cent or two, and quoting its capacity to four decimal places implies a measurement that nobody made. Convert exactly, then round to the precision the object actually has, and say which of the two gallons you meant.