What each Volume unit is
Millilitre ml
A millilitre is exactly one cubic centimetre, which is why medicine and laboratory work quote both and mean the same thing.
Litre l
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.
US fluid ounce fl oz
A US fluid ounce is 29.57 ml; a UK one is 28.41 ml. They are different units with the same name, and recipes rarely say which they mean.
US cup cup
A US customary cup is 236.6 ml, but American nutrition labels use a "legal" cup of exactly 240 ml, and a metric cup in Australia is 250. A recipe scaled up tenfold will show which one it assumed.
US pint pt
A US pint is 473 ml and an imperial pint is 568 ml — a fifth larger. This is the single most consequential difference between the two systems for anybody following a recipe or ordering a drink.
US quart qt
A quart is a quarter of a gallon, which is where the name comes from. Like the pint and the gallon it carries the US-versus-imperial difference with it.
US gallon gal
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.
Notes on the common Volume conversions
cups to fluid ounces
A cup is eight fluid ounces by definition, not by measurement
The US customary cup is defined as one sixteenth of a US gallon, and the US fluid ounce as one hundred and twenty-eighth of the same gallon. Eight fluid ounces to the cup therefore falls out of the definitions rather than being measured, which makes this one of the few conversions on this site with no rounding anywhere in it. Two cups is 16 fl oz, three is 24, and there is no decimal to lose.
The gallon those definitions sit on is itself exact: 231 cubic inches, a figure inherited from an English wine gallon and fixed in metric terms once the inch became exactly 25.4 mm. So a cup is exactly 236.5882365 ml, and the exactness runs all the way down. Everything difficult about this conversion happens above the arithmetic, in what the reader is holding.
cups to gallons
Sixteen cups to a gallon, and the two rungs in between
The chain is 2 cups to a pint, 2 pints to a quart, 4 quarts to a gallon, which multiplies to 16. Written that way the number is obvious; recalled cold it is not, because the gallon has three plausible small numbers attached to it — 4 quarts, 8 pints, 16 cups — and they are all correct about different units. Picking the wrong one halves or doubles the answer with nothing to signal the error.
The useful landmarks are the quarters. Four cups is a quart, eight is a half gallon, twelve is three quarters, sixteen is a gallon. Any cup count that is a multiple of four converts to a clean fraction of a gallon, and any that is not will produce a sixteenth — 10 cups is 0.625 gallons, which is a number nobody can act on. Restating it as 2½ quarts is what a cook actually needs.
cups to litres
Cup counts reach litre quantities faster than they read
A cup sounds like a small amount and a recipe rarely contains one. Four cups is 946 ml, eight is 1.89 litres, twelve is 2.84 and sixteen is 3.785 — a full US gallon. A stock recipe, a punch, a brine or a batch of soup crosses into litre territory within the first few lines, and the total is the number that decides what equipment the job needs.
The customary ladder is what makes the counting easy in the original units and awkward in metric. Two cups to a pint, four to a quart, sixteen to a gallon: every step is exact and every step lands on a container an American shop sells. Convert the same ladder and it becomes 0.473, 0.946 and 3.785 litres, three numbers that correspond to nothing on a metric shelf.
cups to ml
The cup converts exactly, and the flour does not
A US customary cup is 236.5882365 ml, and that figure is exact. What is not exact is what a cup of an ingredient contains: flour spooned loosely into a cup and levelled weighs around 120 g, and flour scooped straight from the bag and packed weighs closer to 150 g. Same cup, same flour, a quarter more of it.
That is the number worth carrying away from this page. Every unit question here — customary against legal, American against metric, 236.59 against 250 — lives inside six per cent, and the way the flour got into the cup is worth three times as much. Converting the volume precisely and then filling the cup carelessly is precision applied at the wrong end.
cups to pints
Two cups to the pint, and why the answer is usually a half
The factor is two, so the conversion is a halving and the interesting part is what the halving produces. Even cup counts give whole pints — 2 cups is a pint, 4 is two, 8 is four and also a half gallon. Odd counts always give a half, because there is nothing between a cup and a pint to absorb the remainder: 3 cups is 1.5 pints, 7 cups is 3.5, and no amount of restating turns either into a whole number.
That is why cup counts in recipes cluster on even numbers. A recipe asking for 2 or 4 cups of stock was written by somebody thinking in containers; one asking for 3 was written by somebody thinking in ratios. Neither is wrong, but the second will always leave part of a pint in the fridge, and it is worth knowing that before shopping rather than after — particularly for cream, which does not keep long once the carton is open.
cups to quarts
Counting cups up to a quart, two rungs at once
Four cups to a quart is a single number hiding two steps. Two cups make a pint, two pints make a quart, and the pint in the middle is the rung that vanishes because no American kitchen owns a pint measure — it is a container size and a glass, not a thing anybody fills. Every error in this conversion is the same error: one of those two doublings goes missing, and the batch comes out at twice or half the intended size.
The fix is to hold the ladder as a whole rather than the single factor. Eight fluid ounces to a cup, 16 to a pint, 32 to a quart, 128 to a gallon. Once those four numbers are in place, four cups to a quart stops being a fact to remember and becomes something that can be rebuilt in a second, which is what makes it survive a distracted afternoon in a kitchen.