How this instrument works
A US liquid gallon is exactly 231 cubic inches, a figure inherited from the wine gallon that Parliament standardised under Queen Anne in 1707 and that the young United States kept after Britain discarded it. The pint is one-eighth of that: 28.875 cubic inches, or 473.176473 millilitres precisely, once the inch was pinned to 25.4 mm by the 1959 international yard and pound agreement.
That eight is why this factor is 0.125 with nothing hiding after it. A pint is not weighed or measured against its gallon; it is defined as a fraction of one, so the conversion is a division by eight and carries no uncertainty whatever. Usefully, the same ratio survives the Atlantic crossing: an imperial gallon also holds eight imperial pints, though each of those is 568.26125 mL, roughly one-fifth larger than its American cousin. The ratio travels; the units do not.
Where you meet them varies by trade. American dairy cases climb pint, quart, half-gallon, gallon, and supermarket ice cream is sold by the pint almost by convention. Donated units of whole blood sit near one. Paint, wood stain, and motor oil still ship in pint tins across the States, while brewers, cider makers, and anyone scaling up homebrew recipes switch to gallons the moment batches outgrow several bottles.
- Type your volume into the Pints (pt) field — 16 is preloaded, one case of pint cartons.
- Read your answer from the US gallons (gal) line; it recomputes as you type, with no button to press.
- Working in imperial pints? The same 0.125 applies, but your result is imperial gallons of 4.54609 L each, not US gallons of 3.785411784 L.
- Need other measures? Halve your pints figure for US quarts, or multiply it by 16 for US fluid ounces.
Worked example — bottling a two-gallon batch
A cider maker racks her finished batch into 16 standard US pint bottles and wants the batch size for her log book. Entering 16 in Pints (pt) returns 2.0 in US gallons (gal): 16 × 0.125, landing dead on because eight pints make one gallon and 16 is two clean eights.
That volume in other terms: 462 cubic inches, 7.570823568 litres, 256 US fluid ounces. Had those 16 bottles been imperial pints — the British 568.26125 mL sort — you would still read 2 gallons, but imperial ones, and 9.09218 litres of cider instead. Same ratio, different bottles.
Questions
Is 0.125 exact, or has it been rounded?
Exact. A US liquid pint is defined as one-eighth of a US liquid gallon, which is itself exactly 231 cubic inches, so dividing by eight introduces no error at all. Nor is there much to round in practice: any whole number of pints divisible by eight lands on whole gallons, and every other whole number terminates within three decimal places, because 1/8 is 0.125 and nothing more.
Does this work for imperial pints and gallons?
Yes, provided you stay inside one system. Imperial and US customary measure both put eight pints in their gallon, so 0.125 converts either — feed it imperial pints and read imperial gallons. What you cannot do is mix them. An imperial pint is 568.26125 mL against a US pint's 473.176473 mL, and an imperial gallon is 4.54609 L against 3.785411784 L. Roughly 20% separates each pair, which is enough to spoil recipes and fuel figures alike.
Why is a US gallon 231 cubic inches?
It is inheritance rather than design. English law fixed the wine gallon at 231 cubic inches in 1707, close to the capacity of a cylinder seven inches across and six deep. The United States kept that wine gallon after independence; Britain scrapped its own tangle of corn, ale, and wine gallons in 1824 for a single imperial gallon defined from the volume of ten pounds of water. Two systems have run in parallel ever since.
Is a US dry pint the same as a liquid one?
No, and produce is where that bites. A dry pint is one-eighth of the US dry gallon descended from the Winchester bushel: 550.610475 mL against the liquid pint's 473.176473 mL, about 16% larger. Berries, mushrooms, and shucked shellfish are commonly sold by dry pint in American markets. This sheet handles liquid measure, so dry pints entered here read low.
How many fluid ounces are in each unit?
A US pint holds 16 US fluid ounces and a US gallon holds 128. An imperial pint holds 20 imperial fluid ounces and an imperial gallon 160 — which is why British beer looks generous to American visitors, and why US recipes calling for pints of stock underfill UK jugs. Even the ounces differ: 29.5735295625 mL US, 28.4130625 mL imperial.
Does the rhyme 'a pint's a pound' actually hold?
Close enough for cooking, not for trade. A US pint of cold water weighs 473.18 g, or 1.043 lb, so the rhyme runs about 4% light. Britain's version is blunter and more accurate: an imperial pint of water weighs a pound and a quarter, 1.253 lb, right to within a gram or two. Neither shortcut helps with milk, honey, or oil, whose densities stray far enough from water to matter on a scale.