Article — Gallons to Pounds Converter
Gallons to Pounds: How Much Does a Liquid Weigh?
A US gallon of water weighs 8.345 lb at 20°C. A gallon of gasoline weighs 6.20 lb, diesel weighs 7.10 lb, whole milk weighs 8.64 lb, and honey weighs 11.85 lb. To convert any liquid from gallons to pounds, multiply the volume by the liquid's density in pounds per gallon. The Imperial gallon (UK) is 20% larger than the US gallon, so an Imperial gallon of water weighs 10.022 lb.
The 8.345 figure is the most-asked number in this whole subject, and it is also the most misquoted. Plumbing books round it to 8.3, freight schedules use 8.34, and physics textbooks list 8.345404. Below is where the number comes from, why it matters for fuel and shipping, and where the common mistakes show up.
Water: the 8.345 lb baseline
The weight of a US gallon of water is not a measurement. It is a derived value that comes out of the way the metric and imperial systems are bolted together. In 1795 the French defined a litre of water as one kilogram. Multiply: 1 US gallon equals 3.78541 L, one kilogram equals 2.20462 lb, and 3.78541 × 2.20462 lands at 8.3454. That is the whole derivation.
Everything else in this article works the same way. Density in g/mL multiplied by 8.345 gives pounds per US gallon. Gasoline is 0.74 g/mL × 8.345 = 6.18 lb/gal (rounded to 6.20 in trade). Honey is 1.42 g/mL × 8.345 = 11.85 lb/gal. The 8.345 constant does all the unit work; the only thing that changes between liquids is density.
The US gallon was inherited from a British unit that was originally a wine measure. In 1707 a London wine merchant was sued by the Crown for unpaid duties; his defense was that no legal definition of the gallon existed, so the customs charge could not be enforced. He won. Parliament responded in 1707 by writing the wine gallon into law as exactly 231 cubic inches — the same volume the US still uses for its gallon today.
Fuel weights and why they matter
Fuel is sold by volume but consumed by mass. The energy in a tank is proportional to mass, not to gallons, which is why aircraft, ships, and freight trucks track fuel in pounds even though they buy it in gallons.
Gasoline has a density of 0.71-0.77 g/mL depending on the blend, with a typical pump average of 0.74 g/mL or 6.20 lb/gal. Diesel sits at 0.82-0.86 g/mL or 7.10 lb/gal — about 14% heavier per gallon than gasoline. Liquid propane is the lightest common fuel at 4.20 lb/gal because at storage temperature it is held just below its boiling point and is much less dense than the petroleum liquids.
- Gasoline 6.20 lb/gal (varies with summer/winter blend)
- Diesel #2 7.10 lb/gal (heavier than petrol by 14%)
- Kerosene 6.82 lb/gal (jet fuel base, similar to diesel)
- Ethanol 6.59 lb/gal (denser than gasoline by 6%)
- Propane (liquid) 4.20 lb/gal (lightest common fuel)
- Heating oil 7.05 lb/gal (essentially diesel)
Service station pumps measure gallons, not pounds. Two stations selling at the same price per gallon can deliver fuel of slightly different mass per gallon if their density differs. Most US states require pumps to be calibrated by volume at 60°F, but if the underground tank is warm the fuel expands and a gallon at the nozzle weighs slightly less than 6.20 lb. The trade calls this the "hot gallon" problem.
US gallon versus Imperial gallon
The two gallons are not the same size. The US gallon is 3.785 L; the Imperial gallon, used in the UK and several Commonwealth countries, is 4.546 L. The Imperial gallon is 20% larger, and so is the weight it implies.
Imperial was defined in 1824 to be the volume of exactly 10 lb of water at 62°F — the kind of round-number definition that physicists love. The US, having broken with British standards by then, kept the older 231-cubic-inch wine gallon. Today, a UK driver who fills a 10-gallon tank with petrol takes on 12 US gallons (or 45 L); the same volume of fuel weighs 74.5 lb instead of 62 lb.
Food liquids: milk, oil, honey, syrup
The dairy and syrup industry has the clearest gallon-to-pound numbers because both are sold heavily in gallons and tracked carefully for nutrition labelling. Whole milk averages 8.64 lb per US gallon, with about a 1% spread between dairies. Skim milk drops to 8.52 lb because removing the fat removes the lightest component. Cooking oils run 7.6-7.7 lb/gal across olive, canola, sunflower and corn varieties; the differences between them are too small to matter outside the laboratory.
Honey and syrup are the heaviest food liquids. Pure honey is 1.42 g/mL and runs 11.85 lb/gal, almost 50% heavier than diesel. Maple syrup is 11.1 lb/gal, and high-fructose corn syrup is 11.0 lb/gal. These weights matter at the wholesale level: a 55-gallon drum of honey weighs about 650 lb, while the same drum of vegetable oil weighs 423 lb.
If you ship liquid in bulk, the freight rate is usually charged by weight, not volume. A 275-gallon IBC tote of water weighs 2,295 lb; the same tote filled with honey weighs 3,259 lb — over a ton more for the same container. Shippers always confirm the product density before booking the carrier.
Temperature and pump-gallon corrections
Density is temperature-dependent. Water density drops about 0.4% from 4°C to 100°C, which is small enough to ignore at home. Petroleum liquids change more — about 0.07% per °C, or 7% across the typical 0-100°C range. Fuel sold in Texas in August is measurably less dense than the same fuel sold in Wisconsin in January.
The US fuel trade handles this with the API gravity scale and temperature-corrected volumes. Wholesale fuel is bought and sold at "60°F equivalent gallons," with refineries and terminals correcting for actual temperature at delivery. Retail pumps are not corrected the same way, which is the source of the long-running "hot fuel" complaint in southern US states.
water (US gal) × 8.345 = lbwater (imp gal) × 10.022 = lbgasoline (US gal) × 6.20 = lbdiesel (US gal) × 7.10 = lbmilk (US gal) × 8.64 = lbhoney (US gal) × 11.85 = lbAviation, freight, and shipping rules
Aircraft fuel is loaded by volume but tracked by mass because the lift capacity of a plane is set by total weight. A Boeing 737 burns about 750 gallons of Jet A per hour, which at 6.71 lb/gal is roughly 5,030 lb per hour or 2,280 kg. Dispatch sheets list the fuel load in kilograms or pounds; the pilot converts to gallons only when telling the truck driver how much to put in.
Ocean freight rates for bulk liquids are quoted in metric tonnes, never gallons. The conversion runs through density: a 6,000-gallon tank trailer of diesel carries about 42,600 lb or 19.3 tonnes; the same trailer of corn syrup carries 66,000 lb or 30 tonnes. Trucking-weight limits are set by axle, so the same trailer cannot legally carry the same volume of every liquid.
"A pint's a pound" and other folk shortcuts
The old American kitchen rhyme says "a pint's a pound the world around." It is almost right. A US pint of water weighs 1.043 lb, which is 4% off a true pound. The British have a longer version that is more accurate: "a pint of pure water weighs a pound and a quarter," which is correct for the Imperial pint (1.25 lb). The shorter version travelled to America with the original immigrants and stuck even though the gallon was redefined.
Two other useful folk shortcuts: a five-gallon bucket of water weighs about 41.7 lb, easy to remember as "five gallons, forty pounds." A 55-gallon drum of water weighs 459 lb, close enough to "half a ton" for industrial estimation. Neither is a precise rule, but both work for back-of-envelope planning when you do not have the calculator handy.
Before 1824, England used at least four different gallons for different liquids — wine, ale, corn, and various local merchant standards. A wine merchant in Bristol could legally sell a "gallon" that contained a different volume than a wine merchant in London. The 1824 Imperial Act consolidated the lot into one definition (the volume of 10 lb of water), which is why every Commonwealth nation now uses the same Imperial gallon. The US, having declared independence in 1776, did not participate in the standardization and kept the older wine gallon.