A US gallon of water weighs about 8.34 pounds (3.78 kilograms). That figure holds for cool tap water around 60°F; at a 70°F room temperature it's 8.33 lb, a difference you'd never notice in the kitchen. A larger UK (imperial) gallon of water weighs about 10.0 pounds (4.54 kg).
Key facts
- 1 US gallon = 3.785411784 liters (NIST), so a gallon of water weighs about 3.78 kg, or 8.34 lb.
- USGS lists water's density as 1.00000 g/cm³ at 4°C (39.2°F) and 0.99802 g/cm³ at 21°C (about 70°F).
- A US cup of water weighs about 236 g (8.3 oz); a liter weighs about 998 g at room temperature.
- An imperial gallon holds 4.54609 L (NIST), about 20% more than a US gallon, so it weighs about 10.0 lb.
- USDA's Agricultural Marketing Service uses 8.60 lb per gallon for whole milk, a little heavier than water.
How much does a gallon of water weigh?
A US gallon of water weighs about 8.34 pounds, or 3.78 kilograms. The math is simple. NIST defines the US gallon as 231 cubic inches, which is exactly 3.785411784 liters. Water weighs very close to 1 kilogram per liter, so a gallon comes out just under 3.79 kg. Divide by 0.45359237 kg per pound (the exact NIST definition of the pound) and you get about 8.34 lb.
The weight shifts slightly with temperature because water expands as it warms. At typical kitchen temperatures the change is in the second decimal place, so 8.3 or 8.34 lb is the right working number for anything you'll carry, ship or cook with.
How many pounds is a gallon of water at different temperatures?
Warm water weighs slightly less per gallon than cold water. The U.S. Geological Survey (USGS) publishes water density by temperature; multiplying each value by the NIST gallon gives these weights:
| Water temperature | Density (USGS) | US gallon weighs | Imperial gallon weighs |
|---|---|---|---|
| 39.2°F (4°C), densest | 1.00000 g/cm³ | 8.345 lb (3.785 kg) | 10.02 lb (4.546 kg) |
| 60°F (15.6°C), cool tap | 0.99907 g/cm³ | 8.338 lb (3.782 kg) | 10.01 lb (4.542 kg) |
| 70°F (21°C), room temperature | 0.99802 g/cm³ | 8.329 lb (3.778 kg) | 10.00 lb (4.537 kg) |
| 100°F (37.8°C), warm | 0.99318 g/cm³ | 8.288 lb (3.760 kg) | 9.95 lb (4.515 kg) |
| 140°F (60°C), hot tap | 0.98338 g/cm³ | 8.207 lb (3.723 kg) | 9.86 lb (4.471 kg) |
| 212°F (100°C), boiling | 0.95865 g/cm³ | 8.00 lb (3.629 kg) | 9.61 lb (4.358 kg) |
Water is densest at about 39°F (4°C), which is why the heaviest figure, 8.345 lb, belongs to near-freezing water. Even boiling water only loses about a third of a pound per gallon. To convert other temperatures, use the temperature converter.
What is the density of water?
The density of water is about 1 gram per milliliter (1 g/cm³, or 1,000 kg/m³). USGS gives exactly 1.00000 g/cm³ at 4°C and 0.99802 g/cm³ at 21°C, roughly room temperature. In US units, that works out to:
- Pounds per gallon: 8.345 lb/gal at 4°C; 8.33 lb/gal at room temperature.
- Pounds per cubic foot: USGS lists 62.424 lb/ft³ at 39.2°F (4°C).
- Pounds per cubic inch: about 0.0361 lb/in³ (62.43 divided by 1,728 cubic inches per cubic foot).
Engineers also talk about the "unit weight" or "specific weight" of water. In US customary units it's commonly given as about 62.4 pounds-force per cubic foot, the same number as the density in pounds per cubic foot under standard gravity. For cooking, the useful takeaway is that a milliliter of water weighs about a gram, so a scale and a measuring cup agree closely for water.
USGS also notes that dissolved material such as salt makes water denser, so a gallon of seawater or brine weighs more than a gallon of fresh water.
How much does a cup, pint or quart of water weigh?
A cup of water weighs about 236 grams, or 8.3 ounces, at room temperature. Smaller and larger measures scale from the same numbers. These weights use the exact NIST volumes and USGS density at 21°C (70°F):
| Amount of water | Volume | Grams | Ounces (weight) | Pounds |
|---|---|---|---|---|
| 1 tablespoon | 14.79 mL | 14.8 g | 0.52 oz | 0.03 lb |
| 1 fluid ounce | 29.57 mL | 29.5 g | 1.04 oz | 0.07 lb |
| 1 cup | 236.6 mL | 236 g | 8.33 oz | 0.52 lb |
| 1 pint (2 cups) | 473.2 mL | 472 g | 16.7 oz | 1.04 lb |
| 1 quart (4 cups) | 946.4 mL | 944 g | 33.3 oz | 2.08 lb |
| 1 liter | 1,000 mL | 998 g | 35.2 oz | 2.20 lb |
| 1/2 gallon | 1.893 L | 1,889 g | 66.6 oz | 4.16 lb |
| 1 gallon | 3.785 L | 3,778 g | 133.3 oz | 8.33 lb |
| 5 gallons | 18.93 L | 18,890 g | 666 oz | 41.6 lb |
The old saying "a pint's a pound the world around" is close for US pints: a pint of water weighs about 1.04 lb. It doesn't hold for the bigger UK pint, which weighs about 1.25 lb. For gram weights of other liquids and dry ingredients, see cups of water to grams and the cups-to-grams hub.
Why fluid ounces and ounces aren't the same thing
A fluid ounce measures volume; an ounce measures weight. For water, the two nearly match: one fluid ounce of water weighs about 1.04 ounces. That's a coincidence of water's density, not a rule. A cup of flour weighs far less than 8 ounces and a cup of honey far more. See ounces in a cup and our guide to kitchen weight conversions.
How do you convert pounds to gallons of water?
To convert pounds of water to gallons, divide by 8.34. To go the other way, multiply gallons by 8.34. For example, 50 lb of water is about 6 gallons (50 ÷ 8.34 = 6.0), and 3 gallons weigh about 25 lb.
- Gallons to pounds: gallons × 8.34 = pounds.
- Pounds to gallons: pounds ÷ 8.34 = gallons (1 lb of water is about 0.12 gallon, just under 2 cups).
- Gallons to kilograms: gallons × 3.78 = kilograms.
These factors only work for water and liquids with nearly the same density. Oil is lighter than water, and syrups and honey are much heavier, so a "pounds to gallons" answer depends on the liquid. For volume-only conversions, see liters in a gallon, ounces in a gallon and pounds in a kilogram.
How much does an imperial gallon of water weigh?
An imperial (UK) gallon of water weighs about 10.0 pounds, or 4.54 kg. NIST lists the imperial gallon as exactly 4.54609 liters, about 1.2 US gallons, so it weighs about 20% more. At 39°F it's 10.02 lb; at room temperature, 10.00 lb. Recipes and labels from the UK, and older Canadian sources, may use imperial gallons and pints, so check where a measurement comes from. Our guide to US vs metric cups covers the other size differences.
Is a gallon of milk heavier than a gallon of water?
Yes, slightly. USDA's Agricultural Marketing Service uses a factor of 8.60 pounds per gallon for whole milk when converting volumes sold, compared with about 8.33 to 8.34 lb for water. Milk's dissolved sugars, proteins and minerals make it a little denser than water. Other liquids differ more: honey is roughly 1.4 times as heavy as water by volume, while oil is lighter. Check the milk and honey pages for cup weights.
Questions people ask
How much does 1 gallon of water weigh in pounds?
About 8.34 lb. At room temperature (70°F) it's 8.33 lb, and near freezing it's 8.345 lb.
How much does a gallon of water weigh in kg?
About 3.78 kg. A US gallon is 3.785 liters, and a liter of water weighs just under 1 kg at room temperature.
How much does a cup of water weigh?
About 236 grams, or 8.3 ounces by weight. Nutrition labels use a rounded 240 mL cup, which would hold about 240 g of water.
How many gallons is 40 pounds of water?
About 4.8 gallons. Divide the pounds by 8.34 to get gallons.
Does hot water weigh less than cold water?
Per gallon, yes, slightly. Using USGS densities, a gallon of boiling water weighs about 8.00 lb, against 8.345 lb at 39°F.
What is the density of water in lb/gal?
About 8.34 lb per US gallon (8.345 at 4°C, 8.33 at room temperature), or about 10.0 lb per imperial gallon.
Sources
- USGS Water Science School: Water density
- NIST: Guide to the SI, Appendix B.9, factors for units listed by kind of quantity
- NIST: Handbook 44 (2024), Appendix C, general tables of units of measurement
- USDA AMS: Factors for converting volumes sold into number of container units sold
- eCFR: 21 CFR 101.9, nutrition labeling of food
