Quick answer: a US gallon of water weighs 8.33 lb (3.78 kg) at room temperature. A litre weighs 1 kg. A cubic foot weighs 62.3 lb. A cubic metre weighs 1 tonne. An imperial gallon — bigger than a US one — weighs 10.0 lb.
Water weight at a glance
The numbers people look up most often, at room temperature (20 °C / 68 °F). These are the figures to memorise; the calculator above handles anything in between.
| Volume | Pounds | Kilograms | Everyday reference |
|---|---|---|---|
| 1 US gallon | 8.33 lb | 3.78 kg | A large jug of milk |
| 2 US gallons | 16.7 lb | 7.56 kg | Small watering can |
| 5 US gallons | 41.6 lb | 18.9 kg | Water cooler bottle, builder's bucket |
| 10 US gallons | 83.3 lb | 37.8 kg | Small aquarium |
| 20 US gallons | 167 lb | 75.6 kg | An adult human |
| 25 litres | 55.0 lb | 25.0 kg | Jerry can, full |
| 30 US gallons | 250 lb | 113 kg | Half a drum |
| 40 US gallons | 333 lb | 151 kg | Typical domestic water heater |
| 50 US gallons | 416 lb | 189 kg | Rain barrel |
| 55 US gallons | 458 lb | 208 kg | Standard steel drum |
| 100 US gallons | 833 lb | 378 kg | Large stock tank |
| 120 US gallons | 999 lb | 454 kg | Big loft tank — a structural load |
| 150 US gallons | 1,249 lb | 567 kg | Small hot tub |
| 200 US gallons | 1,665 lb | 755 kg | Large aquarium |
| 225 US gallons | 1,873 lb | 850 kg | IBC tote, roughly two-thirds full |
| 300 US gallons | 2,498 lb | 1,133 kg | Over a US ton — needs a slab |
| 500 US gallons | 4,163 lb | 1,888 kg | Small cistern |
| 1,000 litres (1 m³) | 2,201 lb | 998 kg | A full IBC tote — one tonne |
The 120-gallon row deserves a second look. That is a common loft or attic tank size, and it puts almost exactly half a ton of load on domestic ceiling joists that were probably sized for storage boxes. If you are planning a tank anywhere other than a ground-floor slab, work out the weight before the volume — the water tank calculator gives you the volume to start from.
Water weight per unit of volume
Working the other direction — the weight of one unit of each measure, so you can multiply by any quantity you like.
| One unit of… | Weighs (lb) | Weighs (kg) | Weighs (oz) |
|---|---|---|---|
| US gallon | 8.330 lb | 3.779 kg | 133.3 oz |
| Imperial (UK) gallon | 10.004 lb | 4.538 kg | 160.1 oz |
| Litre | 2.201 lb | 0.998 kg | 35.2 oz |
| Millilitre | 0.0022 lb | 0.998 g | 0.035 oz |
| Cubic metre | 2,201 lb | 998.2 kg | — |
| Cubic foot | 62.32 lb | 28.27 kg | 997 oz |
| Cubic inch | 0.0361 lb | 16.36 g | 0.577 oz |
| Cubic yard | 1,683 lb | 763.3 kg | — |
| US pint | 1.041 lb | 0.472 kg | 16.7 oz |
| US quart | 2.083 lb | 0.945 kg | 33.3 oz |
| US fluid ounce | 0.0651 lb | 29.5 g | 1.04 oz |
Two old mnemonics live in that table, and both are nearly right. "A pint's a pound the world around" — a US pint of water weighs 1.041 lb, off by 4%. And the British version, "a pint of water weighs a pound and a quarter" — an imperial pint comes to 1.25 lb, accurate to better than half a percent. The imperial system got that one right because the imperial gallon was defined as ten pounds of water.
How temperature changes the answer
Water is famously badly behaved. Almost every substance gets denser as it cools, all the way to freezing. Water gets denser down to 3.98 °C and then starts expanding again — which is why ice floats, why lakes freeze from the top down, and why fish survive winter.
| Temperature | Density (kg/m³) | US gallon | Cubic foot | Litre |
|---|---|---|---|---|
| 0 °C / 32 °F (liquid) | 999.84 | 8.344 lb | 62.42 lb | 1.000 kg |
| 3.98 °C / 39.2 °F (densest) | 999.97 | 8.345 lb | 62.43 lb | 1.000 kg |
| 10 °C / 50 °F | 999.70 | 8.343 lb | 62.41 lb | 1.000 kg |
| 20 °C / 68 °F (room) | 998.21 | 8.330 lb | 62.32 lb | 0.998 kg |
| 25 °C / 77 °F | 997.05 | 8.320 lb | 62.24 lb | 0.997 kg |
| 40 °C / 104 °F | 992.22 | 8.280 lb | 61.94 lb | 0.992 kg |
| 60 °C / 140 °F (hot tap) | 983.20 | 8.204 lb | 61.38 lb | 0.983 kg |
| 80 °C / 176 °F | 971.79 | 8.109 lb | 60.66 lb | 0.972 kg |
| 100 °C / 212 °F (boiling) | 958.35 | 7.998 lb | 59.82 lb | 0.958 kg |
| 0 °C — as ice | 916.7 | 7.650 lb | 57.22 lb | 0.917 kg |
Across the entire liquid range the weight of a gallon moves by about 4%. For carrying buckets, filling pools or sizing a shelf, ignore it. For three cases it genuinely matters: tank calibration and custody transfer, where a percent is money; hot water systems, where a 300-litre thermal store at 60 °C holds 4.5 kg less than the same volume cold; and the ice row, which is not a rounding difference at all. Water expands roughly 9% on freezing, and it does so with enough force to split cast iron. That is why a pipe bursts, and why the ice figure in the table is the only one in this whole article that has ever caused a flood.
Salt water, and how water compares to other liquids
The calculator's density presets, plus a few comparisons for context. Anything below 1.000 floats on water; anything above sinks.
| Liquid | Density (kg/m³) | Per US gallon | Note |
|---|---|---|---|
| Fresh water (20 °C) | 998 | 8.33 lb | The baseline |
| Seawater (average, 35 ppt) | 1,025 | 8.55 lb | 2.7% heavier — matters for large aquariums |
| Dead Sea water | 1,240 | 10.35 lb | 24% heavier; the reason you float |
| Milk (whole) | 1,030 | 8.60 lb | Slightly heavier than water |
| Petrol / gasoline | 750 | 6.26 lb | Floats — which is why fuel fires spread on water |
| Diesel / heating oil | 850 | 7.09 lb | Also floats |
| Ethanol | 789 | 6.58 lb | Why ABV and ABW differ |
| Honey | 1,420 | 11.85 lb | Sinks straight through water |
| Mercury | 13,534 | 112.9 lb | A gallon weighs more than a person |
For any other material, the volume to weight calculator carries a full density table, and the density–mass–volume calculator rearranges ρ = m/V in whichever direction you need.
Common mistakes & pro tips
- US gallon vs imperial gallon. A 20% error, and the most common one on this topic. A UK gallon is 4.546 L and weighs 10.0 lb; a US gallon is 3.785 L and weighs 8.33 lb. Recipes, tank specs and older engineering tables all vary in which they mean.
- Forgetting the container. Water weight is not total weight. A 55-gallon steel drum adds ~40 lb, a 275-gallon IBC tote adds ~130 lb, and a glass aquarium can add 15–20% of its water weight.
- Assuming a "ton" is one thing. A US short ton is 2,000 lb, a metric tonne is 2,205 lb, a UK long ton is 2,240 lb. 300 US gallons of water is over a short ton but under a tonne.
- Using 8.34 lb/gal for hot water. Fine for a cold tank; 1.5% out at 60 °C, which matters when you are calibrating.
- Pro tip — the one number to remember. One cubic metre of water = one tonne = 1,000 litres = 1,000 kg. Every metric water weight question collapses into that identity.
- Pro tip — check the floor, not just the tank. Residential floors are typically designed for around 40 lb per square foot of live load. A 150-gallon aquarium on a 2 ft × 6 ft stand puts roughly 130 lb/ft² through that footprint. Distribute the load across joists, or put it on a ground-floor slab.
How to use this calculator
- Choose a direction. Volume → weight is the usual one; Weight → volume answers "how many gallons is 500 lb of water".
- Enter the volume and pick its unit — US gallons, imperial gallons, litres, cubic feet, cubic inches, cubic metres and more.
- Set the water type or temperature. Fresh water at your chosen temperature, seawater, or ice.
- Read the weight in your preferred unit, with the alternatives and the density used shown alongside.
Frequently asked questions
How much does a gallon of water weigh?
A US gallon of water weighs 8.33 pounds (3.78 kg) at room temperature, around 20–22 °C. At its densest, 4 °C, it reaches 8.345 pounds, and near boiling it falls to about 8.00 pounds. An imperial (UK) gallon is larger at 4.546 litres, so it weighs 10.0 pounds — which is where the old British rule "a pint of water weighs a pound and a quarter" comes from.
How much does 5 gallons of water weigh?
Five US gallons of water weigh 41.6 pounds (18.9 kg) at room temperature. Add the container: a standard 5-gallon plastic bucket adds about 2 pounds, so a full one is around 44 pounds — right at the edge of comfortable one-handed carrying, which is why they have handles and why water cooler bottles stop at 5 gallons.
How much does a litre of water weigh?
One litre of water weighs almost exactly 1 kilogram — 0.998 kg at 20 °C and 1.000 kg at 4 °C. This is not a coincidence: the kilogram was originally defined in 1795 as the mass of one litre of water at its melting point. The definition has since moved to the Planck constant, but the numbers were chosen to line up and still do to within a fifth of a percent.
How much does a cubic foot of water weigh?
A cubic foot of water weighs 62.3 pounds at room temperature and 62.4 pounds at 4 °C. The 62.4 figure is the one used throughout civil and structural engineering for hydrostatic loads. A cubic metre, by contrast, weighs 1,000 kg — one tonne exactly — which is the single most useful number in this whole subject.
Does temperature change the weight of water?
It changes the weight of a given volume, yes, because water expands when heated. A US gallon drops from 8.345 lb at 4 °C to 8.00 lb at 100 °C — about 4%. Water is unusual in also expanding when cooled below 4 °C, which is why ice floats and why pipes burst. For most practical purposes the variation is negligible; for tank calibration, fuel metering or laboratory work it is not.
How much does salt water weigh compared to fresh water?
Average seawater has a density around 1,025 kg/m³ against 998 for fresh water, so it is about 2.7% heavier — a US gallon comes to roughly 8.55 lb. It matters more than the small percentage suggests: a 10,000-gallon saltwater aquarium holds about 2,200 lb more than the same volume of fresh water, which is a genuine structural consideration for anything above ground floor.
References & further reading
- Kell, G. S. (1975), "Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C", Journal of Chemical & Engineering Data — the density equation used by this calculator.
- NIST Chemistry WebBook — thermophysical properties of water.
- UNESCO/IOC international equation of state of seawater — the basis of the 1,025 kg/m³ average figure.
- NIST Handbook 44 — definitions of the US gallon and the avoirdupois pound.
Density is computed from the Kell equation for pure, air-free water at one atmosphere. Tap water containing dissolved minerals is very slightly denser. Weights given are mass; on Earth's surface the distinction from weight is not practically relevant. See our accuracy policy.