In short: a US gallon of fresh water weighs about 8.34 lb (3.79 kg) at room temperature; a liter weighs almost exactly 1 kg; and a cubic foot weighs roughly 62.3 lb. Exact weight depends on temperature, because warmer water is less dense — use the calculator above for any volume and temperature.
These figures are planning aids based on published density data. For structural, regulatory or safety-critical work, confirm with a licensed engineer.
How to calculate the weight of water
The weight of water depends on two things: how much of it you have (volume) and how tightly its molecules are packed (density). Density is the bridge between volume and weight, and it shifts with temperature. Once you know the density, the calculation is a single multiplication:
This calculator converts your volume to cubic meters, looks up the density of pure water at the temperature you entered, and returns the mass in pounds, kilograms, grams, ounces and tonnes. For everyday questions — “how heavy is a full aquarium?” or “what does a gallon of water weigh?” — the temperature barely matters. For engineering, dosing or laboratory work, it does, which is why the temperature control is front and center.
Worked example. Take 5 US gallons at 20 °C. Five gallons is 18.927 liters, or 0.018927 m³. Water at 20 °C has a density of 998.2 kg/m³. Multiply: 0.018927 × 998.2 = 18.89 kg, which is 41.7 lb. That single multiplication is all the calculator is doing — the work is in getting the density right for the temperature.
Weight vs. mass, in one line. Strictly, kilograms and grams measure mass, while pounds are a unit of force (weight) on Earth. In everyday use on the planet’s surface the two track each other exactly, so this tool follows common usage and reports both side by side.
Why temperature changes water’s weight
Heat a fixed amount of water and its mass does not change — but its volume does. Warm water spreads out, so each liter contains slightly fewer molecules and weighs slightly less. Water is unusual: it reaches its maximum density at about 4 °C (39 °F), not at freezing. Below and above that point it becomes less dense, which is why ice floats and why lakes freeze from the top down.
Under the hood, this calculator uses the Kell equation, the standard polynomial fit for the density of air-free water between 0 °C and 150 °C at one atmosphere. It is accurate to a few parts per million — far tighter than any household need, and dependable enough for lab and engineering estimates. You can read the full method on our methodology page.
Density of water by temperature
The values this calculator uses, from freezing to boiling. Notice how little the density moves across normal indoor temperatures, and how the drop accelerates as water approaches its boiling point.
| Temperature | Density (kg/m³) | Density (lb/ft³) | Weight of 1 US gallon |
|---|---|---|---|
| 0 °C / 32 °F | 999.84 | 62.42 | 8.344 lb |
| 4 °C / 39 °F (densest) | 999.97 | 62.43 | 8.345 lb |
| 10 °C / 50 °F | 999.70 | 62.41 | 8.343 lb |
| 20 °C / 68 °F (room) | 998.20 | 62.32 | 8.330 lb |
| 25 °C / 77 °F | 997.04 | 62.24 | 8.321 lb |
| 37 °C / 99 °F (body) | 993.33 | 62.01 | 8.290 lb |
| 60 °C / 140 °F | 983.20 | 61.38 | 8.205 lb |
| 80 °C / 176 °F | 971.80 | 60.67 | 8.110 lb |
| 100 °C / 212 °F (boiling) | 958.36 | 59.83 | 7.998 lb |
Quick reference: how much water weighs
Common volumes at room temperature (20 °C / 68 °F). Handy when you just need a number without opening the calculator.
| Volume | Pounds (lb) | Kilograms (kg) |
|---|---|---|
| 1 US gallon | 8.33 | 3.78 |
| 5 US gallons | 41.7 | 18.9 |
| 1 imperial gallon | 10.00 | 4.54 |
| 1 liter | 2.20 | 0.998 |
| 1 cubic foot | 62.3 | 28.3 |
| 1 cubic meter | 2,201 | 998 |
| 1 US cup | 0.521 | 0.236 |
| 55-gallon drum | 458 | 208 |
Worked examples
Filling a 40-gallon aquarium
333 lb
40 US gallons of water at room temperature weigh about 333 lb (151 kg) — before you add gravel, rock or the tank itself. Always check what your floor and stand can hold.
A 5-gallon jug for camping
41.7 lb
Full, a 5-gallon jug weighs roughly 42 lb (19 kg). That is why they are near the limit of what most people carry comfortably in one hand.
Topping up a hot tub
2,670 lb
A 320-gallon hot tub holds about 2,670 lb (1,210 kg) of water. Warm water at 40 °C weighs a little less than cold — but not enough to change your deck’s load rating.
What else affects the weight of water
Dissolved salts. Seawater is about 2–3% heavier than fresh water because of dissolved salt — roughly 1,025 kg/m³ versus 998. If you are weighing salt water, brine or pool water, use the density override in the calculator’s advanced options.
Pressure. Water is nearly incompressible, so pressure has a negligible effect outside deep-ocean or high-pressure industrial settings. For any household or building calculation you can ignore it.
Dissolved air and impurities. Tap water carries tiny amounts of dissolved gas and minerals. The effect on weight is well under a tenth of a percent — invisible for practical purposes, which is why the calculator models pure, air-free water.
Frequently asked questions
How much does a gallon of water weigh?
One US gallon of water weighs about 8.33 pounds (3.78 kg) at room temperature (20 °C / 68 °F). At its densest point, 4 °C, it weighs about 8.34 lb. An imperial (UK) gallon is a larger volume, so it weighs about 10.0 lb.
How much does a liter of water weigh?
Almost exactly 1 kilogram (1,000 grams). It is precisely 1 kg at 4 °C and about 0.998 kg at 20 °C, because water expands a little as it warms. This near-perfect match is by design — the kilogram was originally defined as the mass of a liter of water.
Does hot water weigh less than cold water?
For the same volume, yes — slightly. Hot water is less dense, so a full container of hot water weighs a touch less than the same container filled with cold water. Between 4 °C and 100 °C the difference is about 4%. The mass of a fixed amount of water never changes when you heat it; only its volume does.
How much does a cubic foot of water weigh?
About 62.3 pounds at 20 °C, or 62.4 lb at 4 °C. This figure is widely used in plumbing, drainage and structural load calculations.
How is this calculator’s accuracy verified?
Densities come from the Kell equation, the reference standard for water density between 0 °C and 150 °C, cross-checked against NIST data. Unit conversions use exact internationally defined factors. Full detail is on our methodology page.
Sources & references
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)