How Much Does Water Weigh? A Complete Guide by Volume

A US gallon of water weighs about 8.34 pounds (3.79 kg). A liter weighs almost exactly 1 kilogram, and a cubic foot comes in at roughly 62.4 pounds. Those are the numbers most people are after — but they shift slightly with temperature, and the "why" behind them is worth knowing if you're sizing a shelf, filling a tank, or just settling a bet. Here's the full picture, unit by unit.

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Water weight at a glance

For everyday purposes, at normal indoor temperatures, these are the figures to memorize:

Two things jump out. First, a liter of water weighing one kilogram is no coincidence — the kilogram was originally defined as the mass of a liter of water. Second, the imperial gallon is heavier than the US gallon simply because it's a larger volume (4.55 liters versus 3.79), not because the water itself is any different.

The classic "8.34 lb per gallon" and "62.4 lb per cubic foot" figures are quoted at roughly 39–60 °F. Warm the water to a hot 70 °F and a gallon reads closer to 8.33 lb — a difference too small to matter for most jobs.

Common volumes, quick reference

Most real questions aren't about a single gallon — they're about a jug, a tank or a delivery. Here's the weight of pure water for the volumes people look up most, at room temperature:

Notice how quickly it adds up: water is heavier than most people expect, and a full 100-gallon tank weighs about as much as a grand piano. That's why weight, not just capacity, drives where a tank can safely sit.

Why water weight is really about density

Weight follows a simple rule: weight = volume × density. Volume is how much space the water takes up; density is how tightly its molecules are packed. Multiply the two and you get mass. So the only reason a gallon of water has a fixed weight is that water has a fixed density — about 998 kilograms per cubic meter at room temperature, or very close to 1 gram per cubic centimeter.

That single relationship is the whole story. Change the volume and the weight scales with it; change the density and the weight moves too. And the main thing that changes water's density is temperature.

Calculate water weight yourself

You don't need a calculator for a rough figure. For US gallons at room temperature, one multiplier does it:

For an exact answer at a specific temperature, work from density instead. Convert your volume to liters, then multiply by the density of water at that temperature (about 0.998 kg per liter at 70 °F), and convert to pounds if you need them:

Both routes land in the same place for everyday temperatures. The density method only pulls ahead when you're dealing with very hot water, salt water, or a job that needs to be exact — which is what the water weight calculator handles for you.

How temperature changes the answer

Heat a fixed amount of water and its mass doesn't change — but it expands, so each gallon holds slightly fewer molecules and weighs a touch less. Water is unusual here: it's densest at about 4 °C (39 °F), not at freezing. Above and below that point it becomes less dense, which is exactly why ice floats and why lakes freeze from the top down.

The effect is small across the temperatures you'll meet at home, but real:

So a gallon of near-boiling water weighs about 4% less than a gallon of ice-cold water. For a glass of water it's irrelevant; for a large hot-water tank or a precise laboratory measurement, it matters — which is why our calculator lets you set the temperature.

Common real-world examples

Numbers are easier to trust when you can picture them:

What else affects the weight

Dissolved salt. Seawater runs about 2–3% heavier than fresh water — roughly 1,025 kg/m³ versus 998 — because of the dissolved salt. So a gallon of seawater weighs closer to 8.6 lb.

Pressure. Water is nearly incompressible, so pressure has almost no effect on its weight outside deep-ocean or high-pressure industrial settings. For any household question, you can ignore it.

Impurities. Tap water carries tiny amounts of dissolved minerals and gas, but the effect on weight is well under a tenth of a percent — invisible for everyday purposes, which is why the standard figures assume pure water.

The bottom line

For nearly every practical purpose, a US gallon of water weighs 8.34 lb, a liter weighs 1 kg, and a cubic foot weighs 62.4 lb. Temperature nudges those figures by a few percent and salt adds a little more, but unless you're doing precise engineering or lab work, the round numbers are all you need. When you do need an exact figure for a specific volume and temperature, the calculator below has it.