A U.S. gallon of water weighs about 8.3 pounds at typical room temperature.

Quick Scoop

  • 1 U.S. gallon of water ≈ 8.33–8.34 lb.
  • That’s about 3.78 kilograms.
  • The exact weight changes very slightly with temperature (water is densest around 39–40°F, where it’s closer to 8.345 lb per gallon).

Why it’s not one exact number

Water expands a bit as it warms and contracts as it cools, so the same gallon (volume) can weigh a bit more or less depending on temperature.

For everyday use—like aquariums, home water jugs, or quick load estimates—people almost always just use 8.3 lb per gallon as a handy rule of thumb.

If you’re planning something weight‑sensitive (like checking if a shelf or trailer can handle a tank of water), multiplying gallons by 8.3 will get you very close.

TL;DR: 1 gallon of water ≈ 8.3 lb, with tiny variations depending on temperature.

Information gathered from public forums or data available on the internet and portrayed here.