How much does a gallon of water weigh?

How much does a gallon of water weigh? We identify the water weight for different quantities of US gallons. Including how much does a gallon of water weigh in pounds, stones and liters. There is also an explanation of US and UK, differences in measurements for gallons.
How much does a gallon of water weigh?

How much does a gallon of water weigh in pounds, stones and kilos

How much does a gallon of water weigh depends on where it is and the temperature.

Temperature

Let me explain, the temperature is simple, if the water is heated it causes the molecules to expand – to move apart. So a container that has a volume of 1 gallon will weigh less than one that has cold water in it, as the molecules are closer together and so can fit more water into the container.
As an experiment weigh a pint, or a liter, of water at home that is cold and then boil the water – now reweigh the same heated pint, or liter of water, and see the weight has gone down.

Location

The ‘where’ is more complex. Basically the altitude (pressure) also effects the weight as does gravity etc. But far too complex for our needs in this summary.
More importantly the where in terms of US or UK location makes a big difference. You see a liter is the same in both but a gallon is not.

Metric System (Liters or Litres)

The metric system using liters (litres in UK and Europe) is fairly straightforward. At a temperature of 4 degrees Celsius (39.2ºF), exposed to standard atmospheric pressure of 101.325 kPa, 1cm³ of water is 1ml, and weighs 1 gram. 1 Liter of water (1000ml) weighs 1kg (1000g).

Simple to remember that 1 liter of water weighs 1 kilogram. (at 4ºC).

Gallons – the confusion begins!

A gallon in the US and UK are not the same measure!

In the US 1 gallon of water is 128 fluid ounces, or 3.785 liters. In the UK it is 160 fluid ounces, or 4.546 litres.

It is best that we stick to comparing liters because just as the size of a gallon differs so does the size of a fluid ounce.

(1 Imperial fluid ounce = 0.9607603 US fluid ounces.)

The point to note is that a UK gallon is a little over 20% bigger than a US gallon.

For example, a 50 Gallon Tank in UK would hold 60 US Gallons of water.

Water Gets Lighter As It Gets Warmer
Water gets lighter as it gets warmer

Water Weight

The following two tables identify how much water weighs at a room temperature of 70°F / 21°C and 80°F / 26.7°C.  Most aquariums at home will be within this range.

Weight of water in pounds, stones and kilos for US gallons
Gallons of water Pounds Stones Kilos
1 gallon 8.33 lb 0.60 3.78 kg
2 gallons 16.66 lb 1.19 7.56 kg
3 gallons 24.99 lb 1.79 11.33 kg
4 gallons 33.32 lb 2.38 15.11 kg
5 gallons 41.64 lb 2.97 18.89 kg
6 gallons 49.97 lb 3.57 22.67 kg
7 gallons 58.3 lb 4.16 26.45 kg
8 gallons 66.63 lb 4.76 30.22 kg
9 gallons 74.96 lb 5.35 34 kg
10 gallons 83.29 lb 5.95 37.78 kg
15 gallons 124.933 lb 8.92 56.67 kg
20 gallons 166.578 lb 11.83 75.56 kg
29 gallons 241.538 lb 17.25 109.56 kg
30 gallons 249.866 lb 17.85 113.34 kg
40 gallons 333.155 lb 23.80 151.12 kg
50 gallons 416.444 lb 29.75 188.90 kg
60 gallons 499.733 lb 35.70 226.67 kg
100 gallons 832.888 lb 59.49 377.79 kg
120 gallons 999.466 lb 71.39 453.35 kg
150 gallons 1249.332 lb 89.24 566.69 kg
At room temperature (70°F / 21°C)

 

Weight of water in pounds, stones and kilos for US gallons
Gallons of water Pounds Stones Kilos
1 gallon 8.318 lb 0.59 3.773 kg
2 gallons 16.636 lb 1.19 7.546 kg
3 gallons 24.953 lb 1.78 11.319 kg
4 gallons 33.271 lb 2.38 15.092 kg
5 gallons 41.589 lb 2.97 18.864 kg
6 gallons 49.907 lb 3.56 22.637 kg
7 gallons 58.224 lb 4.16 26.41 kg
8 gallons 66.542 lb 4.75 30.183 kg
9 gallons 74.86 lb 5.35 33.956 kg
10 gallons 83.178 lb 5.94 37.729 kg
15 gallons 124.767 lb 8.91 56.593 kg
20 gallons 166.356 lb 11.88 75.458 kg
29 gallons 241.216 lb 17.23 109.414 kg
30 gallons 249.533 lb 17.82 113.186 kg
40 gallons 332.711 lb 23.77 150.915 kg
50 gallons 415.889 lb 29.71 188.644 kg
60 gallons 499.067 lb 35.65 226.373 kg
100 gallons 831.778 lb 59.41 377.288 kg
120 gallons 998.134 lb 71.30 452.746 kg
150 gallons 1247.667 lb 89.12 565.932 kg
At room temperature (80°F / 26.7°C)

 

Aquarium Dimensions and Typical Tank Sizes

If you want to calculate the volume of your tank and need a quick calculator to see how many gallons or liters it holds go to our ‘Aquarium Calculatorpage.

Or check out our article that tells you the typical dimensions for various standard tank sizes HERE.

 

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