How Much Does a Gallon of Water Weigh? Easy Calculation 2


Question: How much does a gallon of water weigh?

Ever wonder how much a gallon of water weighs? A US gallon is 8.34 lbs or 3.78 kg. (David Mulder)

Ever wonder how much a gallon of water weighs? A US gallon is 8.34 lbs or 3.78 kg. (David Mulder)

Answer: A US gallon of water weighs 8.34 lbs or 3.78 kg at 62 °F (17 °C). An imperial gallon (UK) weighs 10.022 lbs or 4.546 kg, at its most dense temperature, which is 2.20456 lbs / L at 4 °C or 39 °F.

Ballpark Calculation (for all temperatures)

1 gallon is about 3.75 liters
1 liters = 1 kilogram (density of water is 1 kg/liter)
1 kg is about 2.2 lbs
so, 3.75 kg is about 8.25 lbs and 1 gallon is about 8.25 lbs

Three Different Units of Gallon

It makes a difference which unit of gallon you are using. There are two US definitions of gallon. The US liquid gallon (most commonly used) is defined as 231 cubic inches, which is 3.785 liters, and weighs 8.344 lbs at its highest density.

The US dry gallon is defined as 1/8th of a US bushel, which is 268.8025 cubic inches or 4.405 L. The weight of a US dry gallon is 9.711 lbs of water at its highest density.

The UK gallon or imperial gallon originally was defined as 10 lbs of water, but the modern definition is exactly 4.54609 L or 10.02 lbs of water at its maximum density.

Effect of Water Temperature on Weight

Cold water is more dense than ice or than warm water or liquid just above freezing. This is an unusual property of the substance, resulting from hydrogen bonding. So, a gallon of warm water would weigh slightly less than a gallon of cold water. The exact difference would depend on the temperatures in question, but it doesn’t affect the value by much.

Of course, gravity affects weight too, so a gallon of water (or anything else) would weigh more on Jupiter than on Earth, while it would weigh less on the Moon than Earth.

Easy Way To Remember the Weight of Water

While you’ll want to keep the exact volume definitions in mind for scientific calculations, for everyday use you can remember the weight of water using the simple rhyme: A pint’s a pound, the world round.

The saying refers to the rough equivalence between 16 fluid ounces (a pint) and 16 ounces avoirdupois weight. A gallon is 4 quarts or 8 pints, so a gallon weighs approximately 8 lbs.


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2 thoughts on “How Much Does a Gallon of Water Weigh? Easy Calculation

  • Rich Anderson

    I usually just try to remember: 8.345 close enough and easy to remember.

    I like your converting gallons to Liters, Liters to Kg (direct) and then Kg to pounds. Simple and accurate solution.

    I have another solution which only uses defined standards…

    By Standard (see end for standard definition):
    1 gallon of water = 231 cubic inches
    1 atmos = 14.696 psi
    1 atmos = 406.782 Inches of Water Column (in. W.C.) (at 4°C)

    Therefore:
    14.696 psi = 406.782″w/c

    Divide both sides by 406.782″w/c to get psi at a depth of 1 inch of water
    14.696 psi / 406.782″w/c = 1″w/c
    0.0361274 psi = 1″w/c

    Which is really:
    0.0361274 pounds over (per) square inches = 1 inch depth (water column)

    Multiply both sides by square inches:
    square inches goes ‘bye-bye’ on left side and the right side becomes cubic inches
    0.0361274 pounds = 1 cubic inch of water

    0.0361274 pounds * 231 cubic inches (in a gallon) = aprox 8.345 pounds per gallon

    So, to simplify:
    231 * 14.696 / 406.782 = 8.345

    This is the standard i found…

    Standard Atmosphere

    The unit “1 Standard Atmosphere” is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0°C (32°F) at sea level, and at standard gravity (32.174 ft/sec2). Atmospheric pressure is the weight of a column of air per area unit as measured from the top of the atmosphere to the reference point being measured. Atmospheric pressure decreases as altitude increases.

    Equivalents to 1 atmosphere are as follows:

    76 centimetres (760 mm) of mercury

    29.921 inches of mercury

    10.3322 metres of water

    406.782 inches of water

    33.899 feet of water

    14.696 pounds-force per square inch

    2,116.2 pounds-force per square foot

    1.033 kilograms-force per square centimetre

    101.325 kilopascals