What This Calculator Does
This tool converts the weight of water, measured in pounds, into volume measured in US gallons. It is based on the well-known rule of thumb that one US gallon of fresh water weighs about 8.34 pounds at room temperature. Whether you are sizing an aquarium, calculating water hauling loads, or working a science problem, this calculator gives you a quick and reliable answer.
How to Use It
Enter the weight of water in pounds and the calculator instantly returns the equivalent volume in US gallons. The conversion factor of 8.34 lb/gal is held constant, so all you need is the weight.
The Formula Explained
The relationship is simple division: $$\text{Gallons} = \frac{\text{Weight (lbs)}}{8.34}$$. A US gallon of water has a mass of roughly 8.34 pounds (\(3.785 \text{ liters} \times 1 \text{ kg/L} \approx 3.785 \text{ kg} \approx 8.34 \text{ lb}\)). Dividing total weight by that per-gallon weight yields total volume. Note this is for fresh water near 4°C; warmer water and salt water are slightly different densities, so results are an excellent approximation rather than an exact lab figure.
Worked Example
Suppose you have 100 pounds of water. Dividing 100 by 8.34 gives 11.99 gallons. So 100 pounds of water is just under 12 US gallons.
$$\text{Gallons} = \frac{100}{8.34} = 11.99$$
FAQ
Why 8.34 pounds per gallon? One US gallon of pure water at about 4°C weighs approximately 8.34 pounds. This is the standard figure used in plumbing, chemistry, and aquarium hobbies.
Does temperature matter? Slightly. Warmer water expands and weighs a touch less per gallon, but 8.34 lb/gal is accurate enough for everyday use.
Is this for US or Imperial gallons? US gallons. An Imperial gallon of water weighs about 10.02 pounds, so use a different factor for UK gallons.
