What Is the Water Weight Calculator?
This tool tells you how much a given volume of water weighs. Because water has a well-known density — approximately 1 kilogram per liter, or about 8.345 pounds per US gallon at room temperature — you can convert any volume directly into a weight. It is handy for cooking, aquariums, fish tanks, plumbing, shipping, fitness, and science experiments.
How to Use It
Enter the volume of water and choose the unit (US gallons, liters, milliliters, or US fluid ounces). The calculator instantly returns the weight in pounds, kilograms, grams, and ounces. It first converts your volume to liters, then applies the density of water.
The Formula Explained
The core relationship is weight = volume × density. Water's density is essentially 1 kg per liter, which is why metric conversions are so clean. For imperial units, 1 US gallon equals 3.78541 liters, and 1 kg equals 2.20462 lb, giving roughly 8.345 lb per gallon.
$$\text{Weight} = \text{Volume} \times 3.785411784 \;\text{kg} \quad\left(\times\,2.20462\;\text{lb}\right)$$
Worked Example
How much does 5 US gallons of water weigh? Convert:
$$5 \times 3.78541 = 18.927 \;\text{liters} = 18.927 \;\text{kg}$$In pounds:
$$18.927 \times 2.20462 \approx \mathbf{41.73 \;\textbf{lb}}$$That is why a full 5-gallon jug is heavy to lift.
FAQ
Does temperature change the weight? Slightly. Water is densest near 4 °C. At normal room temperature the density is about 0.997 kg/L, so this calculator's use of 1.0 kg/L introduces under 0.3% difference — negligible for everyday use.
Is a US gallon the same as a UK gallon? No. This calculator uses the US gallon (3.78541 L). A UK (imperial) gallon is larger at 4.546 L and weighs about 10.02 lb.
Why does 1 liter equal 1 kilogram? The kilogram was originally defined as the mass of one liter of water, so the relationship is essentially exact for pure water.
