How many kilograms does a liter of fluid weigh?
For water 1 kg = 1 liter. For anything else, you will need to
know the density of the liquid. Most things are close enough. A
liter of water have a density of 1 g/cm³ and a liter of milk has a
density of 1.026g/cm³ not much of a difference.
For standard water under standard conditions: 1 liter has 1 kg of
mass, weighs 9.8 newtons or 2.20462 pounds on earth.