Question: how much does a 20 litre water bottle weigh?
Answer: Liter is a volume unit, to convert it to a weight (in kg), the base is 1 (this is 1 kg equals one Liter of water at 4 deg Celsius, at 1 bar, there are minimal changes at other temperatures or other pressure). Because the density of compost ist 0.2, the calculation is pretty simple: 20 Liters(=kg) * 0.2 = 4 kg.