Asked by: Stephen Sykes, Nuneaton
At standard temperature and pressure, helium weighs about 0.18g/litre, and air 1.29g/litre. The difference is what a litre of helium should lift: 1.11g. So 1000 litres lifts a little over a kilogram, and a 70kg human needs around 70,000 litres: very roughly 10,000 normal balloons, or approximately 300 of those party balloon cylinders at supermarkets. ‘Cluster ballooning’ aficionados use 50-150 balloons, each larger – over a metre wide – than a standard balloon. Not one to try at home though.