Question: How many ballons would i need to lift me 20foot in the air if i weighes 9 stone

Joseph Cook answered on 21 Jun 2010:

It depends on how big the balloons are and what they contain. I’ll assume they contain helium, which they usually do, although hydrogen is less dense so fewer balloons would be needed of the same size if you used hydrogen (but it’s also less safe – see the Hindenburg disaster). Also, bear in mind that it would be difficult to stop at exactly 20 foot. You would probably need to weigh yourself down with some additional weights, use some additional balloons to compensate for the extra weight, and then drop the weights when you got to 20 feet so that everything balanced perfectly. So you would need to calculate everything very carefully, otherwise you would either fly off higher into the atmosphere, or fall back down to ground. I’ll just do a basic calculation to estimate how many you would need to get off the ground.

1 litre of nitrogen (air is mostly nitrogen) weighs about 1.25 grams and 1 litre of helium weighs about 0.18 grams, so the difference between them is about 1.07 grams. This means that a 1 litre helium balloon can support up to around 1.07 grams.

9 stones of weight, which is about 57 kilograms or 57,000 grams, would therefore need around 53,000 1 litre helium balloons. Helium balloons are probably larger than 1 litre though. I have no idea how big they usually are, I’m sure they vary, but I’ll guess they are 20 cm in diameter. This would give them a volume of around 4 litres each. So you would ‘only’ need 13,250 balloons of 20 cm diameter to get you off the ground. If the balloons were larger, you could use fewer.

This is only an estimate, but it shows that you would need a LOT of balloons to get off the ground!