# Q & A: Floating, helium and Archimedes’ Principle

Q:
How do I calculate the amount(volume) of helium I would need to keep an object of X weight afloat in water?
- Greg Sandy
Salem,MA
A:
Hi Greg,

According to Archimedes' Principle, the buoyant force on an object is equal to the weight of the water it displaces. Subtract off the weight of the material used to displace the water -- it is pulled down by gravity. Water has a density of 1 kg per liter. Helium's density is very small compared to that of water. Plain old air has a density very small compared with that of water and is easier to come by than helium, although helium is less dense even than air and therefore will make things float better.

So... you just find the mass of the object you want to float in kg, and that tells you how many liters of water you need to displace with something that weighs much less than water. Be sure to add in the mass of the boat or balloon or tank or whatever it is you are using to contain the helium (and also the mass of the helium, which likely can be ignored here because it is small). It is good to make the floatation device bigger than needed -- boats usually extend above the water -- they aren't completely submerged. But not so much extra that the helium will cause the object of weight X to float away in the air. This is one other main reason it is more convenient to use air for floatation in water -- you will not lose your floatation device watching it float away in the sky when you take off your object of weight X.

Tom

(published on 10/22/2007)