I need to find a formula between the weight of an object and the volume of the buoy that can float it on water So my problem is that i am working on an art instalation that needs to float. The idea is that i need to find a formula between the volume (V) of the buoy and the total weight of the whole instalation. 
 A: Density is your friend! So is Buoyancy!
Okay, you can find the appropriate formulas there, but let's walk through what you'd like to do. There is some art installation which you want to float; it will be supported by a buoy and only by a buoy.
I assume you can figure out what the mass of the whole installation is, and you can properly measure the volume of a buoy, but you want to know what size buoy to use. Too large a buoy, and it's too large or expensive for whatever you're making. Too small, and the art installation is now at the bottom of some body of water.
This is a good case in which to apply Archimedies' Principle. Archimedies' Principle claims you need to displace an amount of water that equals the weight of the thing you're trying to float. So you figure out how much the installation weighs, and then you divide by the density of water to get the volume of water you need. If your buoy is the same size or larger than that volume of water, it will float!
You could also get tricky, and add in the weight of the buoy to this calculation. This would ensure that you can support the art installation, or you can just add in a safety factor to compensate for the additional weight of the buoy. If you want to be thorough, you could do both methods.
Safety factors are numbers you multiply different values in your equation to give them more "wiggle room," making your overall product more safe. In this case, if you displace only the perfect amount of water, your art installation would just barely float. A bird resting on the installation or a light breeze could cause it to go under. If you assumed your installation weight was, say, 50% heavier than it actually weighs, your installation will float higher, and be better able to withstand things like people working on it, birds landing on it, breezes, etc. Whatever factor you use depends on the environment the installation will be in, but it's better to be safe than sorry!
