How can you use the density of an object to predict whether it will sink or float in water?

If the density of the object is greater than 1 the object will sink, if the object is less than 1 the object will float.

If you know two densities u can compare them and the object a higher density than water - [latex](1000 frac{kg}{m^3})[/latex] - it will float and a smaller sink.