The upward buoyant force on a body immersed in a fluid is equal to what?

The upward buoyant force is simply equivalent to the weight of an amount of the fluid that would occupy the same space (same volume). The total upward force on the body, if freely floating, would be found by subtracting the downward force of the body's own weight.

So for example, the buoyant force on a balloon filled with air submerged in water would be equal to the weight of the same-size balloon filled with water suspended in air.