How can I calculate if the air from a motor/fan will be enough to lift a hovercraft I'm building?

Question

Also how can I know that it can compensate for the air lost that comes out from under the skirt?

Answer 1

Better to measure.
Direct the output of the fan into a closed plenum.
Measure the pressure in the plenum.
If the pressure times the footprint area of your hovercraft is greater than the weight, it will lift.
The air loss at the skirt is self regulating.
If the pressure drops, the craft sinks making the gap and thus the loss smaller.
(It would be prudent to allow some extra flow for sideways movement.)

Answer 2

try to search in the specs. for the process of calculation regarding air ventilation of bldgs..