Pounds of thrust divided by 2.5 equals horsepower.
That's very simplistic - but believe it or not, it's not too bad a generalization for static (early takeoff roll) approximation, given a typical prop.
What is missing is any accounting for propeller efficiency, or for vehicle speed. The old rule of thumb was I think HP = F(lb) * V(mph)/375. Note that at zero speed the useful HP is zero; all you're creating is heat and noise.
Do your own units conversion.
