This paper deals with the eective computation of normal forms, centre manifolds and rst integrals in Hamiltonian mechanics. These kind of calculations are very useful since they allow, for instance, to give explicit estimates on the diusion time or to compute invariant tori. The approach presented here is based on using algebraic manipulation for the formal series but taking numerical coecients for them. This, jointly with a very ecient implementation of the software, allows big savings in both memory and execution time of the algorithms if we compare with the use of commercial algebraic manipulators. The algorithms are presented jointly with their C/C++ implementations, and they are applied to some concrete examples coming from celestial mechanics.