A special case of Haiman´s identity for the character of the quotient ring of diagonal coinvariants under the diagonal action of the symmetric group yields a formula for the bigraded Hilbert series as a sum of rational functions in q,t. In this talk I will show a summation identity of Garsia and Zabrocki for Macdonald Pieri coefficients can be used to transform Haiman´s formula for the Hilbert series into an explicit polynomial in q,t with integer coefficients. An equivalent formulation expresses the Hilbert series as the constant term in a certain multivariate Laurent series.