  For an almost split Kac-Moody group G over a local non-archimedean field, the
last two authors constructed a spherical Hecke algebra H (over the complex
numbers C, say) and its Satake isomorphism with the commutative algebra of Weyl
invariant elements in some formal series algebra C[[Y]].In this article, we
prove a Macdonald's formula, i.e. an explicit formula for the image of a basis
element of H. The proof involves geometric arguments in the masure associated
to G and algebraic tools, including the Cherednik's representation of the
Bernstein-Lusztig-Hecke algebra (introduced in a previous article) and the
Cherednik's identity between some symmetrizers.
