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Sr3Ru2O7 is itinerant metamagnet, exhibiting a sharp increase in magnetisation in a field of 5.5 – 7.7 T (depending on the applied field direction) at temperatures around 1 K. This metamagnetic transition is thought to be related to the behaviour of the density of states near the Fermi level. We have performed a study of the magnetisation density in the metamagnetic state combining a magnetic Compton scattering experiment with electronic structure calculations. Our initial density functional calculations were fixed to reproduce the value of the experimentally determined spin moment, but do not agree well with the experimentally measured spin density. In the talk, we will show how it is possible to gain a good agreement with the experimental spin densities from the calculations and the consequences of this in relation to the electronic structure and underlying physics.