  Magnetars are compact stars which are observationally determined to have very
strong surface magnetic fields of the order of $10^{14}-10^{15}$G. The centre
of the star can potentially have a magnetic field several orders of magnitude
larger. We study the effect of the field on the mass and shape of such a star.
In general, we assume a non-uniform magnetic field inside the star which varies
with density. The magnetic energy and pressure as well as the metric are
expanded as multipoles in spherical harmonics up to the quadrupole term.
Solving the Einstein equations for the gravitational potential, one obtains the
correction terms as functions of the magnetic field. Using a nonlinear model
for the hadronic EoS the excess mass and change in equatorial radius of the
star due to the magnetic field are quite significant if the surface field is
$10^{15}$G and the central field is about $10^{18}$ G. For a value of the
central magnetic field strength of $1.75\times10^{18}$ G, we find that both the
excess mass and the equatorial radius of the star changes by about $3-4\%$
compared to the spherical solution.
