  We present a formalism to calculate the macroscopic magnetoelectric
monopolization from first principles within the density functional theory
framework. An expression for the monopolization in the case of insulating
collinear magnetism is derived first in terms of spin-polarized Wannier
functions then recast as a Berry phase. We propose an extension to the general,
non-collinear case which we implement computationally in the Wannier function
form and use to calculate the magnetoelectric monopolizations of LiMnPO$_4$ and
Cr$_2$O$_3$. We find that, while the former is well approximated by a summation
over the formal local spin moments, the latter shows significant deviations
from this approximation. We suggest that equating the Berry phase value with a
sum over local moments provides an unambiguous route to defining the size of
the local magnetic moment in magnetoelectric antiferromagnets containing only
one type of magnetic ion.
