The surface mass density of a cluster of galaxies, and thus its total mass, can be estimated from its lens magnification. The magnification can be determined from the variation in number counts of its background galaxies. In the weak lensing approximation the surface mass density is a linear function of the magnification. However, most observational data is concentrated in the central parts of clusters, so one needs to go beyond the weak lensing approximation, and consider the lens shear as well, which is unknown from the variation in number counts alone. We studied the lensing properties of a catalogue of numerical cluster models in order to find the best possible approximation for the shear which still allows straightforward determination of the surface mass density. We show that by using such an approximation one can fairly well reconstruct the surface mass distribution from the magnification alone. It is demonstrated that the mass estimated using the weak lens magnification approximation is usually at least twice the true mass. We illustrate our technique on existing data, and show that the resulting masses compare well to other estimates.
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