We derive a first-order linear differential equation describing the shape of the probability density function (PDF) of magnetic field strengths in the quiet Sun. The modeling is very schematic. It considers convective motions which continuously supply and withdraw magnetic structures. In addition, a magnetic amplification mechanism increases the field strength up to a threshold that cannot be exceeded. These three basic ingredients provide PDFs in good agreement with the PDFs produced by realistic numerical simulations of magnetoconvection, as well as with quiet-Sun PDFs inferred from observations. In particular, the distribution is approximately lognormal, and it produces an excess of magnetic fields (i.e., a hump in the distribution) right before the maximum field strength. The success of this simple model may indicate that only a few basic ingredients shape the quiet-Sun PDF. Our approach provides a concise parametric representation of the PDF, as required to develop automatic methods of diagnostics.