In accelerator magnet design, important criteria are the magnetic field distribution and the magnetic field quality. Accelerator magnets are usually designed for the ideal case in which no magnetic material is present in the field region. In practice, however, the presence of a vacuum chamber inside the magnet aperture can affect the magnetic field in case its material has nonnegligible relative permeability. We quantify this effect based on numerical simulations with a conventional quadrupole magnet model. The results show that there is a measurable effect of the configuration, the material choice, and the presence of welding seam in particular.