Applying a magnetic field B to a double-barrier resonant-tunnelling diode, perpendicular to the layer structure, introduces oscillations in current density and capacitance that are periodic in 1/B. A derivation of this periodicity is given, based on coherent wave propagation. Two magneto-periods are found, corresponding to the electron concentration in emitter and well, respectively. Numerical calculations are presented for a semiconductor model with self-consistently determined electron potential.