The interplanetary magnetic field is known to be highly helical. Although the detailed spatial structure of the fields has yet to be elucidated, the helicity spectrum has been conjectured to result from a random walk in direction of a constant magnitude magnetic field vector. We demonstrate that a model using three-dimensional fluctuations with variations in ‖B‖ gives a good fit to the helicity spectrum as well as to other properties of the interplanetary magnetic field.