We determine a simple expression for the ramp width of a collisionless fast shock, based upon the relationship between the noncoplanar and main magnetic field components. By comparing this predicted width with that measured during a shock observation, the shock velocity can be determined from a single spacecraft. For a range of low-Mach, low-&bgr; bow shock observations made by ISEE-1 and -2, ramp widths determined from two-spacecraft comparison and from this noncoplanar component relationship agree within 30%. When two-spacecraft measurements are not available or are inefficient, this technique provides a reasonable scale size estimation for low Mach shocks.¿ 1997 American Geophysical Union