A recently developed data analysis tool allows direct measurement of the total root-mean-square noise in magnetometer data. Since the total is a combination of intrinsic instrument noise and solar wind magnetic field fluctuations, we can put upper limits on the instrument noise during periods of relatively quiet solar wind fields. The technique involves taking vector and scalar products of successive magnetic measurements; the resulting scalar quantity is insensitive to slow field variations as well as sudden ones (i.e., tangential discontinuities) and provides a measure of total noise. We illustrate this analysis by determining the noise levels for the Mars Global Surveyor magnetometer. Far from Mars and any obvious waves, we find that the magnetic field measurements noise level is ¿0.05 nT. This is consistent with the ground-based instrument's measured noise level of ¿0.01 nT expected under ideal conditions.