  High-dispersion, archival spectra of magnetic Ap stars with resolved Zeeman
components in Stokes I are used to derive a simple relation that can be
utilised to estimate the mean surface field strengths of stars with v sin i >
10 km/s.
  For each star, the mean surface field, as measured from the observed
splitting in Fe II at 6149 A, is compared to the differential broadening of
spectral lines with large and small Lande factors in order to produce a
relation to estimate the field strengths of magnetic stars with unresolved
Zeeman patterns.
  The method is shown to be reliable for rotational velocities up to about 50
km/s for field strengths down to about 5 kG.
  These results should allow for better constraints to be placed on the mean
surface magnetic fields of Ap stars where Zeeman patterns are unresolved.
