Your question suggests that you might be interested in setting tabular rules (
\hline's) of different weight in order to improve your typography rather than for the purposes of some simple "one-off" adjustment. If so, you should consider the booktabs package (if you haven't already done so). It provides canned weighted rules (
\midrule, etc) which, for typographical reasons, are defined in terms of fractions of
em's rather than
pt's, although these are very easy to define in
pt's as well. Each rule can be locally or globally parameterised for your own specific weighting requirements. The package also provides macros like
\lightrulewidth which you can use "as is" or can redefine to suit your particular needs. (Just as the author uses the term 'rule' where others might call it 'line', the author also uses the term 'width' where others might prefer to use 'thickness'.) The package is exceptionally easy to use. As a side-benefit, the documentation contains some very insightful guidelines about the ins and outs of good tabular typography.