It depends on rim width... This page has a chart.
http://www.bicycletires.com/a_49/tire_width/article.htm
There is also a formula, but I don't recall what it is... You can probably derive it from the table here and/or Sheldon Brown's site. It is like tire width = rim inner width * 1.x Where 1.x is a ratio... a different ratio for minimum and maximum.