The general rule-of-thumb is that it ordinarily doesn' t make sense to switch jobs for less than an increase of at least 10% to 15% more in salary.  It' s also commonly accepted that the value of benefits associated with full-time jobs is anywhere from 25% to 35% of the salary being earned.  So, one way to come up with a reasonable idea of what you should charge is to figure out what your annualized salary would be with at least a 10% to 15% increase and then add on another, say, 30% to reflect a reasonable amount for the value of benefits you won' t be receiving as a contract employee.  Whatever that annualized amount turns out to be you can break it down to an hourly rate if you want to and let it represent the bottom of your anticipated contract rate range.  Call it $X.  Add another 10% to $X to come up with the top of your anticipated contract rate.  Call it $Y.  It will be a pretty narrow range, but it should represent a reasonable, albeit narrow hourly range in which you need to be to seriously consider switching jobs from an established company to a start-up - always a somewhat risky thing to do...