The rule of thumb for conversion from a full-time salaried position with benefits to an hourly rate as a contractor or freelancer is
( annual salary / 2000 ) X (2, 3, or 4)
Your target annual salary, divided by 2000, which is the number of working hours in the year, will give you an hourly rate. Multiplying that by 2 is intended to reflect the cost of benefits. That multiplier may be more or less depending on what your needs are and the number of dependents you have.
The figure of 2000 hours assumes that you will be working all but 2 weeks out of the year, which is a reasonable expectation for a full-time employee. As a freelancer or contractor, however, you may be very well working much less than that, in which case you may want to figure in a number of hours lower than 2000.
The trick is to set a rate that's not too low -- sometimes a rate can sound very high, but depending on what you need to earn and what you need to pay for, it can be easy to fall into a money-losing situation.
This calculation is merely a shortcut -- you'll want to do a more detailed analysis of what benefits will actually cost you before entering into a contract.
See also https://www.quora.com/Compensation/How-should-a-contractor-or-consultant-arrive-at-an-hourly-rate