There was recently a question about moving in the opposite direction (salary to contractor) that you may find helpful. Especially this answer that links to a calculator where you can find the contractor rate corresponding to a given salary plus some variables. I've done the calculation a couple of different ways, and the calculator was eerily close to the numbers I found in my spreadsheet even though it didn't account for some of the things I used in the spreadsheet.
As a second method, Turbotax has a calculator you can use to estimate your taxes.
What you want to do is enter your expected annual salary in the wages box, fill in the other boxes (mortgage interest, dependents, etc), leave withholding/payments at zero, and see how much tax you'd owe -- the total number. Write this down.
Then go back and zero out the wages box and enter your contractor income in the business income box. Leave the other figures the same. Look at the total number.
Then subtract the tax amounts from the respective earnings figures to get an after-tax earnings number for each.
The equation gets a bit more complex if you are currently paying for your own benefits, and these benefits will be paid by your prospective employer. Add/subtract these from the after-tax numbers you just calculated to get a better idea of your take-home comparison.
Another factor to consider is any 401k match you might get.