The Net Interest Calculator approaches the challenge of evaluating a lease from a different angle. New federal rules compel dealers to disclose all the elements of a lease -- except for the interest rate you'll be paying. This worksheet will calculate one for you, factoring in onetime fees along the way. We've provided space so you can plug in the numbers for two leases and compare them side by side. The lower the net interest rate, the better the deal. You can also compare this figure to competitive interest rates on bank or credit union auto loans.