There are two important differences between the calculations for an AMT and regular income tax. The first one is the presence of an AMT exemption limit. While everyone is required to file regular income tax, the AMT was meant to ensure that the very wealthy pay their tax dues. The AMT exemption limit is a means to exclude the lower income and middle class from being penalized. The second difference lies in the treatment of deductions and gains. AMT adds back tax preference items or items that are excluded while calculating regular income tax.