The alternative minimum tax sets a limit on how low tax benefits can lower a person's regular tax amount under current tax laws, according to the IRS. If a taxpayer's total tax amount drops below the AMT because of tax benefits, then that person has to pay the AMT.
If a taxpayer utilizes tax software to prepare his tax return, the software automatically calculates the alternative minimum tax, explains the IRS. Individuals who complete their own Form 1040 tax return without the use of software may have to complete Form 6251 in order to find out if they have to pay the alternative minimum tax. The AMT assistant helps people determine whether or not they even need to fill out Form 6251.