Obtain a copy of form 6251 from the IRS website. You can download the form and print it out or request a copy from the IRS.
Report your adjusted gross income, found on line 38 of your form 1040, on line 1 of form 6251. If you itemized your deduction, report the amount on line 41 instead of line 38.
Complete lines 2 through 28 with the appropriate values from your tax return. These are deductions that are disallowed under the AMT, including state and local sales and income taxes, mortgage deductions and miscellaneous tax deductions.
Enter the total from lines 1 through 28 on line 29 to calculate your income subject to the alternative minimum tax.
Determine your AMT exemption amount using the table on form 6251 or the AMT exemption worksheet on page 8 of the form 6251 instructions and enter the amount on line 30. The exemption amount is determined by your income and filing status.
Subtract the amount of your AMT exemption (line 30) from your income subject to the AMT (line 29) to calculate your AMT taxable income and report it on line 31.
Calculate your AMT by following the instructions next to line 32. For 2010, if your AMT income is below $175,000 ($87,500 or less if married filing separately), multiply your AMT income by .26 to calculate your AMT. If your income exceeds those limits, multiply your AMT income by 0.28 and subtract $3,500 ($1,750 if married filing separately) to calculate your AMT.
Enter the amount of your alternative minimum tax foreign tax credit, if any, on line 33.
Subtract the line 33 from line 32 to calculate how much you would have to pay under the AMT system and write the result on line 34.
Write the amount of tax you would owe under the standard income tax calculations on line 35. If this amount exceeds the amount on line 34, you do not owe any AMT. However, if the AMT exceeds your standard income tax, you must enter the difference between the AMT and the standard tax on line 36 of form 5261 and line 45 of form 1040. For example, if you would owe $30,000 under standard tax calculations but $35,000 using the AMT rules, you would owe an addition $5,000 due to the AMT.