So if someone has a AGI of $90,000 and contributed $72,000 in cash to a public charity. What is the deduction allowed to be used for AMAT purpose? By my calcualation it is $45,000. Am I correct? Thanks!

I assume AMAT is AMT (Alternative Minimum Tax).

AMT is NOT an issue for charitable contributions.

However, you can give NO MORE than 50% of your AGI to a 50% charity.

The limit is 30% if the charity is designated a 30% charity.

So if your public charity is 50% (most likely), your answer is correct!

Yes, I did not notice that I added an extra "A", but it is suppose to be AMT and NOT AMAT. Sorry. Thanks. The amount of the deduction also depends on the type of charity receiving the contribution, the type of property contributed, and the applicable limitations. For individuals, the general overall limitation applicable to public charities is 50% of the taxpayer's AGI for the year. Contributions of capital gain property, in which the capital assets held over one year on which a gain would be realized if sold to public charities are generally valued at the property's FMV, but the deduction may not exceed an overall limit of 30% of AGI instead of a 50% limit. Thanks!

You seem to grasp the essentials on this one! :-)