It's actually 35%, if you don't count people who file with just the standard deduction. While technically a "deduction", the standard deduction is pretty dead-simple to compute. It only gets more complicated if you're taking itemized deductions, and that's about 1/3 of taxpayers (though that's still a high proportion of itemized filers compared to many countries).

Source: http://taxfoundation.org/article/most-americans-dont-itemize...