Pulling this out of the Downsized thread since it will certainly drag it off topic
Let me break it down for you (current system, two individuals with no dependents living on their own):
$25,000 per year pre-tax income
$17,825
minimum standards living expenses
$7,175 left over
$3,844 taxes (current system)
$3,331 left over
$150,000 per year pre-tax income
$17,825 minimum standards living expenses
$132,175 left over
$22,372 taxes (current system)
$109,803 left over
flat tax system (20%):
$25,000 per year pre-tax income
$17,825 minimum standards living expenses
$7,175 left over
$5,000 taxes
$2,175 left over
$150,000 per year pre-tax income
$17,825 minimum standards living expenses
$132,175 left over
$30,000 taxes
$102,175 left over
Did you see that? $150,000 is paying 7% more of his excess, and $25,000 is paying 34.5% more of his excess.