Why dont we used a flat tax to calculate how much people should pay?

Given a 10% tax rate, if I make $10 and you make $100, I pay $1 tax and you pay $10 tax, so the richer person pays more tax. So far, so good. But presumably, somewhere along the line, somebody decided that the rich(er) person should not only pay mathematically more tax ($10 vs $1), but percentage-wise more (you pay 15% tax, I pay only 10%). At first blush, that sort of seems ok too, but the problem is the issue of tax credits. They may have been more or less even-handed at one time, but over the course of time, they have come to favour the rich in the sense that they have access to deductions that somebody making less money doesn't have. So while they may be liable for 15% initially, with their deductions, they are able to whittle this down to 7%, while the poorer person is stuck at 10%. And naturally enough, as taxes and deductions increase over the course of time, this disparity grows. There is also the argument (which I'm not entirely sure I buy) is that if the rich, especially the billionaire class, are too excessively taxed (albeit too excessively perhaps only in their minds), they will take their billions off-shore / overseas and the government and we the people through social programme funding will end up getting nothing.

A flat tax would be great if you were in the upper income levels but disastrous to lower income people.

Because a flat tax rate of around 25% would be necessary to generate the same revenue that the current graduated tax does. The poor would be decimated by a flat tax, and the middle class would bear the major portion of the cost. The first rule of taxation is that a fair tax must take into consideration the taxpayer's ability to pay. To put it very simply, the wealthy can afford to pay more and therefore they SHOULD pay more. The ultra-wealthy would LOVE a flat tax rate as they'd save millions. But the working class and the poor would take it in the shorts.

Just what the first guy said...it's simply unfair for a person making $10,000 to have to pay the same amount as a person making $400,000.

I agree. Flat tax of about 10% with no deductions combined with a federal sales tax on everything but food and primary rent or housing. This way everyone pays some as they should, and those who spend money pay the most. It is a little like Florida and Navada which have no income tax. Just sales tax.