Answer by Wray Rives:
A flat tax in its basic form just means all taxpayers pay tax at the same rate or percentage of whatever number the tax is based on. That is as opposed to a progressive tax rate which is what the US currently has, which means that as your income (or whatever factor you choose to tax) increases the rate that you pay also increases. see http://rivescpa.co/?p=522
If you get down to an individual level, the pros and cons really depend on where you fall on the income scale, because generally with a flat tax, people higher on the scale will tend to pay less tax and people lower on the scale will tend to pay more tax. If you can get enough information about how a flat tax proposal works, then you could take your tax return from last year and estimate if you are pro or con.
If you follow the link I gave above at the bottom of the page there are two links one is to a current House bill for a flat tax and you can find the calculation of taxable income about half way down the page.
I guess in order to "sell" the concept what you usually see is a flat tax proposal combined with some element of "tax simplification". In my opinion, that is smoke and mirrors, because tax simplification, usually means we took away your deductions, but look we lowered your tax rate to 20% for everybody and it ultimately has nothing to do with the flat tax rate.
The reality is if I pay $30,000 in taxes it does not make a big difference if you calculate that as 30% of $100,000 taxable income or 20% of $150,000 of taxable income. Just because you define taxable income differently, I am still paying $30,000 in tax and none of the tax proposals you see for the US right now are saying they plan to lower how much tax is collected in total.
Most of them go to great lengths to explain why their plan will raise the exact same amount of revenue for the government. The basic math in that tells you that either everyone is going to pay the same amount of absolute dollars in tax as they already pay or for every dollar that someone at the higher end of the scale saves, someone at the lower end of the income scale is going to pay more.