I've heard different theories ranging from 17% to 23% on what the flat tax would need to be to cover current expenditures.
Here's how rates come out for a flat individual/corporate income tax, based on maintaining constant levels of revenue to pay the nation's debts etc. (not including SS/Medicare payments)
http://www.library.unt.edu/govinfo/subject/vital.html
Standard Allowances | Option 1 | Option 2 | Option 3 | Option 4 | Option 5 |
Single | $13,100 | $13,100 | $ 6,550 | $ 6,550 | $0 |
Joint | $26,200 | $26,200 | $13,100 | $13,100 | $0 |
Head of Household | $17,200 | $17,200 | $ 8,600 | $ 8,600 | $0 |
Dependent Exemption | $ 5,300 | $ 2,650 | $ 5,300 | $ 2,650 | $0 |
Revenue Neutral Tax Rate | 19.9% | 19.4% | 16.8% | 16.3% | 13.1% |
Source: Congressional Budget Office, 1995.
If it were to be a Flat individual income tax alone(without personal exemptions, or any deductions), the rate would be the effective total federal tax rate with respect to gross family income.
Total federal taxes as a percentage of gross family income is 23.5% (taxfoundation)