Turbotax calculates the blended tax rate by dividing total tax by your Adjusted Gross Income (AGI) on Line 11.
This AGI is before deductions including the standard deduction of $14,600 (or $29,200 for married couples).
The blended tax rate considers that the effective tax rate is 0 percent for the first $14,600 of income. Therefore, the blended tax rate can be lower than the minimum tax rate published of 10 percent.
