To create an Excel spreadsheet that calculates the marginal tax rate, begin by opening a spreadsheet and create columns with the titles "Taxable Income," "Marginal Tax Rate," and "Base Tax." Under This is 0% of your total income of $0. 0% would also be your average tax rate. Your income puts you in the 0% tax bracket. At higher incomes many deductions and many credits are phased out. This increases your tax bill and your marginal tax rate. With these phase outs, adding $1,000 to your income would result in a 0% marginal tax rate. Marginal tax rate is the income tax rate that applies to each additional dollar of taxable income. It can be calculated by dividing increase in tax payable in response to a $1 increase in taxable income. In a progressive tax structure, it is the income tax rate applicable to the highest tax bracket in which the last dollar of taxable income falls.