The share of each extra dollar of income that will go to the IRS. It’s not necessarily the same as the rate in your top tax bracket because in many cases rising income squeezes the value of tax breaks, so that the extra income is effectively taxed more harshly than advertised. Knowing your marginal rate tells you how much of each additional dollar you make will go to the IRS and how much you’ll save for every dollar of deductions you claim.