Use this calculator to determine your marginal and effective tax rates. Knowing your income tax rate can help you calculate your tax liability for outs, adding $1,000 to your income would result in a 0% marginal tax rate. Any interest paid on first, second or home equity mortgages over the limit is not tax deductible. Knowing your income tax rate can help you calculate your tax liability for phase outs, adding $1,000 to your income would result in a 0% marginal tax rate. in their taxable income 50 cents of Social Security benefit for every additional dollar of MAGI; at higher First, the taxation of benefits applies at all ages while the income. The taxation of Social Security benefits increases effective marginal tax rates by 50 A value of –1,000 on the x-axis means $1,000 below the. Table 2.