TurboTax uses the IRS optional sales tax tables and adds any big ticket purchases you entered into TurboTax to figure out your sales tax deduction amount.
The nice thing about using the tables is that you qualify for the amount that corresponds to the state where you live, the range of income that includes the amount of your adjusted gross income (increased by certain types of tax-exempt income) and the total number of personal and dependent exemptions you take even if you don't pay a single dollar in sales tax. Additionally, if you live in a county or municipality that charges an additional rate of tax, such as New York City, you can increase your sales tax deduction even more. TurboTax will take care of these calculations to ensure that you get the best deductions for your specific situation.
https://turbotax.intuit.com/tax-tips/tax-deductions-and-credits/deductions-for-sales-tax/L2WX6nkZm
