Payroll tax calculations withheld on employee's paycheck are based on the payroll data you entered in the system. Thus, QuickBooks automatically calculate the taxes for you. To ensure accurate calculation of wages and tax amounts, I recommend checking your set up.
You can also use our Paycheck Calculator to have an idea of how the system calculates your pay. Since at Intuit, we do our best to be compliant by the IRS and your state agency. QuickBooks Online correctly calculates taxes according to the IRS tax table. This is to make sure we can avoid any penalties due to a variety of errors and omissions related to withholding.
I invite you checking the IRS Publication 15 for more details about the Federal Income tax withholding methods.