The smartasset paycheck calculator seems grossly inaccurate. I started with the default example, single taxpayer with $66,833 annual salary. For such a taxpayer, with no dependents, no other income, and standard deduction, in 2023 the effective federal tax rate would be 10.4%, while this calculator shows federal income tax withholding at 15.44%, quite a huge difference. This example taxpayer is greatly over-withheld and would receive a nice refund at tax filing time, but the report fails to take that into account.