My Fed Tax calculated by TT is less than 50% of the amount comparing to a manual calculation using taxable income and IRS tax rate. What could cause this?

I am surprised to see TT calculated Fed tax is less than 50% of the amount  that I manually calculated using my taxable income and IRS tax rate.  How do I know if TT calculation is correct?  What could cause such a big gap?  Is there any place I should check?  Thanks.