when I started preparing tax returns decades ago, there were no computers so they had to be done by hand. I could carry a copy of the Internal Revenue Code - 1 volume that could be carried in one hand. over the next 5 decades the US Congress has seen fit to make the code so complicated only the simplest returns can be prepared correctly by hand. Your concern about accuracy is the same that pro's using software costing many thousands of dollars have. There is no way to be sure. For a fact, this year's IRS's original worksheet for computing the Capital Gains Tax contained an error that affected some filers. Since some software providers like TT followed the IRS worksheet, some of its filers were affected. The IRS didn't catch or fix the error until after the 4/15 filing deadline. During my career I have seen IRS notices saying taxpayers returns were wrong to be in error. So you see even the IRS gets it wrong. At least TT offers to pay penalties and interest if its computations are wrong.