To make the estimating simpler, TaxCaster makes various assumptions. TaxCaster generally calculates only income and self-employment taxes. It does not calculate various penalties such as penalties for early distributions from retirement accounts.
Even though 2017 TurboTax is not finalized yet, in most cases 2017 TurboTax will give a more accurate result than will TaxCaster. Errors due to 2017 TurboTax not being finalized will be small in almost all cases since these errors are generally the result of cost-of-living adjustments. (Of course this likely won't be true for 2018 taxes when there will be major changes resulting from the new tax law.)
