There are many sites that allow you to roughly estimate your 2018 taxes...but almost all are very simplistic....for example they don't take into account special tax provisions for Dividends and Capital Gains and they don't allow you to itemize your deductions or tax credits, etc so their estimate may be way off.
I just stumbled on a company/site OLT (online Taxes)[olt.com] which allows you to get a much better estimate of your taxes....allowing you to break down your income sources, deductions, etc. It is surprisingly complete and it's a slick deal....it's free. One other major plus...you don't have to enter any personal information (email, etc) or even open an account to use this estimator.
But a warning....unlike TurboTax and TaxAct, it doesn't take you through questions...you just follow their steps and enter your best estimate of your 2018 income, deductions, etc.
Note: It's very interesting to take your 2017 return and enter all the numbers to see what you would have paid, if the new 2018 tax changes had been in effect for 2017....Actually, that's an excellent way to see if the new tax law actually helped (or hurt) YOU...and how much you would have saved if the current tax cut had been in effect last year!
If you file a 1040EZ (short form), this is overkill, but if you file a more complex 1040A, you might want to look at this estimator.
Alternatively, you can, of course, just buy TaxAct, Turbo Tax or other software which can do the same estimates. This is just a free option.