Take your last year's 1040 and calc your effective tax - total tax (line 24)/total income (line 9) and see where you land, should be in that ballpark but of course taxes are so variable just make sure you include social security/medicare in your total tax if you are a W2 and don't forget state - you really just want all the taxes you paid (not witheld!) divided by your gross income. You can also (if you use it) just take the Turbo Tax software and run a "simulation" assuming her income but no estimates paid and that will tell you about what it's going to cost of course assumption is tax law will stay the same. Again, there is a bit of guess work here. I know folks that are 1099s that have no clue and when they get that tax bill in April they shit - don't be them, hold some back and at least make a calculated effort at getting it close. I have seen others hold way too much back (40-50% which is crazy high) and giving the man their money for free for the year. ~20% is going to be about right for lots of people.