W-4 and withholding taxes for self-employed spouse
With your income so high, your marginal tax rate should be pretty easy to determine.  You are very likely in the 33% tax bracket (married filing jointly income range of $231,450 to $413,350), so your wife's additional income will effectively be taxed at 33% plus 15% for self-employment taxes. Rounding to 50% means you need to withhold $19,000 over the year (or slightly less depending on what business expenses you can deduct). You could use a similar calculation for CA state taxes. You can either just add this gross additional amount to your withholdings, or make an estimated tax payment every quarter.  Any difference will be made up when you file your 2017 taxes. So long as you withhold 100% of your total tax liability from last year, you should not have any underpayment penalties.