Get your taxes done using TurboTax

Let me clarify my assumptions. I am assuming that you have one job that has tax withholdings (you are issued a W-2 for it). That's why you technically, have one job. I am also assuming that the side gig is treated as self-employment income and there are no tax withholdings (instead of a W-2, you will be receiving a 1099-NEC). 

Based on that, you probably want to add your expected net income from the side gig on line 4 of the W-4.  Alternatively, you don't include that income on line 4 but instead pay estimated tax payments to cover that income. 

As an example for estimated tax payments, I ran the numbers through the TurboTax Calculator (see screenshot), and based on my numbers, I would owe an additional $3,699. This should be paid quarterly to the IRS (remember, there is also state tax due, if applicable to you). Read more about Estimated Tax Payments here.