How to fill out W4 for second job

There are instruction on the W-4 regarding how to complete the form if you have more than one job. They also have an online tool: https://www.irs.gov/individuals/tax-withholding-estimator

The general idea is that you have to account for both jobs or your taxes will be under-withheld, which could cause you to have to write a big check when you file next spring. This is very important if you have multiple jobs at the same time.

The IRS has a FAQ on their website:

FAQs on the 2020 Form W-4

10. Why do I need to account for multiple jobs (Step 2)? I have never done that before.

Tax rates increase as income rises, and only one standard deduction can be claimed on each tax return, regardless of the number of jobs. Therefore, if you have more than one job at a time or are married filing jointly and both you and your spouse work, more money should usually be withheld from the combined pay for all the jobs than would be withheld if each job was considered by itself. Adjustments to your withholding must be made to avoid owing additional tax, and potentially penalties, when you file your tax return. All of this has been true for many years; it did not change with the recent tax law changes. The old Form W-4 accounted for multiple jobs using detailed instructions and worksheets that many employees may have overlooked. Step 2 of the redesigned Form W-4 lists three different options you should choose from to make the necessary withholding adjustments. Note that, to be accurate, you should furnish a 2020 Form W-4 for all of these jobs.

11. Which option in Step 2 should I use to account for my multiple jobs? Which is most accurate? What if I don't want to reveal to my employer on my W-4 that I have a second job?

Step 2 allows you to choose one of three options, which involve tradeoffs between accuracy, privacy, and ease of use:

Step 2(a): For maximum accuracy and privacy, use the Tax Withholding Estimator at www.irs.gov/W4app. You will generally be guided to enter an additional amount to withhold in Step 4(c). While you will need to know the approximate amount of pay for each job, you will enter the additional amount of withholding in Step 4(c) on the Form W-4 for only one of the jobs. If pay for any of the jobs changes significantly, you will need to use the Tax Withholding Estimator again and furnish a new Form W-4 to change the amount in Step 4(c) to have accurate withholding.

Step 2(b): If you do not have access to the Tax Withholding Estimator but wish to have roughly accurate withholding and retain privacy, you may use the Multiple Jobs Worksheet on page 3. You will be guided to enter an additional amount to withhold in Step 4(c). While you will need to know the approximate amount of pay for each job, you will enter the additional amount of withholding in Step 4(c) on the Form W-4 for only one of the jobs. If a change in pay for any of the jobs changes the additional withholding amount in the lookup table used with this worksheet, you will need to furnish a new Form W-4 to change the amount in Step 4(c) to have accurate withholding. If you (and your spouse) have a total of only two jobs and the pay at the higher paying job is more than double the pay at the lower paying job, this option is generally more accurate than choosing Step 2(c). If the pay at each job is more similar, choosing Step 2(c) is more accurate than choosing Step 2(b).

Step 2(c): If you (and your spouse) have a total of only two jobs held at the same time, you may check the box in Step 2(c) on the Forms W-4 for both jobs. That is, to use this option, you should complete a Form W-4 for each job with the box in Step 2(c) checked. The standard deduction and tax brackets will be cut in half for each job to calculate withholding. You will not need to furnish a new Form W-4 to account for pay changes at either job. This option is accurate for jobs with similar pay; otherwise more tax than necessary may be withheld from your wages. This extra amount will be larger the greater the difference in pay is between the two jobs.

12. The instructions above Step 3 say that in multiple job households, adjustments in Steps 3 – 4b are to be made on only one form, and that withholding will be most accurate if the adjustments are made on the W-4 for the highest paying job. But what happens if pay at two jobs is relatively similar or if changes in pay over time result in another job becoming the highest paying?

In general, making these adjustments on the Form W-4 for the highest paying job increases accuracy. However, if the jobs in your household pay about the same or if changes in pay over time change which is the highest paying job, it is less important which Form W-4 is used to make the adjustment.