It's best to use a tax withholding estimater like our W-4 Calculator. The IRS also has a withholding calculator Carefully following the steps on the W-4 and being sure to indicate that there are multiple jobs in Step 2 will help facilitate more accurate withholding. There is also a Multiple Jobs Worksheet as part of the W-4. This will also help determine an additional withholding amount that to enter in Step 4(c) on one of the W-4 forms (typically the higher earner's).