For those of you who are married, you may want to check what you have filed on your W-4 at work – especially if you recently got married. I have seen something like five posts a day that go something like
My spouse and I each file as married with 0 allowances on our W-4 but somehow we owe $3,000! What went wrong??
There is a simple thing that went wrong here. If you list your W-4 filing status as Married (2019 version) or Married filing jointly (2020 version), the IRS is set up to assume that you are the sole breadwinner of your family. If both you and your spouse work, your household income is going to be a lot higher than your employer thinks, and you will not have enough withheld in taxes.
There are two easy solutions here depending on your relative incomes:
Quick Solution (similar incomes): On your 2020 W-4, file as married but check the "two jobs" box on line 2(c). This will withhold as if you have a spouse who makes exactly as much as you do, which is close enough for most purposes. If you have a 2019 or older W-4, you simply choose a filing status of "Married, but withhold at higher single rate".
Detailed Solution (more correct, or less similar incomes): You can either complete the IRS Calculator (requires a lot of details) or the Multiple Jobs Worksheet and enter the results. For the 2019 version, use the Two Earners/Multiple Jobs worksheet. This will exactly calculate the right withholding for you based on your situation.