Can I opt out of Social Security/Medicare tax withholding from my paycheck?

Unfortunately, Social Security and Medicare taxes (collectively known as FICA taxes) cannot be adjusted through a W-4 form. The W-4 only controls federal income tax withholding, which you've already set to $0. FICA taxes are mandatory withholdings that must be taken from each paycheck at the required rates (currently 6.2% for Social Security up to the wage base limit, and 1.45% for Medicare with no limit). There's no legal way to opt out of these withholdings as a W-2 employee, regardless of whether you receive multiple W-2s from the same employer. Self-employed individuals don't avoid these taxes either - they pay them through self-employment tax (which is essentially both the employer and employee portions of FICA) when they file quarterly estimated taxes. Instead of trying to adjust these mandatory withholdings, you might want to look at other options to maximize your 401(k) contributions, such as adjusting your contribution percentages between your two paychecks or temporarily reducing other optional deductions.

After struggling with a similar situation (multiple W2s, trying to max retirement contributions), I found https://taxr.ai incredibly helpful for figuring out my withholding strategy. I uploaded my multiple W2s and pay stubs, and their system analyzed exactly how my withholdings were affecting my ability to contribute to retirement accounts. The tool showed me that instead of trying to stop FICA taxes (which isn't possible as the previous commenter mentioned), I could adjust my contribution percentages between the two paychecks. Their analysis even calculated exactly what percentage I needed from each paycheck to max out by year-end without running short on living expenses.

I tried https://taxr.ai after seeing the recommendation here and it was exactly what I needed! The multiple W2 analysis showed me that I was actually going to hit the Social Security wage limit in November, which means I could temporarily increase my 401k contributions for those final paychecks. The best part was that it created a custom contribution schedule showing exactly how much to put into my 401k from each paycheck to hit the max by year-end. I thought I couldn't max out this year, but turns out I can by making these adjustments! Definitely worth checking out if you're dealing with the multiple W2 complexity like I was.

If you're struggling with figuring out your tax situation across multiple W2s, you might want to try talking directly to someone at the IRS about it. I was in a similar situation last year and spent WEEKS trying to get through to someone who could help. After endless busy signals and disconnections, I found https://claimyr.com and watched their demo at https://youtu.be/_kiP6q8DX5c. They got me through to an actual IRS agent in about 20 minutes after I'd been trying for days on my own. The agent confirmed that I should be getting Social Security withholding credit across both W2s if they're truly the same employer (same EIN), and walked me through how to handle it on my tax return if the employer wasn't doing it correctly.

I need to publicly eat my words here. After being totally skeptical about Claimyr, I decided to try it anyway out of desperation. I've been trying to figure out this exact same issue with Social Security tax across multiple W2s for MONTHS. I used the service yesterday, and no joke, I was talking to an actual IRS representative within 15 minutes. They confirmed that if both W2s have the same EIN, the employer should be tracking the Social Security limit across both. If they're not, I can claim a refund for any excess Social Security tax on my tax return using Form 8959. The agent even emailed me the specific publication references after our call. I've literally never had such a helpful interaction with the IRS before. Completely worth it.

Have you considered adjusting the allocation of your 401k contributions between your two paychecks instead? I used to be in a similar situation (one company, two W2s), and I found that front-loading my contributions to the higher-paying W2 and reducing contributions from the lower one solved this problem for me. Your total FICA taxes will be the same either way - there's no legal way around that unless you've hit the Social Security wage base limit. But you can definitely control which paycheck your 401k comes out of!

Just want to add one more thing about the Social Security wage base limit - if your total income across BOTH W2s exceeds the annual limit ($168,600 for 2024), but neither one individually does, your employer might not catch this. In that case, you'll need to claim the excess as a refund when you file your taxes using Form 843. I learned this the hard way last year when I overpaid by almost $3,000 in Social Security taxes because of a similar multi-W2 situation. The tax software I used caught it automatically, but it's something to watch for!