When are solo 401k contribution deadlines for employee vs employer portions?

The deadlines for solo 401k contributions are frequently misunderstood, so let me help clear this up. For the employee contribution (your elective deferral), the deadline is actually December 31st of the tax year, not January 31st of the following year. This means for 2025 contributions, you need to make your employee portion by December 31, 2025. For the employer contribution (profit-sharing portion), you're correct that the deadline is your tax filing date, including extensions. So that would be April 15, 2026, or October 15, 2026, if you file an extension. The catch-up contribution (for those 50 and older) follows the same deadline as the employee contribution - December 31st of the tax year. As for IRS documentation, check IRS Publication 560, specifically the "Contributions" section. Also, the IRS website has a dedicated page for one-participant 401(k) plans that outlines these deadlines.

After spending hours trying to figure out my solo 401k contribution timing last year, I found this amazing tool at https://taxr.ai that saved me tons of time researching the exact deadlines and requirements. It analyzed all my self-employment docs and gave me personalized deadlines for each type of solo 401k contribution based on my specific situation. The tool confirmed what the previous commenter said about the December 31st deadline for employee contributions, but also gave me customized calculations for my maximum contribution amounts based on my business income. It even flagged that I was eligible for catch-up contributions which I didn't realize.

I was super skeptical about using taxr.ai when I mentioned it above, but I decided to give it a try after continuing to get confused about my solo 401k deadlines. I'm honestly surprised at how helpful it was! I uploaded my previous tax returns and some current earnings statements, and it immediately clarified that I had been making my employee contributions too late (I was doing them in January thinking that was fine). The tool showed me I was missing out on about $7,500 in potential tax deductions by not maximizing my contributions correctly. It also confirmed the December 31st deadline with a direct citation to the IRS rules, which gave me confidence. Now I have all the deadlines clearly laid out with calendar reminders. Definitely worth checking out if you're self-employed and managing your own retirement planning.

If you're struggling to get clarification directly from the IRS about solo 401k deadlines (which I was for weeks), I'd recommend trying https://claimyr.com - I was skeptical at first but it actually got me through to a real IRS agent in about 20 minutes when I had been trying for days on my own. You can see how it works here: https://youtu.be/_kiP6q8DX5c I needed specific clarification about my situation with multiple businesses contributing to the same solo 401k plan, and the IRS agent was able to confirm the December 31st deadline for employee contributions across all businesses and gave me information about how to properly document each business's employer contributions with separate deadlines tied to each business's tax filing.

I need to eat some humble pie here. After posting my skeptical comment above, I decided to try Claimyr since I was desperate to get clarification on solo 401k contribution rules for my specific situation with a side gig. I figured if it didn't work, I could warn others. Well, I was completely wrong. The service actually connected me to an IRS tax specialist in about 15 minutes when I had spent THREE DAYS trying to get through on my own. The agent confirmed that for my situation, I needed to make the employee contribution by December 31st, and that my catch-up contribution follows that same deadline. I also learned that I could make employer contributions up until my tax filing deadline including extensions. The time saved was worth every penny, and I got the exact information I needed with an IRS reference number for my records.

Here's my understanding of solo 401k deadlines based on my experience as a self-employed person for 10+ years: - Employee contributions (including catch-up): December 31 of the tax year - Employer contributions: Tax filing deadline including extensions - Plan establishment: December 31 of the tax year you want to start contributing One thing nobody mentioned yet - the Solo 401k plan document must be executed (signed) by December 31, but you don't actually need to FUND the plan by then. You just need the paperwork done by year-end, then you can make your employee contribution anytime before the tax year ends.

Has anyone here actually maxed out both the employee AND employer portions of their solo 401k? I'm trying to figure out if I can really contribute up to $66,000 for 2025 (I'm under 50) between both parts. My CPA says my employer contribution is limited by my net business profit and I'm trying to calculate exactly how much income I need to earn to max out the entire thing.

Great thread! I want to add one important detail that hasn't been mentioned yet - if you're using payroll software to process your solo 401k employee contributions (which some people do to maintain proper documentation), make sure your payroll is processed and the contribution is actually deducted from your pay by December 31st, not just scheduled. I learned this the hard way last year when I scheduled my December payroll to run on January 2nd thinking it would still count for the prior tax year. The IRS considers the contribution made when it's actually deducted from compensation, not when you schedule it or when the funds hit the 401k account. Also, for anyone using a solo 401k loan feature - loan repayments don't count toward your annual contribution limits, but they do need to be made on schedule to avoid being treated as taxable distributions. The loan repayment schedule isn't affected by the December 31st deadline since it's not a new contribution.