Turn 2025 profit into a solo 401(k) before the extension deadline

The extension changes the deadline, not the math

If you filed a 2025 business return extension, the solo 401(k) funding window can run into September or October 2026. The move that still matters is the employer profit-sharing contribution. You read the bank balance daily, and you know profit on the books and cash in the account are different animals.

A low salary can leave real profit in the business, and the solo 401(k) is one of the last places to put it before the return is due. The extension gives you time, but it does not remove the need to do the math while the cash is still visible.

The employee deferral route is familiar, but it depends on compensation you have already paid. The profit-sharing route looks at what the business earned, so it can matter more when you kept your salary low. You are not trying to maximize a number for its own sake. You are moving profit into a retirement account while the extension still allows it.

The checklist turns profit into a funded contribution

• Step 1: Confirm the extended return deadline. For a sole proprietor, the extended business return deadline is October 15, 2026. If your entity has a different filing date, use the date on your extension. Finished means a single date in your calendar, with a reminder set for the funding transfer.
• Step 2: Calculate the net self-employment earnings for the tax year. Start with business income, subtract the deductions that reduce it, and use the net figure that supports the contribution. The employee deferral limit for 2025 is $23,500, but a low salary can make that number less useful. The employer profit-sharing piece can reach 25% of compensation, with a total contribution ceiling of $70,000 for 2025. For a founder who kept personal pay low, the profit-sharing piece can be the larger lever when the business has net earnings. You are ready when the worksheet starts with net earnings, not gross revenue, and shows the contribution amount before you move cash.
• Step 3: Establish the plan if it was not in place at year-end. If the plan was not in place by December 31, 2025, you can still set it up in 2026 and fund the 2025 employer profit-sharing and voluntary after-tax amounts by the extended deadline. The plan needs to be opened in time to receive the contribution, and the account should be labeled for the tax year. The setup is done when you have a plan document, a custodian account, and a clear label.
• Step 4: Fund the profit-sharing amount before the deadline. The extended window does not cover every contribution type; it covers employer profit-sharing and voluntary after-tax money. That limitation is where people go wrong: they assume the whole plan can wait, then discover the contribution they wanted is outside the window. If the amount is large, make the transfer in a way that leaves a clear record. The step is complete when a cash transfer leaves the business account, not a promise to pay later.
• Step 5: Document the earnings tie-in. The 2026 funding must tie to 2025 earned self-employment income, not 2026 earnings. Keep the earnings figure, the contribution calculation, the plan document, and the transfer record in one folder. If the business had a rough quarter, the worksheet should show why the amount is what it is. Finish it when a saved schedule shows the earnings figure, the contribution percentage, and the amount funded.

The cash move has to cover the next month

Slower growth is a deliberate choice, and this contribution should not force a cash shortage. If the transfer would leave the operating account thin, fund what the business can actually support. The tax benefit is useful, but it should not make next month's payroll hard to cover. A bootstrapped company depends on cash, not on a future tax refund.

If you need the money for inventory, payroll, or a slow month, leave it in the business. The contribution is a choice you make from surplus, not a way to spend next year's cash.