How flexible is the SEP-IRA contribution deadline?

The SEP-IRA's deadline flexibility is one of its best features: you can open and fund it as late as your business tax-filing deadline, including extensions, for the prior tax year. That means a contribution for 2025 can typically be made up to April 15, 2026, or as late as October if you file a valid extension. This is enormously useful for variable-income freelancers, because you don't have to commit a number during the year — you can wait until your profit is final and your tax preparer shows you exactly how much a contribution will save, then fund accordingly. You also choose the amount each year, from zero up to the limit, with no obligation to repeat it. By contrast, a Solo 401(k) generally must be established by year-end (though the employer portion can be funded later). That deadline grace makes the SEP a flexible last-minute tax lever.