For a self-employed person, the SEP IRA contribution limit is roughly 20% of net self-employment income (after the deduction for half of self-employment tax), capped at the annual IRS maximum ($69,000-ish range in recent years, adjusted yearly). This calculator uses the simplified 20% approximation most freelancers use for quick planning.
This is a simplified estimate, not tax advice. The exact SEP IRA calculation involves your specific self-employment tax deduction, confirm with a tax professional or IRS Publication 560 before contributing. See the SEP IRA vs. Solo 401(k) guide for help deciding which account fits you better.