The IRS charges an underpayment penalty if your quarterly estimated tax payments fall short of what you actually owe. The safe harbor rule lets you avoid the penalty by hitting one of two simple targets:
- Path A: 100% of last year's total tax. Total up everything you paid in tax last year — federal income tax + SE tax + any AMT. If your prior-year AGI was over $150,000 ($75K MFS), bump that to 110%.
- Path B: 90% of this year's total tax. If you're paying close to what you'll actually owe (within 10%), you're safe.
Path A is the more popular choice because you can lock in the target on April 15 — you know exactly what last year's tax was. Path B requires forecasting current-year income, which is harder for gig workers with variable income.
To use the safe harbor, your four quarterly payments must equal the target divided by four (or weighted by quarter using the annualized income installment method on Form 2210).
One important caveat: even if you meet the safe harbor and owe no penalty, you still owe the actual tax balance at the April 15 filing deadline. The safe harbor only protects you from the underpayment penalty, not from owing tax.