Safe harbor rule
The IRS rule that you won't owe an underpayment penalty if you paid at least 90% of this year's tax or 100% of last year's, whichever is smaller.
The safe harbor counts both withholding and estimated payments, and each quarterly due date is checked separately. Paying 100% of last year's total tax in four equal parts is the simplest way to meet it.
If your adjusted gross income last year was over $150,000 ($75,000 married filing separately), the prior-year figure rises to 110%. Different rules apply to farmers and fishers.
Example
Last year's tax was $10,000. Paying $2,500 on each of the four due dates meets the 100% safe harbor, even if you owe more when you file.