One rule, three parts. Estimated tax is paying during the year, the penalty is interest for paying late, and safe harbor is the amount that avoids it.
Start with the requirement. Federal income tax is pay-as-you-go. Withholding handles it for an employee. For income with no withholding, the IRS wants four payments during the year, and the requirement applies once you expect to owe $1,000 or more at filing time. That is estimated tax.
Next, the consequence. If your payments and withholding by each due date fall short of what was required, the IRS charges the underpayment penalty. Form 2210 figures it, and it is interest, currently 7% a year, on each shortfall for the days it was late. It is not a percentage of your tax and not a flat fine.
Last, the target. The required amount for the year is the smaller of two figures, 90% of this year's tax or 100% of last year's total tax, rising to 110% when last year's adjusted gross income was over $150,000. The second figure is the safe harbor. People use it because the prior return fixes it in January. Pay a quarter of it on each due date and the penalty cannot apply no matter how much you owe in April.
So take the safe harbor number from last year's return, compare it to 90% of what this year will be, and pay a quarter of the smaller one at each deadline. The safe harbor calculator does that comparison on your own figures.
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Sources: IRS Publication 505· IRS, Estimated taxes