What is the tax safe harbor?

The safe harbor is a set amount you can pre-pay through withholding and estimated payments to avoid an IRS underpayment penalty — even if you end up owing more. You're safe if you pay at least 90% of this year's tax, or 100% of last year's tax (110% if your prior-year AGI was over $150,000).

How much should I pay in estimated taxes each quarter?

Take your safe-harbor amount, subtract expected withholding, and divide the rest into four equal quarterly payments due in April, June, September, and January.

What is the penalty for underpaying estimated taxes?

The IRS charges interest on the underpaid amount for each quarter you were short, currently around 7% annualized, compounded to the filing deadline.

Safe Harbor Estimated Taxes: How Much to Pay to Avoid a Penalty

If you have income that isn't fully withheld — self-employment, 1099, K-1, big capital gains — the IRS expects you to pre-pay tax during the year. Miss it and you owe an underpayment penalty. The "safe harbor" is how you guarantee you won't.

The safe harbor, in one sentence

You will not owe an underpayment penalty as long as your total withholding + estimated payments for the year is at least the smallest of these:

90% of this year's total tax, or
100% of last year's total tax (the number on last year's return), or
110% of last year's tax if your prior-year AGI was over $150,000.

Hit any one of those and you're safe — even if you still owe a balance in April. The penalty is about the timing of payments, not the final bill.

The prior-year rule (100% / 110%) is the easy button: it's a fixed number you already know, so you don't have to forecast this year's income. Great if your income jumped this year.

How much to pay each quarter

Pick your safe-harbor target (usually the prior-year rule — it's certain).
Subtract the federal tax you expect to be withheld from paychecks.
Divide what's left into four equal payments.

The 2026 estimated-payment due dates are roughly April 15, June 15, September 15, and January 15 (of the following year). Withholding counts as paid evenly across the year, which is why bumping your W-4 late in the year can still fix an underpayment.

A quick example

Say last year's tax was $20,000 and your AGI was under $150k. Your safe harbor is 100% = $20,000. If your job will withhold $12,000, you need $8,000 more in estimates — $2,000 per quarter. Do that and no penalty, no matter what you actually owe.

Run your own numbers

Figuring out the 90%-of-this-year number means estimating this year's tax. Two free tools help:

Have side income? Start with the self-employment tax calculator to size the SE-tax piece.
Want the whole picture — income tax + SE tax together? Use the full tax calculator.
Prefer to fix it through your paycheck instead of quarterly checks? The W-4 calibrator tells you exactly what to put on your W-4.

FAQ

What's the penalty if I miss it?: Interest on the shortfall for each quarter you were under, currently ~7% annualized, compounded to the filing deadline. It's not huge, but it's avoidable.
Can I just pay it all in Q4?: Not cleanly — estimates are expected each quarter, so a big Q4 payment can still leave earlier quarters short. Withholding, though, is treated as paid evenly, so a late-year W-4 bump is the exception.
Does this cover state taxes?: No — states have their own estimated-tax rules and safe harbors. This is federal.