What is the estimated tax safe harbor?

You avoid the underpayment penalty if prepayments reach the smaller of 90% of the current year's tax or 100% of last year's total tax — 110% of last year's when prior-year AGI exceeded $150,000.

If I pay the safe harbor, do I still owe more at filing?

Usually, when income grew — the harbor only shields you from the penalty. In the example, $15,000 prepaid against a $22,287.77 projected bill leaves $7,287.77 due with the return, penalty-free.

Quarterly Estimated Tax Calculator

Quarterly Estimated Tax Calculator — Safe-Harbor Payments

The penalty-proof view: the IRS waives the underpayment penalty if your withholding plus estimated payments reach the smaller of 90% of this year's tax or 100% of last year's (110% when prior-year AGI topped $150,000). For the $90,000 filer who owed $15,000 last year, that floor is $15,000 — four payments of $3,750 instead of $5,571.94 — with the balance settled at filing. Ideal for income that's growing or lumpy.

2026 quarterly estimated tax

ViewNet self-employment profit ($/yr)W-2 / other income ($/yr)Federal tax already withheld ($/yr)Filing statusLast year's total tax ($)Prior-year AGI over $150,000?

Safe-harbor quarterly payment

$3,750.00

4 payments covering the $15,000 required annual payment

Breakdown

SE tax base (92.35% of profit)$83,115

Social Security portion$10,306.26

Medicare portion$2,410.34

Federal income tax$9,571.17

Total estimated tax$22,287.77

Prior-year safe harbor$15,000.00

2026 estimate using the IRS Form 1040-ES method with the standard deduction only — no QBI deduction, credits, or state tax. Not tax advice; confirm with a tax professional.

Pay the floor, not the forecast

The safe harbor exists because nobody knows their current-year tax in February. Anchoring to last year's known number makes the quarterly obligation certain even when income is volatile: pay $3,750 a quarter in the example and no penalty applies regardless of how the year actually lands. The remaining $7,287.77 of the projected bill is still owed — at filing, without penalty — so the harbor is a cash-flow strategy, not a discount.

The harbor cuts the other way when income drops: 90% of this year's smaller tax may be far below last year's 100%, and paying the current-year leg instead frees cash months earlier. The calculator reports both legs and uses the smaller, which is exactly the 1040-ES rule.

Questions

What is the estimated tax safe harbor?: You avoid the underpayment penalty if prepayments reach the smaller of 90% of the current year's tax or 100% of last year's total tax — 110% of last year's when prior-year AGI exceeded $150,000.
If I pay the safe harbor, do I still owe more at filing?: Usually, when income grew — the harbor only shields you from the penalty. In the example, $15,000 prepaid against a $22,287.77 projected bill leaves $7,287.77 due with the return, penalty-free.