Definition
The safe harbor rule protects you from the IRS underpayment penalty even if you owe taxes when you file — as long as you've met either of two conditions by year-end.
Rule 1 (90% of current year): Your total withholding and estimated tax payments equal at least 90% of your actual tax liability for the current year.
Rule 2 (100% of prior year): Your total withholding and estimated payments equal at least 100% of your prior year's tax liability. If your prior year AGI exceeded $150,000, the threshold is 110%.
You only need to meet one rule. Rule 2 is often more useful because you know the exact prior-year number from your previous return — Rule 1 requires estimating your current-year liability before the year ends.
How this affects your W-4
The 'Check If I'm On Track' tab in this calculator evaluates whether your projected year-end withholding meets the 90% Rule 1 safe harbor. If it doesn't, it shows the Step 4c amount to add per paycheck to close the gap.
If you have last year's Form 1040 handy (line 24), enter that amount in the prior year tax field to also check the prior-year rule.
See how safe harbor rule affects your withholding
Use the free W-4 calculator to calculate your exact withholding with your specific situation.
Open the W-4 Calculator →Related terms
Underpayment Penalty
An IRS penalty charged when you owe more than $1,000 and your withholding falls below safe harbor thresholds.
Step 4c — Extra Withholding
A line on the W-4 where you specify a flat dollar amount to withhold from every paycheck in addition to the calculated amount.
Withholding
Taxes taken out of each paycheck before you receive it.