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RSU withholding fell short: avoid the underpayment penalty
Safe harbor can protect you from an underpayment penalty, but it doesn't erase the tax you owe. For penalty calculations, federal income tax withholding is generally spread across the four payment periods. Form 1040-ES payments must meet each applicable due date.
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Safe harbor is a payment threshold, not a December 31 deadline. Withholding is generally treated as paid one-fourth in each payment period. Estimated payments must be made by the applicable due dates below, and a late payment can still leave an earlier period underpaid. Meeting a safe harbor removes the underpayment penalty, not the balance you may owe when you file.
The two safe harbors
For most taxpayers, the required annual payment is the smaller of these two thresholds:
- 100% or 110% of last year's tax. Use 100% if prior-year AGI was $150,000 or less ($75,000 if married filing separately). Use 110% above that threshold. This gives you a fixed prior-year number instead of requiring a forecast of this year's tax.
- 90% of this year's tax. This path requires an accurate projection of the current year's total tax.
Extra W-4 withholding vs. 1040-ES vouchers
W-4, line 4(c): extra withholding per paycheck
For penalty calculations, the IRS generally treats one-fourth of your full-year withholding as paid by each payment-period due date. You can choose to use the actual withholding dates instead. Under the default treatment, extra late-year W-4 withholding can help cover an earlier shortfall.
Ask HR to update your W-4 to add a flat extra federal withholding amount for the rest of the year. Divide the shortfall by the number of remaining paychecks. Done.
1040-ES vouchers: direct estimated payments
Pay directly to the IRS at IRS Direct Pay, or by mailing a 1040-ES voucher. Estimated payments are credited on their payment dates, so a late payment does not retroactively meet an earlier period's deadline.
Estimated payment due dates
| Period | Due date |
|---|---|
| Q1 (Jan–Mar) | April 15 |
| Q2 (Apr–May) | June 15 |
| Q3 (Jun–Aug) | September 15 |
| Q4 (Sep–Dec) | January 15 (next year) |
Note the uneven periods. Q2 is two months, Q3 is three, Q4 is four.
A practical playbook after a big vest
- If you're using the prior-year safe harbor, start with last year's Form 1040, line 24. For most taxpayers, use 100% of that tax, or 110% if prior-year AGI was more than $150,000 ($75,000 if married filing separately).
- Add up federal withholding on all pay stubs so far and project withholding through year-end.
- Compare that total with the required annual payment. Extra W-4 withholding can cover the gap, or you can make 1040-ES payments by the applicable payment-period due dates.
- Project your final tax bill separately. Safe harbor addresses the underpayment penalty, not the balance you may still owe when you file.