W-4 vs 1040-ES
RSU gap: extra W-4 or 1040-ES before September 15?
Reviewed August 21, 2026. About. Not tax advice.
Use extra W-4 withholding if payroll can still take it from remaining salary checks — the IRS treats withholding as paid evenly through the year, so a December dump can still satisfy estimated-tax rules. Use Form 1040-ES if you need a dated payment by September 15 or January 15, or if payroll will not raise withhold. Many HENRYs do both: W-4 for the rest of the year, one ES voucher for the vest that already happened. Vestgap’s desk shows the still-due figure, the next ES slice, and an extra-per-remaining-biweekly-check estimate. It does not file the W-4 or the voucher.
On the Bay Area sample
Still due $34,925. Next 1040-ES slice $17,463 on September 15, 2026. Spreading the same gap across about 9 remaining biweekly checks is about $3,881 extra federal withhold each — an estimate, not a completed W-4.
FAQ
- Is extra W-4 better than a 1040-ES check?
- Withholding is simpler for safe harbor because it is deemed paid ratably. 1040-ES is better when the vest already posted and there is not enough remaining salary, or when you want the payment dated before September 15.
- Can I wait until December to fix RSU under-withholding?
- Often yes for the federal underpayment penalty if extra W-4 in December is large enough. You still need cash. State rules can differ. Ask a CPA before you treat December as a plan.
Sources
- IRS Publication 15 (Circular E) — Supplemental wage withholding: 22% up to $1 million, 37% above.
- IRS tax year 2026 inflation adjustments (Rev. Proc. 2025-32) — 2026 federal brackets and standard deduction.
- IRS estimated taxes — 1040-ES installment dates and underpayment penalty.