Last Updated: April 2026
The IRS underpayment penalty is entirely avoidable — but thousands of tech workers pay it every year because they do not know their RSU vest has created a gap between what was withheld and what they owe. This guide provides a step-by-step workflow: from running the calculator to determine your gap, to paying via IRS Direct Pay or EFTPS, to the California FTB estimated payment system, to the W-4 adjustment alternative that eliminates the need for separate payments. It also covers the two safe harbour methods and which one is better for RSU recipients with variable vesting income.
This is the recommended workflow for managing RSU-related estimated tax payments. Execute after each vest event.
Step 1 — Determine your gap: Go to the RSU Tax Calculator. Enter your annual salary, RSU vest value, filing status, and state. The calculator returns (a) total estimated tax on the vest at your marginal rates, and (b) estimated withholding. The gap = actual minus withheld. If the gap is under $1,000 and you are on track to meet safe harbour via Method B (last year's tax), you may not need to make a payment.
Step 2 — Check safe harbour Method B: Look at last year's Form 1040, Line 24 (Total Tax). If your 2025 AGI exceeded $150,000, multiply that total tax by 110%. If not, use 100%. This is the target. Subtract your current year-to-date withholding (from your most recent paystub, Box 2 YTD). If current withholding already exceeds the target, you are covered — no estimated payment needed for this vest. If the target is not yet met, the deficit (after considering remaining salary withholding for the rest of the year) is your estimated payment amount.
Step 3 — Pay federal via IRS Direct Pay: Go to directpay.irs.gov. Select 'Estimated Tax' as the reason for payment. Select the applicable tax year (2026) and the estimated payment period (the quarter in which the vest occurred). Enter payment amount and bank details. No registration or account required. Confirmation number issued immediately. Payments can be made up to the quarterly deadline or earlier — pay within 2–3 weeks of the vest while the sell-to-cover cash is still accessible.
Step 4 — Pay California via FTB Web Pay: Go to ftb.ca.gov/pay. Select 'Estimated Tax Payment (Form 540-ES)'. Enter your CA tax year, payment amount, and bank details. Same-day processing. The CA Q2 deadline requires that 70% of your estimated annual CA shortfall be paid by June 15 — calculate and pay the full Q2 amount at that time even if subsequent vests happen in Q3.
Step 5 — Record for your tax return: Keep confirmation numbers. Your federal estimated payments appear on Form 1040 Schedule 3, Line 8 (Estimated Tax Payments). California payments appear on Form 540, Line 113. Your tax preparer will need these amounts — they do not appear on any third-party form the IRS sends you.
Instead of making separate quarterly estimated payments, you can increase payroll withholding to cover the RSU gap. This is often simpler and eliminates the need to track quarterly deadlines.
How to do it: Submit a new Form W-4 to your employer's HR or payroll system. On Step 4(c) ('Extra withholding'), enter the additional dollar amount to withhold from each paycheck. Calculate: RSU gap ÷ remaining pay periods in the year = additional withholding per pay period.
Example: You vest $50,000 in RSUs in March 2026 with a $3,000 estimated gap. After the Q1 vest you have approximately 18 remaining semi-monthly pay periods. Additional withholding per period = $3,000 ÷ 18 = $167. Entering $167 on Step 4(c) distributes the catchup across the remaining paychecks, eliminating the need for a separate Q1 estimated payment.
Advantages of W-4 adjustment: No separate payments, no tracking of quarterly deadlines, withholding counts toward safe harbour just like estimated payments. The IRS treats additional withholding as if it were paid ratably throughout the year — this is actually more favourable than a single lump payment, because for underpayment penalty purposes withholding is deemed paid equally across all quarters even if it actually hits late in the year.
When quarterly payments are better: If you prefer to keep the cash in a high-yield savings account between vest and quarterly deadline (earning 4–5% APY while the IRS is not yet owed); if you only have a few large vest events per year and find it simpler than W-4 math; or if you are self-employed or a contractor without payroll withholding.
If you have quarterly or monthly vest events, the estimated tax calculation becomes a running exercise rather than a one-time event. Here is how to track it.
The rolling tally method: After each vest, add the marginal tax on that vest to a running total. Subtract cumulative withholding (from paystubs). The running gap is your current underpayment exposure. Compare to the quarterly deadlines and pay accordingly.
Key insight — vests later in the year are cheaper to underpay: The underpayment penalty is calculated from the quarterly due date to April 15 of the following year. A Q1 underpayment accrues approximately 12 months of penalty. A Q4 underpayment accrues only 3.5 months. If you have limited cash flow and must choose which vest gaps to cover, covering Q1 and Q2 gaps is most important — the penalty for missing Q3 and Q4 is much smaller.
The annualized income instalment method (Form 2210, Part II): If your income is uneven — a large Q1 vest with little income thereafter — the annualized method may result in lower required estimated payments than the standard method. Under this method, each quarter's required payment is based on actual annualized income to date rather than an equal fraction of the estimated annual total. Most people do not use this method because it requires detailed quarterly income tracking and more complex Form 2210 calculations. If you have a single large vest early in the year followed by no additional vest income, ask your CPA whether the annualized method saves you money.
The prior-year safe harbour is the easiest way for most RSU recipients to avoid the underpayment penalty. Here is exactly how to apply it.
Step 1: Find your 2025 Form 1040, Line 24 (Total Tax). Example: $28,000.
Step 2: Check your 2025 AGI (Line 11). If it exceeded $150,000, multiply the $28,000 by 110% = $30,800 required. If AGI was under $150,000, the target is simply $28,000.
Step 3: Estimate your 2026 total withholding for the full year. Check your most recent paystub for YTD federal withholding (Box 2), extrapolate to year-end. If you earn $150,000 and your employer withholds ~$3,000 per semi-monthly period, annual salary withholding is approximately $72,000. The RSU withholding at vest (22%) is also included.
Step 4: If projected total withholding already exceeds the $30,800 target, you are covered. No estimated payments needed regardless of what bracket your RSU vests land in.
The safe harbour sweet spot: High earners with large RSU vests are often fully covered by the prior-year safe harbour because their salary withholding alone exceeds last year's total tax. A $300,000/year salary at a large company typically generates $80,000+ in withholding — easily covering the safe harbour target for most prior-year income levels. If you are in this position, the withholding gap creates a balance due at filing but no penalty.
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File Your RSU Return and Estimate Payments with TFX →The underpayment penalty is calculated at the federal short-term interest rate plus 3 percentage points, applied to the underpaid amount for the period it remained underpaid. In 2026 this is approximately 8% annualised (verify at IRS.gov — the rate adjusts quarterly). On a $5,000 underpayment for 9 months, the penalty is approximately $300. It is not catastrophic, but it is avoidable. The penalty is assessed automatically via Form 2210 when you file.
You avoid the underpayment penalty if total withholding plus estimated payments equals the lesser of: (A) 90% of your 2026 actual federal tax liability, or (B) 100% of your 2025 federal tax (Form 1040, Line 24) — or 110% of 2025 tax if your 2025 AGI exceeded $150,000. Method B (prior-year) is simpler: you do not need to estimate 2026 income at all. Just ensure total 2026 payments reach the prior-year target.
As soon as possible after the vest while you have cash from the sell-to-cover proceeds. The IRS quarterly deadlines are April 15, June 15, September 15, and January 15 — you want your vest-related payment to arrive before the deadline for the quarter in which the vest occurred. Paying within 2–4 weeks of vest is a good rule of thumb. There is no penalty for paying early.
Yes. All federal income tax withheld — from salary and from RSU sell-to-cover — counts toward your safe harbour total. Withholding is treated as paid ratably throughout the year, which is more favourable than an equivalent lump estimated payment for penalty calculation purposes. If your employer is withholding enough total tax throughout the year (salary withholding + RSU 22% withholding) to meet the safe harbour target, no separate estimated payments are required even if a gap exists.
Use IRS Direct Pay (directpay.irs.gov) — available from any country, accepts international bank accounts? No: Direct Pay only accepts US bank accounts (routing and account number). If you only have foreign bank accounts, use EFTPS (eftps.gov) with a US bank account, or pay by check mailed to the IRS. Alternatively, some expats maintain a US bank account specifically for this purpose. Wire transfer from a foreign bank to the IRS is not supported directly — you would need a US intermediary account.
Yes. California estimated tax payments go to the FTB (ftb.ca.gov/pay) and are separate from federal payments to the IRS. California's schedule is different: the Q2 payment (due June 15) must cover 70% of the estimated annual CA liability — far front-loaded compared to federal. If you have a Q1 vest in California, your June 15 FTB payment should be 70% of the estimated full-year CA tax on that vest (and all other CA income). Pay via FTB Web Pay — no account required.
Pay what you can by the deadline to minimise the penalty — the penalty is based on the unpaid balance, so a partial payment reduces the exposure. If you sold shares via sell-to-cover and received net shares (not cash), you may have received fewer shares than the pre-tax value and may not have liquidity equal to the full tax gap. In that case, consider selling additional vested shares to fund the payment, or adjust your W-4 to catch up through future payroll withholding. The underpayment penalty is smaller than the cost of selling shares at the wrong time — model both scenarios.
It provides a good estimate of the federal and state withholding gap using your salary, vest value, filing status, and state. For most straightforward RSU situations (one state, standard deduction, no other significant income sources), the estimate is reliable enough to base an estimated payment on. If you have significant other income (rental income, spouse's salary, capital gains), the calculator may understate your total marginal rate. When in doubt, apply a slight buffer or use the safe harbour Method B (prior-year tax) instead of Method A (90% of current year) to avoid precision risk.