Estimated Tax Safe Harbor After an ISO Exercise

Part of Nauma's complete guide to The ISO AMT Tax Trap in 2026.

Exercising a large block of incentive stock options can create a substantial AMT liability — and unlike a paycheck, nothing is automatically withheld from that liability along the way. The estimated tax safe harbor rules determine how much has to be paid, and by when, to avoid an underpayment penalty on top of the tax itself.

The Three Federal Safe Harbor Tests

The IRS will not assess an underpayment penalty if a taxpayer meets any one of the following three tests over the course of the year:

1. Owe less than $1,000 after subtracting withholding and refundable credits from the total tax shown on the return.
2. Pay at least 90% of the current year's tax liability through withholding and estimated payments combined.
3. Pay 100% of the prior year's total tax liability (using the prior year's Form 1040, line 24) — or 110% of the prior year's tax if the prior year's adjusted gross income (AGI) exceeded $150,000 (or $75,000 if married filing separately).

Meeting any one of these avoids the penalty — it does not mean the tax bill itself is smaller. A taxpayer who meets safe harbor through the 110% prior-year test can still owe a large balance when filing, if the current year's actual liability is higher.

Why This Specifically Matters for an ISO Exercise

A large ISO exercise-and-hold creates an AMT preference item — and therefore potential AMT liability — with no associated payroll withholding, unlike a bonus or RSU vest, where the employer withholds automatically. The entire liability generated by the exercise has to be covered through estimated payments or additional withholding elected on other income, or it falls outside safe harbor and triggers a penalty on the shortfall.

This creates a specific timing trap: because ISO exercises often happen mid-year, and because many employees don't calculate the resulting AMT liability until they prepare their return the following spring, it's easy to discover the safe harbor requirement was missed only after the year in question has already ended — at which point the only fix (increasing withholding) is no longer available for that year.

Why Paying It All in April Doesn't Work

The federal system is not evaluated as a single annual balance — it's checked quarterly, using Form 2210, against four cumulative benchmarks: 25% of the required annual payment by the first due date, 50% by the second, 75% by the third, and 100% by the fourth. Falling short at an earlier checkpoint generates a penalty for that period even if a large payment is made later in the year to "catch up." For calendar-year 2026, the federal due dates are April 15, 2026, June 15, 2026, September 15, 2026, and January 15, 2027 (the fourth payment can be skipped if the full return is filed and paid by January 31, 2027).

The one meaningful exception: withholding is treated differently from estimated payments. Withholding — even a lump sum withheld from a December bonus or year-end distribution — is deemed to have been paid evenly across all four periods, regardless of when it was actually withheld. This means increasing W-4 withholding late in the year, or requesting extra withholding on a year-end distribution, can retroactively cover an earlier-quarter shortfall in a way that a late estimated payment cannot. For someone who exercised ISOs mid-year and realizes only in Q4 that they're short of safe harbor, adjusting withholding is often the more effective fix than an estimated payment for the same dollar amount.

Choosing Between the 100%/110% and the 90% Tests

The prior-year test (100% or 110%) has the advantage of being a known, fixed number — it doesn't require forecasting the current year's income at all. This makes it the more commonly used approach for anyone with a large, hard-to-predict income event like an ISO exercise. The 90%-of-current-year test can be more favorable when the current year's income will clearly be lower than the prior year's, but it requires an accurate projection of the current year's total liability — a harder thing to do reliably in a year that includes a large, one-time exercise.

A worked example: an employee with 2025 AGI of $210,000 and 2025 total tax of $52,000 exercises a large ISO position in mid-2026, expecting the AMT impact to push 2026 total tax to roughly $95,000. Because 2025 AGI exceeded $150,000, the prior-year safe harbor requires 110% of $52,000 — $57,200 — paid across the year through withholding and estimated payments. Paying that $57,200 on schedule avoids the underpayment penalty entirely, even though the actual 2026 liability of roughly $95,000 leaves a real balance due at filing. This example is illustrative only and does not reflect any specific taxpayer's actual liability.

What This Page Does Not Cover

This page covers how much to pay and by when, to avoid a penalty — not how to calculate the underlying AMT liability itself, which depends on the specifics of the exercise and the AMT exemption and phase-out rules described in The ISO AMT Tax Trap in 2026 and What Is the Alternative Minimum Tax (AMT)?. It also does not cover estimated tax planning for self-employment or investment income unrelated to equity compensation, though the same three-test framework applies to those situations as well.

California Note

California's safe harbor rules are both stricter and structured on a different payment schedule than the federal rules — a detail that catches many California filers off guard specifically because they assume the federal and state timelines match.

The payment schedule is uneven across quarters, not just front-loaded — and the third installment is easy to get wrong. Per the FTB's own 2026 Form 540-ES instructions, California's required installment percentages are 30% for the first (April) installment, 40% for the second (June) installment, 0% — no installment at all — for the third (September) installment, and 30% for the fourth (January) installment. This is a materially different shape than the federal schedule's four roughly equal installments: a filer who simply mirrors the federal 25%-per-quarter pattern will underpay the June installment and overpay in September, since California requires nothing at all on the September date. Confirming a $0 balance is due in September (rather than assuming a smaller payment is still owed) is worth double-checking directly, since the assumption that every quarter requires some payment is the single most common California-specific mistake in this area.

The prior-year safe harbor follows the same $150,000 / 110% tier as the federal rule — up to a $1,000,000 cutoff, above which it disappears entirely. California's own Form 540-ES worksheet applies the identical structure to the federal rule described above: if prior-year AGI was $150,000 or less ($75,000 or less if married filing separately), the prior-year safe harbor is 100% of the prior year's California tax; if prior-year AGI exceeded that threshold, the requirement rises to 110% of the prior year's tax — not 100% — exactly mirroring the federal 110% rule. Separately, and specific to California, once current-year California AGI reaches $1,000,000 (or $500,000 if married filing separately), the prior-year safe harbor is not available at all, regardless of the 100%/110% distinction — the only way to avoid a penalty at that point is paying at least 90% of the current year's actual California tax liability, which requires accurately estimating a number that, for someone with a large ISO exercise, may not be fully known until later in the year. This is exactly the income range a substantial ISO exercise can reach when combined with salary, bonus, and RSU vesting in the same year — making California's stricter rule a frequent and avoidable trap for precisely the households this cluster is written for.

See Capital Gains Taxes for how California taxes investment and equity-related income more broadly.

This article is for educational purposes only and does not constitute investment, tax, or financial advice. Safe harbor rules involve specific calculations based on individual income, filing status, and state of residence, and are subject to change. Always consult a qualified tax advisor before relying on any specific safe harbor calculation, particularly in a year involving a large equity compensation event.