Updated July 28, 2026. Quick answer: With options a sell-to-cover has to raise the exercise price as well as any withholding, so far more shares are sold than at an RSU vest. With incentive stock options there is a further effect that is widely reported incorrectly: selling shares in the same taxable year as the exercise brings IRC §422(c)(2) into the alternative minimum tax calculation for those shares, which caps what is included rather than leaving the full exercise-date spread in it.
First: options are not RSUs
At an RSU vest there is nothing to pay. At an option exercise there is: the exercise price. A sell-to-cover at exercise must therefore raise the exercise cost plus any withholding, and the number of shares consumed is correspondingly larger. People who reason from their RSU experience routinely underestimate this.
Non-qualified options
The spread at exercise is compensation, reported through payroll, and withholding applies to it as a supplemental wage — the same flat-rate mechanics, with the same shortfall risk for high earners, as an RSU vest.
Incentive stock options: no withholding, and a different problem
ISOs do not go through withholding at all. IRC §3121(a)(22) excludes from wages any remuneration on account of “a transfer of a share of stock to any individual pursuant to an exercise of an incentive stock option” or “any disposition by the individual of such stock”. So a sell-to-cover on an ISO exercise is not funding withholding — it is funding the exercise price. The tax problem arrives separately, and later, as alternative minimum tax.
The two holding periods
Favourable ISO treatment requires, in the words of IRC §422(a)(1), that “no disposition of such share is made by him within 2 years from the date of the granting of the option nor within 1 year after the transfer of such share to him”. Selling any shares at exercise breaks the one-year test for those shares. That is a disqualifying disposition, and the spread on them becomes ordinary income.
The part that is commonly reported backwards. It is often written that selling some shares at exercise does not reduce the AMT adjustment, because the whole exercise is a preference item regardless. Read together, IRC §56(b)(3) and §422(c)(2) do not say that. Section 56(b)(3) provides that “section 422(c)(2) shall apply in any case where the disposition and the inclusion for purposes of this part are within the same taxable year and such section shall not apply in any other case.” And §422(c)(2) provides that the amount includible “shall not exceed the excess (if any) of the amount realized on such sale or exchange over the adjusted basis of such share.” For shares sold in the same taxable year as the exercise, the amount is capped at what you actually realised on them — it is not the exercise-date spread.
Note the exact condition: the same taxable year. Exercise in November and sell in January and you get neither treatment — the shares still carry the full exercise-date spread into the prior year’s AMT calculation, and you have missed the window by a matter of weeks. This is the single most expensive date in an ISO exercise and it is a calendar fact, not a strategy.
What follows from this
| What you do | Regular tax | AMT on those shares |
|---|---|---|
| Exercise and hold past year end | Nothing yet | Full exercise-date spread is a preference item |
| Exercise and sell some in the same year | Disqualifying disposition on the sold shares — ordinary income | §422(c)(2) applies: capped at what you realised |
Which of those is better is not a general question — it depends on the spread, your other income, and whether you want the position at all. It is worth an actual calculation rather than a rule of thumb, and it is one of the few equity decisions where the cost of getting it wrong is large enough to justify paid advice.
Sources
IRC §422(a)(1) (holding periods); IRC §422(c)(2) (limit on the amount includible on a disqualifying disposition); IRC §56(b)(3) (alternative minimum tax treatment of incentive stock options, and the same-taxable-year rule); IRC §3121(a)(22) (FICA exclusion for statutory options). All quoted from the United States Code as in force July 2026.
This states what the cited authority says and what plan documents actually do. It is not tax advice, and your employer’s plan controls which of these elections you are offered at all.