RSU, NSO, and ISO Withholding: How They Actually Differ

Restricted stock units create ordinary income at vesting, based on the fair market value of the shares on that date, and the employer has a withholding obligation at that moment, not at grant, when nothing has actually been earned yet. Supplemental wage withholding rates apply, which are flat rates that often don't match an employee's actual marginal tax rate, meaning many employees end up under-withheld and owe a true-up when they file their return. Net settlement, where the company withholds or sells a portion of the vesting shares to cover the tax obligation, is the common mechanism for actually satisfying this without asking the employee to write a check. Large vesting events involving many employees at once are worth flagging to payroll ahead of time, since a sell-to-cover transaction across a big batch of shares takes coordination with a broker that a single employee's exercise doesn't.

Non-qualified stock options create ordinary income at exercise, equal to the spread between the stock's fair market value and the strike price the employee pays, and the employer withholds on that spread the same way it would on RSU vesting income. This means an employee who exercises when the stock has appreciated significantly can trigger a meaningful withholding obligation and a meaningful cash outlay at exercise, well before there's any actual liquidity event, which is a real cash planning issue for the employee even though it's the company's withholding mechanics that create the timing.

Incentive stock options are different from both: exercising an ISO doesn't create regular taxable income and doesn't trigger a withholding obligation, but the spread at exercise is a preference item for alternative minimum tax purposes, which can still create a real tax bill for the employee even though nothing shows up on a W-2 and the company withholds nothing. If the employee later sells the shares in a disqualifying disposition, generally within one year of exercise or two years of grant, some or all of that gain converts to ordinary income retroactively, and depending on the circumstances, withholding obligations can come into play at that later point instead.

The pattern across all three comes down to when ordinary income is actually recognized: RSUs generally at vesting or settlement, NSOs at exercise, and ISOs generally never, unless a disqualifying disposition happens. Withholding follows recognition, so a payroll system built around one event, vesting dates for RSUs, say, can completely miss an NSO exercise that happens off the normal vesting calendar, or a disqualifying ISO disposition that an employee doesn't even report to the company until tax season, if at all.

For example, an employee exercises NSOs in the middle of the year, well outside the company's normal vesting dates. A payroll process built around vesting events never sees it, so no withholding is taken on the spread and the employee faces a surprise bill. A common mistake is assuming the equity platform passes every event to payroll automatically. The fix is a standing rule that any exercise triggers a same-week notice to payroll, and that finance reviews the option ledger each pay period for exercises and for sales that might be disqualifying dispositions. This keeps withholding tied to recognition instead of the vesting calendar.

Track vesting events, option exercises, and any disqualifying ISO dispositions as separate events, because they carry different withholding and reporting treatment, rather than assuming your equity administration platform handles all of them automatically. A payroll platform such as Rippling, which has built out equity administration integration, can handle a meaningful share of the RSU and NSO withholding mechanics directly, while cross-border teams add another layer: a platform such as Deel matters specifically when equity compensation reaches employees outside the U.S., where withholding and reporting obligations follow that jurisdiction's own rules rather than the ones described here. Whichever platform you use, someone still has to notice a disqualifying ISO disposition when an employee reports one, since that's rarely something a payroll system surfaces on its own.

Track these events separately in payroll and equity administration:

• RSU vesting or settlement, where withholding applies at vesting based on the fair market value of the shares, not at grant.
• NSO exercises, where withholding applies to the spread between fair market value and the strike price, often well before any liquidity event.
• ISO exercises, which create no regular income or withholding but produce an alternative minimum tax preference item for the employee.
• Disqualifying ISO dispositions, generally sales within one year of exercise or two years of grant, which can convert gain to ordinary income.
• Employees outside the U.S., where equity withholding and reporting add another layer that a global payroll platform may need to handle.