Supplemental Wage Withholding

Supplemental wage withholding is how a US employer determines the federal income tax to withhold from a payment that is not the employee's regular pay. The governing rule is Treasury Regulation section 31.3402(g)-1, headed "Supplemental wage payments," and it begins from a definition by exclusion: "Supplemental wages are all wages paid by an employer that are not regular wages."

Regular wages, by contrast, are "amounts that are paid at a regular hourly, daily, or similar periodic rate (and not an overtime rate) for the current payroll period or at a predetermined fixed determinable amount for the current payroll period." Anything that is wages and does not fit that description is supplemental. The category is therefore not a list somebody drew up; it is what remains, which is why it collects things as unlike each other as a signing bonus and imputed income for a partner's health coverage. Bonus tax withholding covers the single most common case, a bonus withheld at a flat 22 percent; this page covers the rules that reach everything else.

The regulation's own list is much longer than "bonuses," and its last entries connect this rule to equity compensation. Supplemental wages include, in the regulation's words, "reported tips ..., overtime pay ..., bonuses, back pay, commissions, wages paid under reimbursement or other expense allowance arrangements, nonqualified deferred compensation includible in wages, wages paid as noncash fringe benefits, sick pay paid by a third party as an agent of the employer, amounts that are includible in gross income under section 409A, income recognized on the exercise of a nonstatutory stock option, wages from imputed income for health coverage for a non-dependent, and wage income recognized on the lapse of a restriction on restricted property transferred from an employer to an employee." Those last two are why the vest of a restricted stock unit and the exercise of a nonqualified option are withheld at a flat rate rather than through the employee's Form W-4 settings.

Two items on the list are elective, which is worth knowing because it explains inconsistent treatment between employers. Paragraph (a)(1)(iv) permits an employer to treat overtime pay as regular wages, and paragraph (a)(1)(v) permits the same for reported tips. Neither is required, so two employers can classify identical overtime differently and both be correct.

A related exclusion catches people out. Paragraph (a)(1)(iii) provides that an amount which is not wages subject to income tax withholding at all is neither regular nor supplemental, and gives as its example income from the disqualifying disposition of shares acquired under a statutory stock option. That income is real and taxable; it simply has no withholding attached, which is a different situation from being withheld at a flat rate.

The regulation supplies three procedures, and the order of analysis matters.

Mandatory flat rate. Under paragraph (a)(2), once an employee's cumulative supplemental wages for the calendar year from one employer exceed $1,000,000, the excess is withheld at "the highest rate of tax applicable under section 1," which IRS Publication 15 states is 37 percent, and does so "without regard to whether income tax has been withheld from the employee's regular wages, and without regard to any entries on Form W-4." The $1,000,000 figure is fixed by the regulation and carries no inflation adjustment. Paragraph (a)(4)(iv) makes the mandatory rate apply only to the portion above $1,000,000, though an employer may choose to apply it to the whole payment.

Aggregate procedure. Paragraph (a)(6) is the fallback and the default where the optional flat rate is unavailable. The supplemental payment is combined with regular wages for the payroll period and withheld as though the total were a single ordinary paycheck, using the employer's normal method and taking the employee's Form W-4 into account. This usually withholds more, because a one-off payment is being treated as though it recurred every period.

Optional flat rate. Paragraph (a)(7) allows a flat percentage, which Publication 15 gives as 22 percent and describes as the only percentage permitted, but only where three conditions are met: the mandatory rate does not apply; the supplemental wages are "either not paid concurrently with regular wages or are separately stated on the payroll records of the employer"; and "income tax has been withheld from regular wages of the employee during the calendar year of the payment or the preceding calendar year."

That third condition is the least-known rule in this area and it bites the lowest earners. An employee from whom no federal income tax has been withheld all year, because their pay is low enough or because they claimed exempt status, cannot have the flat rate applied to their bonus. The employer must use the aggregate procedure instead. The regulation works the case itself, with two employees on identical salaries receiving identical $2,000 bonuses in December: the one whose regular wages have had income tax withheld may be given the flat rate; the one whose have not may not be.

"One employer" is wider than one company. Paragraph (a)(3)(i) treats all persons who are a single employer under section 52(a) or (b), the controlled group and common control rules, as one employer for the $1,000,000 test. Paragraph (a)(3)(ii) adds payments made by a third party acting as the employer's agent. A narrow relief at paragraph (a)(4)(iii) lets an agent paying an individual less than $100,000 in total wages for the year be disregarded, unless an employer is using five or more agents and a principal effect is to avoid the mandatory rate.

The 22 percent is not written into the regulation, which is why it has moved without the regulation being amended. Paragraph (a)(7)(iii) sets out the history: 20 percent from 1966, 28 percent from 1994, 27.5 percent and then 27 percent in the early 2000s, 25 percent from 2003, and then, for supplemental wages paid after December 31, 2004, "28 percent (or the corresponding rate in effect under section 1(i)(2) for taxable years beginning in the calendar year in which the payment is made)." The rate tracks a statutory bracket rather than standing on its own, and the bracket changed. Publication 15 for 2026 states that the rate "remains 22% (37% if supplemental wages paid to an employee during the calendar year exceed $1 million) because P.L. 119-21 permanently extended the individual tax rates enacted in P.L. 115-97."

How an employer works through a payment.

1. Classify the payment. Is it wages at all? If it is wages, is it paid at a regular periodic rate for the current payroll period, or is it something else? Something else is supplemental.

2. Test against the $1,000,000 threshold, counting all supplemental wages paid to the employee this calendar year by every entity in the same controlled group and by the employer's agents.

3. Withhold the excess above $1,000,000 at the top rate, currently 37 percent, ignoring the Form W-4 entirely.

4. For the rest, check whether the optional flat rate is available. It requires the payment to be separate from or separately stated alongside regular wages, and income tax to have been withheld from the employee's regular wages this year or last.

5. Apply 22 percent if it is available, or the aggregate procedure if it is not.

A hypothetical example of the controlled-group rule, built on the facts of the regulation's own worked example. Dele is an employee of both X Corporation and Y Corporation, which are treated as a single employer under section 52. X pays his regular monthly wages, and income tax is withheld from them. In March, X pays Dele a bonus of $600,000. In November, Y pays him a bonus of $500,000.

The March bonus is under the threshold, so X may use the optional flat rate of 22 percent, withholding $132,000.

The November bonus has to be split, because $600,000 has already been paid this year by the same employer for this purpose. The first $400,000 of it brings the cumulative total to $1,000,000 and can still take the optional flat rate, since income tax has been withheld from Dele's regular wages: $400,000 times 0.22 is $88,000. The remaining $100,000 is above the threshold and takes the mandatory rate of 37 percent, ignoring anything on Dele's Form W-4: $100,000 times 0.37 is $37,000. Withholding on the November payment is therefore $88,000 plus $37,000, or $125,000.

Had Y been an unrelated company with no common control, the November bonus would have started a fresh count and none of it would have reached the mandatory rate.

What the rules get right

• A single residual definition covers every irregular payment an employer can make, so no new rule is needed each time a new form of compensation appears.
• The flat rate is simple, predictable, and applied to the payment itself, so an employee can see exactly what was taken and why.
• The mandatory rate above $1,000,000 stops very large one-off payments from being systematically under-withheld through Form W-4 settings.
• Treating a controlled group as one employer closes the obvious route around that threshold, and the agent rule closes a second one.

Where it goes wrong for people

• The flat rate is a single number applied to everyone below the threshold, so it under-withholds for a higher earner and over-withholds for a lower one.
• Which method an employer uses is the employer's choice within the rules, so two people with identical payments can see very different amounts withheld.
• The optional flat rate is unavailable to an employee whose regular wages have had no income tax withheld, which means the aggregate procedure applies to some of the lowest-paid recipients of a bonus.
• Equity compensation is swept in by the definition, so a large restricted stock vest can be withheld at 22 percent against a much higher marginal rate, leaving a balance due.
• Nothing here changes the tax; it changes the prepayment. A shortfall shows up in April, and a large one can produce an estimated tax problem as well.