Supplemental Wage Withholding Rules for Bonuses and Commissions

Supplemental wages are a defined category under federal tax law, not a loose term payroll teams apply to anything that looks like a bonus. The definition itself decides which withholding rules apply to a given payment, so getting it right is the first step in the whole exercise. IRS Publication 15 (Circular E), Section 7, defines supplemental wages as any wages paid to an employee that are not regular wages, a category far wider than most people assume. It covers bonuses, commissions, overtime pay, severance, back pay, retroactive pay increases, awards, prizes, accumulated sick leave payouts, taxable fringe benefits, and payments for nondeductible moving expenses.

The classification has nothing to do with the size of the check or what a company calls it internally. A payment counts as supplemental because it is not a scheduled, recurring wage, not because of its dollar amount or its label on an offer letter. That single fact, scheduled versus not, decides the entire withholding path a payment takes.

One distinction inside this category deserves separate attention: a genuinely discretionary bonus is unannounced and carries no prior promise, and is treated differently from a nondiscretionary bonus or commission. Nondiscretionary amounts have to be folded into the regular rate when calculating overtime, a step payroll teams miss often enough that auditors flag it as a recurring finding. Overtime pay itself sits in an odd position, since employers can treat it as either supplemental or regular wages, and that choice determines which withholding method applies and how the math runs from there. Restricted stock units add a further wrinkle: they vest as ordinary income, and at the moment of vesting they are treated as supplemental wages, so the same withholding rules that govern a cash bonus apply to an equity payout. For any company using equity as a regular part of compensation, that detail connects stock administration directly to payroll withholding policy.

The IRS definition reaches further than most payroll operators assume it does. Severance, prizes, retroactive raises, and sick-leave payouts all fall inside the same bucket as a holiday bonus. Correct classification is the first control point in the entire process, because every withholding decision that follows depends on getting this first one right.

The two permitted federal withholding methods

Federal law allows employers exactly two ways to withhold income tax on supplemental wages, and the method chosen changes what an employee actually takes home. Both methods are legitimate. Each comes with its own conditions, and an operator needs to know which conditions attach to which payment before running payroll.

The first is the flat-rate method, sometimes called the percentage method. Under IRS Publication 15 (Circular E), employers withhold a flat 22% federal income tax on supplemental wages up to the annual cumulative supplemental wage ceiling for that calendar year. Two conditions have to be met before this method is available. The payment must be made separately from regular wages, or clearly identified as a separate amount on the same check, and the employer must have withheld federal income tax from that employee's regular wages at some point during the current or prior calendar year. A new employee receiving a bonus as their very first paycheck fails that second condition, so the flat-rate method is off the table and the aggregate method has to be used instead. In practice, the flat-rate method is the one most companies reach for, since it needs no lookup against an employee's W-4 and is simple to check after the fact.

The second is the aggregate method. Here the supplemental payment gets added to the employee's most recent regular-period wages, withholding gets calculated on that combined total using the annualized percentage method and the employee's W-4 filing status, and the withholding already taken from the regular wages alone gets subtracted out. What's left is the withholding applied to the bonus. This approach tracks closer to an employee's real marginal rate at that income level, but it takes more computation, and it can push withholding higher than the flat rate would if the combined amount lands in a higher bracket for that pay period. Payroll systems without a flat-rate toggle default to this method automatically, often without anyone choosing it on purpose.

The employer picks the method, not the employee, and that choice carries real cash consequences for whoever receives the check. A company running several payment types in parallel, regular payroll, commissions, equity vests, needs one documented policy that says which method applies where, rather than letting the payroll system's defaults decide it by accident.

The high cumulative threshold's different behavior from the 22% rate

The cumulative supplemental wage threshold works nothing like a higher tax bracket. It is mandatory, it accumulates across the full calendar year, and no employer preference can override it, which makes it a separate compliance obligation from the standard supplemental withholding rules described above. Once an employee's cumulative supplemental wages for the year cross the high cumulative threshold, the federal withholding rate on everything above that line jumps to 37%, confirmed for 2026 under IRS Publication 15 (Circular E).

That threshold counts across every supplemental payment made to that employee during the year, not per individual payment. A large commission check early in the year followed by a sizable equity vest later on can push the employee over the line partway through the second payment, well before anyone expects it. And once the threshold is crossed, the higher rate on the excess is not optional. Employers cannot choose the aggregate method instead or apply the standard flat rate to the overage; the excess has to be withheld at 37%, full stop on the rate itself.

This failure occurs most at scaling companies, where payroll systems often track withholding per payment. A system built that way will not catch the threshold when supplemental payments land across different pay cycles, different payment types, or separate payroll runs. The payments most likely to trip this threshold quietly are large sales commissions, RSU vest events, one-time buyouts, severance packages, and year-end bonuses, particularly when several of these land for the same senior employee in the same calendar year. The common failure is not that a payroll team doesn't know the mandatory 37% rate exists; it is the absence of a running, year-to-date counter of cumulative supplemental wages per employee that fires a flag before the next payment gets processed.

Getting the rate right on a given payment still leaves an open question: whether that rate, 22%, 37%, or anything in between, actually matches what the employee owes the IRS at filing time.

Withholding rate versus actual tax liability

Supplemental withholding is a prepayment toward taxes, not a final calculation of what's owed. Treating the two as the same number causes employee relations problems, and for high earners, it creates genuine under-withholding exposure.

For most employees, income falls in brackets below the 22% flat rate, so the flat-rate method takes out more than necessary and the difference comes back as a refund. That is the most common outcome, and it's also the root of the common complaint that bonuses get "taxed more" than regular pay, a complaint that reflects withholding mechanics. High earners face the opposite problem. When an employee's combined income reaches federal marginal rates above 22%, the flat-rate method under-withholds: it produces a fixed withholding amount on the bonus, but the employee's real liability on that income, once the rest of their tax picture is factored in, can run higher.

Withholding gets harder to coordinate when an employee receives both a bonus and an RSU vest in the same year. A payroll system only sees the wages it processes directly. It has no visibility into investment gains, a spouse's income, or any estimated payments the employee is making outside of payroll, so under-withholding risk can build up entirely unseen by the employer.

The phrase "my bonus was already taxed" is technically off the mark. Money was withheld toward taxes when the bonus was paid, but whether that amount matched the actual liability only gets settled at filing. Payroll operators need this distinction clearly in mind because employees will complain about over-withholding, and some high earners will later receive IRS underpayment notices. Both outcomes follow from a correctly applied withholding rule. Neither is evidence that the rule was applied wrong.

Getting federal income tax withholding right, through either method, still leaves FICA untouched, since FICA runs on its own separate track regardless of which income tax method applies.

FICA obligations on supplemental wages and the 2026 Social Security wage base

The choice between the flat-rate and aggregate methods governs federal income tax withholding only. FICA applies to bonuses and commissions exactly as it applies to regular wages, and it runs in parallel no matter which income tax method a company uses.

Social Security tax applies to wages up to the 2026 wage base of $184,500. Once an employee's year-to-date wages, regular pay plus supplemental pay combined, reach that ceiling, no further Social Security tax is owed on anything paid after that point in the year, bonuses included. Medicare tax works differently: it applies to all wages with no ceiling at all, plus an additional surtax once wages cross the single-filer threshold. That additional Medicare tax is the employer's withholding obligation the moment the employee's wages from that employer cross the threshold, even if the employee's full household income, counting a spouse's wages, would trigger the surtax at a lower combined total than payroll can see.

A large bonus paid to a high earner who has already crossed the Social Security wage base for the year gets FICA withholding consisting only of Medicare tax, since Social Security withholding has already maxed out. That detail changes the net take-home math on the bonus, and it can confuse employees if payroll communications don't explain why Social Security withholding looks different from what they expected. FUTA, the federal unemployment tax, also applies to supplemental wages, but under its own separate annual wage base per employee, distinct from the FICA wage bases.

Once the federal rules, income tax and FICA both, are applied correctly, a second and often messier layer of rules kicks in at the state level.

How state supplemental withholding rules diverge

State supplemental withholding rules are not a simple extension of the federal framework. States take genuinely different approaches to the same category of payment, and no single method works the same way across every jurisdiction a company operates in.

States generally fall into three groups. Some states set a specific flat supplemental withholding rate separate from their regular wage tables, New York and California among them, with California applying a higher rate to bonuses and stock options paid separately from regular wages and a lower rate to commissions, severance, and vacation or PTO payouts paid separately. Other states have no published supplemental rate at all, which effectively forces employers into the aggregate method for state purposes even if the company uses the federal flat-rate method for federal withholding. A third group of states has no state income tax on wages, so no state supplemental withholding applies there.

California's split rate illustrates the complexity well: a higher rate applies to bonuses and stock options, a lower rate applies to commissions and severance, and the type of payment, not just the state, determines which rate is correct. Applying a single rate to every supplemental payment in California produces errors in both directions, over-withholding some employees and under-withholding others depending on what kind of payment they received. Some states layer local income taxes on top of state withholding, adding yet another variable tied to the specific municipality where an employee works.

These rules change often enough that any reference guide carries an expiration date. One state supplemental tax rate guide, last updated January 20, 2026, explicitly tells operators to confirm the current instructions from the relevant state tax agency for the year they're running payroll.

Manual lookups against a state-by-state table work when a company operates in one or two states with a handful of supplemental payments a year. That approach breaks down as the employee count, the state footprint, and the variety of payment types all grow at once.

How multi-state supplemental withholding breaks down as companies scale

The gap between knowing these rules and executing them correctly on every payment widens fast as a company adds employees, adds states, and adds new forms of compensation. Supplemental wages are where payroll errors concentrate disproportionately, precisely because so many separate variables, federal method, the high cumulative threshold, FICA caps, and state-specific rates, all have to line up correctly on the same check.

A single new hire in a state the company has never operated in before triggers its own set of obligations: tax registration, withholding setup, unemployment insurance, workers' compensation, all of it before a single supplemental wage payment has even been processed for that person. The supplemental withholding obligation stacks directly on top of that existing registration and compliance burden. A sales organization with commissioned employees in a dozen states, a company issuing RSU vests to a distributed engineering team, and a business paying year-end bonuses across every department at once are all running the same underlying set of federal and state rules simultaneously, for every employee, on every payment, in every state where someone works.

Manual tracking cannot reliably catch a cumulative threshold that crosses pay cycles, apply California's split rate correctly to a mixed batch of bonuses and commissions, and reconcile FICA wage base limits across employees who changed jobs mid-year, all at the same time, without producing an error somewhere. Each of these rules is manageable in isolation. Running all of them correctly, for every employee, on every payment type, across every state, in the same payroll cycle is where scaling companies most often lose the thread, and where the rules described above stop being theoretical and start generating under-withholding notices and employee disputes.