Why Is My Paycheck Different Every Week?
Most week-to-week variation comes from one mechanism almost nobody explains: federal withholding annualizes a single paycheck. The formula takes the wages in front of it, multiplies them by the number of pay periods in the year, finds the bracket that annual figure lands in, and divides back down. A week with overtime is therefore withheld as though every week of your year contained that overtime.
That is not a payroll error and it is not your employer being clever. It is the percentage method in IRS Publication 15-T, working exactly as written. Below are the five things that actually move the number on your stub, in rough order of how often they are the culprit.
1. Withholding annualizes each paycheck separately
Publication 15-T’s Worksheet 1A — the Employer’s Withholding Worksheet for the Percentage Method — runs like this:
- Take taxable wages for the current payroll period.
- Multiply by the number of pay periods in the year. This is the annualizing step.
- Apply adjustments to reach the “Adjusted Annual Wage Amount.”
- Look that annual figure up in the Annual Percentage Method tables.
- “Divide the amount on line 2g by the number of pay periods on line 1b. This is the Tentative Withholding Amount.”
Step 2 is where the surprise lives. Payroll is not forecasting your year. It is treating this week as a representative sample of all of them.
| Week | Gross | What the formula assumes you earn annually |
|---|---|---|
| Ordinary week | $1,000 | $52,000 |
| Week with overtime | $1,800 | $93,600 |
The heavy week is withheld at the rate a $93,600 earner pays, so your take-home does not rise in proportion to the extra hours. This is the mechanism behind the very common and very wrong belief that overtime “pushes you into a higher bracket and isn’t worth it.” It does not. Your bracket is determined by your actual annual income when you file. What happened is that withholding ran high for one period, and the excess comes back as refund.
The money is timing, not loss. That distinction is worth holding onto, because the paycheck really is smaller and being told “brackets are marginal” does not explain why.
2. Overtime follows the workweek, not your pay period
These are different units and they often do not line up. 29 CFR 778.104 is explicit:
“The Act takes a single workweek as its standard and does not permit averaging of hours over 2 or more weeks.”
So an employee who works 30 hours in one week and 50 in the next is owed overtime for the ten hours over forty in the second week. They are not averaged to 40 and 40 with no overtime owed, and an employer that does average them has not paid what is due. The regulation applies the same way to swing shifts, pieceworkers and commission-based employees.
The practical consequence for a variable paycheck: if your two-week pay period straddles the workweek boundary, identical total hours can produce different overtime and a different check depending purely on which side of the boundary they fell. If you want to check your own numbers, our overtime calculator works per workweek for this reason.
3. Bonuses and commissions are withheld under a different rule
IRS Publication 15 sets a flat rate for supplemental wages:
“The withholding rate on supplemental wages remains 22% (37% if supplemental wages paid to an employee during the calendar year exceed $1 million).”
If your ordinary marginal rate is 12%, a bonus withheld at a flat 22% looks like confiscation on the stub. It is a withholding rate, not a tax rate. We go through the two permitted methods and when each applies in the aggregate versus percentage method.
4. Deductions do not hit every check
This one has nothing to do with tax and is probably the most common non-tax cause. Employers frequently take benefit premiums from a fixed number of checks a month, typically the first two. In a month with three biweekly paychecks, one check carries no premium at all and arrives noticeably larger — and because that happens roughly twice a year rather than monthly, it rarely gets connected to the cause.
Nothing about your rate changed. A fixed cost simply landed on a different check. If your employer pays biweekly, you can see which months these are in the three-paycheck months.
5. Late in the year, a cap can switch off
The 6.2% Social Security (OASDI) half of FICA applies only to earnings up to an annual wage base. Once your year-to-date wages pass it, that 6.2% stops for the rest of the calendar year and your net rises with no change to your gross. Medicare has no cap and keeps going.
We are deliberately not quoting the current wage-base figure here. It changes almost every year, and the Social Security Administration’s own page was not reachable to us at the time of writing, so any number we printed would have been copied from somewhere else and presented as if verified. Check ssa.gov for the figure that applies to you. An article that quotes a stale cap with confidence is worse than one that tells you where to look.
Working out which one it is
| What you noticed | Most likely cause |
|---|---|
| Worked more hours, take-home barely moved | Annualized withholding (§1) |
| Overtime appeared on an unexpected check | Workweek vs pay period (§2) |
| A bonus was taxed far above your usual rate | Flat 22% supplemental withholding (§3) |
| One check was larger and gross was identical | A deduction skipped that check (§4) |
| Every check got bigger around November | Social Security wage base reached (§5) |
| Gross itself differs and hours were the same | Not withholding — check the rate and hours on the stub |
The one that catches nearly everyone is the first, because it is counterintuitive and because the usual explanation offered — “tax brackets are marginal” — is true but answers a question nobody asked. The bracket is not the problem. The formula treating one week as your whole year is.
This is general information about how withholding is computed, not tax advice for your circumstances.
Sources
- IRS Publication 15-T, Federal Income Tax Withholding Methods — Worksheet 1A, percentage method. irs.gov/publications/p15t
- IRS Publication 15, (Circular E), Employer’s Tax Guide — supplemental wage withholding rates. irs.gov/publications/p15
- 29 CFR 778.104 — the workweek as the standard; no averaging over two or more weeks. law.cornell.edu/cfr/text/29/778.104