Your paycheck is different every week because the pieces that build it — hours worked, supplemental pay, tax withholding, benefit deductions, garnishments, and even the calendar — each move on their own schedule. A single check reflects whatever combination happened to land in that pay period, so the deposit can shift by dozens or hundreds of dollars without anything being wrong. Most of the swings are explainable once you know where to look on your stub.
Hours Worked and Overtime
If you’re paid hourly, this is usually the biggest driver. Your pay comes from the exact time your employer records, and small differences compound. Federal regulations let employers round your clock-in and clock-out times to the nearest five minutes or quarter hour, provided the rounding doesn’t consistently shortchange you.1eCFR. 29 CFR 785.48 – Use of Time Clocks Clocking in two minutes early one day and three minutes late the next can quietly shift your weekly total.
Overtime is where the math really moves. Federal law requires at least one and a half times your regular rate for every hour beyond 40 in a workweek.2Office of the Law Revision Counsel. 29 USC 207 – Maximum Hours Working 45 hours instead of 40 doesn’t just add five hours of pay; those extra hours are worth 50% more than your base rate. A jump from a 38-hour week to a 45-hour week can produce a check that’s 25% larger than the previous one. Shift differentials and hazard pay work the same way, adding a premium to certain hours without changing the rest of your schedule.
Bonuses, Commissions, and Tips
Supplemental pay lands on your check irregularly, which creates the obvious spikes. What catches people off guard is how it’s taxed. The IRS lets employers withhold federal income tax on bonuses and commissions at a flat 22%, regardless of your usual bracket. If your normal withholding rate is lower than that, a bonus check looks like it was taxed harder than your regular pay, because it was — at least upfront. You reconcile the difference when you file. If your supplemental pay exceeds $1 million in a calendar year, the rate on the excess jumps to 37%.3Internal Revenue Service. Publication 15 (Circular E), Employer’s Tax Guide
Tips create a different kind of volatility. Earning $600 in tips one week and $250 the next swings your gross pay by $350 before taxes enter the picture. All cash and non-cash tips are taxable income, and you’re required to report cash tips to your employer whenever they total $20 or more in a calendar month.4Internal Revenue Service. Tip Recordkeeping and Reporting Your employer then withholds income tax and FICA from your wages to cover the tax on those tips, so a high-tip week pulls more out of your base pay for taxes.
How Tax Withholding Reacts to Each Check
Federal income tax withholding is calculated each pay period based on how much you earned that period and the information on your Form W-4.5Internal Revenue Service. Tax Withholding for Individuals The payroll system essentially annualizes each check to estimate your yearly income, then withholds the tax it thinks you’ll owe. When you work extra hours or earn a commission, the software projects a higher annual income and withholds at a steeper rate. The following week, when pay drops back to normal, the withholding drops too. This is the single biggest reason net pay bounces around even when gross pay only changes modestly.
State and local income taxes follow similar logic, though rates and brackets vary by jurisdiction. If you recently updated your W-4, changed your filing status, or added dependents, your withholding will shift on every check going forward until you change it again.
FICA Thresholds That Kick In Mid-Year
Social Security and Medicare taxes — together called FICA — create predictable shifts later in the year for higher earners. The Social Security tax rate is 6.2% on earnings up to $184,500 in 2026.6Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates Once your year-to-date earnings cross that threshold, the 6.2% deduction disappears from your remaining paychecks for the year. If you earn $4,000 per biweekly check, that’s roughly $248 that suddenly stops being withheld — a noticeable bump in take-home pay that reverses in January when the clock resets.
Medicare tax has no wage cap, so 1.45% comes out of every dollar all year. But if your wages exceed $200,000 in a calendar year, your employer must withhold an additional 0.9% Medicare tax on earnings above that mark.6Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates That extra deduction kicks in mid-year with no warning on your stub beyond a larger Medicare line item.
Benefit and Retirement Deductions
Health insurance premiums are usually a fixed dollar amount, but they don’t always hit every paycheck. If your employer bills premiums monthly and you’re paid weekly, the full deduction might come out of just one check per month, leaving the others noticeably fatter. Dental and vision coverage can work the same way. When open enrollment closes each fall, any changes you made — upgrading to a family plan, adding an FSA or HSA, switching tiers — will change your deductions starting in January, sometimes by enough to make your first check of the new year feel like a pay cut.
Retirement contributions are where gross-pay swings echo into your deductions. If you contribute a fixed percentage to a 401(k), the dollar amount deducted rises and falls with your earnings.7Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans Contributing 6% of a $1,200 gross check sends $72 to your retirement account; 6% of a $1,500 overtime check sends $90. The difference is small per check, but it compounds the week-to-week variation. For 2026, you can defer up to $24,500 into a 401(k), with an additional $8,000 catch-up if you’re 50 or older, or $11,250 if you’re between 60 and 63.8Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Once you hit that annual cap, contributions stop and your take-home pay jumps for the rest of the year.
Garnishments Tied to Your Disposable Pay
Wage garnishments are involuntary deductions ordered by a court, and they fluctuate because they’re calculated as a percentage of your disposable earnings, meaning what’s left after taxes. For most consumer debts, federal law caps garnishment at 25% of your disposable earnings for the week, or the amount by which those earnings exceed 30 times the federal minimum wage, whichever is less.9Office of the Law Revision Counsel. 15 USC 1673 – Restriction on Garnishment If your disposable pay dips below a certain floor, the garnishment shrinks or pauses.
Child support, alimony, federal tax debts, and student loan garnishments follow different formulas with higher limits, but the same principle applies: because they’re tied to disposable earnings, any change in your pre-tax pay cascades into a different garnishment amount and a different net check.9Office of the Law Revision Counsel. 15 USC 1673 – Restriction on Garnishment Some states also let your employer deduct a small administrative fee for processing each payment, adding one more moving part.
Pay Schedule and Calendar Quirks
The calendar itself creates variations that have nothing to do with your work. If you’re paid biweekly, you receive 26 paychecks most years, but roughly every 11 to 12 years the calendar produces a 27th pay period. For salaried employees, employers sometimes spread the annual salary across 27 checks instead of 26, making each check slightly smaller that year. If your employer doesn’t adjust and pays your usual per-check amount 27 times, you’d be overpaid for the year, and some employers claw back the difference.
Even in a normal year, biweekly pay means two months contain three paydays instead of two. For people whose fixed bills assume a twice-monthly rhythm, that third paycheck can feel like bonus money, but the annual total is the same. Semi-monthly pay (the 1st and 15th, for example) always delivers exactly 24 checks, though the number of working days in each half-month varies, which can create odd-looking checks for hourly workers.
Holidays add another wrinkle. If payday falls on a bank holiday, your deposit typically lands a business day early, which can temporarily shorten or lengthen the gap between checks. Some employers also pay premium rates for hours worked on federal holidays, bumping gross pay for that period.
How to Check and Fix a Paycheck Problem
Most fluctuations are legitimate, but actual payroll errors — wrong hours, a missing shift differential, a deduction applied twice — happen more often than you’d think. Start by reviewing your pay stub line by line and comparing the hours listed against your own records. Even a note in your phone counts. Federal law requires your employer to keep detailed payroll records, though the right to inspect your own records depends on your state.10United States Department of Labor. Fact Sheet 21 – Recordkeeping Requirements Under the Fair Labor Standards Act (FLSA)
If something looks wrong, go to your payroll department or HR first. Most errors are clerical and get corrected on the next check once flagged. For problems that don’t get resolved internally, especially unpaid overtime or missing wages, you can file a complaint with the Department of Labor’s Wage and Hour Division online or by calling 1-866-487-9243.11Worker.gov. Filing a Complaint With the U.S. Department of Labor’s Wage and Hour Division (WHD) The nearest field office typically contacts you within two business days to evaluate whether an investigation is warranted. If the investigation finds you were underpaid, you’ll receive a check for the lost wages.
For withholding that seems too high or too low, the IRS Tax Withholding Estimator on irs.gov lets you plug in your year-to-date figures and see whether you’re on track. If you’re consistently over-withheld, submitting an updated W-4 to your employer brings your paychecks closer to your actual tax liability instead of waiting for a large refund at filing time.5Internal Revenue Service. Tax Withholding for Individuals