To calculate retro pay, find the difference between your old and new hourly rate, multiply it by the regular hours you worked during the underpayment period, then add time-and-a-half of that same difference for every overtime hour in the gap. That gives you the gross amount owed before taxes. The rest is knowing how supplemental-wage withholding, FICA caps, and retirement deferrals reshape what actually lands in your account.
What to Pull Together Before You Do the Math
Four pieces of information drive the calculation: the effective date of the new pay rate, the date it actually started appearing in your paychecks, your old rate, and your new rate. You also need timecards or pay stubs showing every hour worked in the gap, broken out by regular and overtime hours. If you worked at more than one rate during any of those weeks, get those splits too — they change the overtime math.
Federal law requires employers to keep basic payroll records for at least three years and supplementary time records for at least two years, so payroll should have the underlying data even if you don’t.1eCFR. 29 CFR Part 516 – Records to Be Kept by Employers Request copies if you’re missing anything.
The Base Calculation
Subtract the old hourly rate from the new one. Going from $20.00 to $22.50 gives you a difference of $2.50 per hour, which is what you were shorted on every regular hour during the gap.
Multiply that difference by your total non-overtime hours during the underpayment period. One month at 160 regular hours produces $400.00 in gross base retro pay ($2.50 × 160). Overtime hours get handled separately in the next step.
If You’re Salaried
Convert the annual salary increase into a per-pay-period amount. A $5,200 annual raise divided across 26 biweekly pay periods is $200 per period. If the delay covered two periods, you’re owed $400. Exempt salaried employees don’t get overtime retro pay, but the base math is the same.
The Overtime Step Most Employers Miss
You cannot simply pay the hourly rate difference for overtime hours. When the base rate rises retroactively, the regular rate that overtime is computed from also rises for the entire period, and the time-and-a-half premium has to be applied to the increase itself.2eCFR. 29 CFR Part 778 – Overtime Compensation – Section 778.303
Take the same $2.50 hourly increase. For each overtime hour in the gap you’re owed the $2.50 plus a 50% premium on that $2.50, which is another $1.25, for a total of $3.75 per overtime hour. Twenty overtime hours during the delay adds $75.00 to the earlier $400.00 base, bringing the gross to $475.00.
Lump-Sum Retroactive Increases
When the retroactive raise comes as a flat lump sum rather than a per-hour figure — common in union settlements — the employer prorates the lump sum across the hours it covers. Divide the lump sum by total hours worked in the retroactive period to get an effective per-hour increase, then apply the 1.5x multiplier to that figure for every overtime hour.2eCFR. 29 CFR Part 778 – Overtime Compensation – Section 778.303
Two Rates in the Same Week
If you worked at more than one rate in the same workweek, say $20/hour on one task and $18/hour on another, overtime is based on a weighted average. Total your earnings across all rates for the week, divide by total hours, and that weighted average is the regular rate used for overtime.3eCFR. 29 CFR Part 778 – Principles for Computing Overtime Pay Based on the Regular Rate A retroactive bump to any of those rates changes the weighted average, which ripples through the overtime calculation for every affected week.
What Taxes Will Take Off the Top
The IRS treats retro pay as supplemental wages, the same category as bonuses, commissions, and severance.4Internal Revenue Service. Publication 15 (2026), Employer’s Tax Guide That classification changes how federal income tax is withheld.
Federal Income Tax Withholding
Employers use one of two methods. The percentage method applies a flat 22% federal income tax withholding to the retro pay, regardless of your bracket, and it’s the common choice for a standalone retro check.4Internal Revenue Service. Publication 15 (2026), Employer’s Tax Guide The aggregate method combines the retro pay with a regular paycheck and withholds on the total as if it were one payment, which can temporarily bump you into a higher withholding tier for that period. Your actual tax bill is settled when you file, so any overwithholding comes back as a refund.
If your total supplemental wages for the year exceed $1 million, everything above that threshold is withheld at 37%.4Internal Revenue Service. Publication 15 (2026), Employer’s Tax Guide
Social Security and Medicare
FICA withholding on the employee side is 6.2% for Social Security and 1.45% for Medicare, or 7.65% combined.5Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates Social Security tax only applies up to the annual wage base, which is $184,500 for 2026.6Social Security Administration. Benefits Planner – Social Security Tax Limits on Your Earnings If your year-to-date wages are already near that cap, a retro check may cross it, and no further Social Security tax is owed on the excess. Medicare has no ceiling and keeps applying.
Once your calendar-year wages exceed $200,000, an Additional Medicare Tax of 0.9% is withheld on wages above that threshold, and retro pay counts toward the total.7Internal Revenue Service. Publication 926 (2026)
401(k) Deferrals
If you’re enrolled in a 401(k) with automatic deferrals, your elected percentage will typically come out of the retro check the same way it does from a regular paycheck. A large lump sum can push you toward the annual limit fast — $24,500 for 2026, or $32,500 if you’re 50 or older with the $8,000 catch-up.8Internal Revenue Service. Notice 25-67, 2026 Amounts Relating to Retirement Plans and IRAs
If retro pay drives your deferrals past the limit, the excess has to be distributed back to you by April 15 of the following year to avoid being taxed twice, once in the deferral year and again when distributed, and possibly hit with early distribution penalties.9Internal Revenue Service. 401(k) Plan Fix-It Guide – Elective Deferrals Under IRC Section 402(g) If a big retro check is landing late in the year, check where your year-to-date deferrals stand and adjust your contribution rate if needed.
Which Year the Retro Pay Belongs To
Retro pay is reported as wages in the year it’s paid, not the year the work was performed. If the hours were worked in 2025 but the check is cut in 2026, it goes on the 2026 W-2 in Box 1.10Internal Revenue Service. Publication 957, Reporting Back Pay and Special Wage Payments to the Social Security Administration Employers often issue retro pay as a separate check to keep the tax reporting clean, though it can also appear as a labeled line on a regular paycheck. Check the pay stub to confirm the gross matches your calculation, the withholding method used, and that FICA lines up with any wage-base cap you’ve hit.
If Your Employer Won’t Pay
If payroll has been notified and the retro pay still hasn’t shown up, the Department of Labor’s Wage and Hour Division investigates unpaid wage claims at no cost and treats them confidentially regardless of immigration status.11U.S. Department of Labor. Information You Need to File a Complaint Under the FLSA, a court can award unpaid wages plus an equal amount in liquidated damages, effectively doubling what was owed, on top of attorney’s fees and court costs.12Office of the Law Revision Counsel. 29 US Code 216 – Penalties
Watch the clock. The federal statute of limitations is two years from the violation, or three years if the underpayment was willful.13Office of the Law Revision Counsel. 29 US Code 255 – Statute of Limitations State deadlines vary. Filing sooner is safer than waiting.