Retro Pay Calculator

What is retro pay?

Retro pay (also called retroactive pay or back pay) is the wage difference owed when an employee was paid less than they should have been over a defined period — typically because a raise was approved but not processed in time, a wrong rate was entered in payroll, or a missed bonus should have been paid earlier. The math: (correct rate − actual rate) × hours worked at the wrong rate. The result is treated as wages by the IRS and DOL, subject to the same income-tax and FICA withholding as regular pay.

Is retro pay taxed differently from regular pay?

Retro pay is taxed at the same rate as regular pay over the course of the year — the IRS treats it as wages, not a separate category. But withholding is different: most employers use the supplemental-wage flat method (22% federal) for retro pay instead of running it through the W-4 aggregate method. So your paycheck may withhold more or less than your eventual tax bill, depending on your W-4 and total earnings. The difference reconciles on your tax return.

Source: IRC § 3402(g)(1) · IRS Publication 15 § 7

What's the difference between retro pay and back pay?

Colloquially, "retro pay" and "back pay" are used interchangeably. Technically, the IRS and DOL distinguish: retro pay is the wage difference when the wrong (lower) rate was paid for work actually performed — the employee was paid, just at the wrong amount. Back pay is wages owed for work the employee was prevented from performing — wrongful termination cases, DOL recovery actions for unpaid overtime, federal-court judgments. Same withholding treatment for both: both are supplemental wages under IRS Publication 15. This calculator works for either case — the math is identical.

Does my state have to follow the 22% federal supplemental rate?

No — that's the federal rate only. State withholding on supplemental wages varies by state: 9 states have no income tax, 13 are flat-tax states where the supplemental rate equals the ordinary rate, and the rest have either a separate supplemental rate (California 6.6% for non-bonus wages, New York 11.7%) or a bracketed schedule based on the employee's W-4. The calculator models the no-tax and flat-tax states directly and surfaces "varies — see note" for the rest.

What if the rate change covered overtime hours?

The calculator handles this — when total hours exceeds 40 (a single FLSA workweek can't have more than 40 straight-time hours), it asks whether the period included overtime, and if so, reveals an OT-hours input. The math under 29 CFR §778.303: the straight-time rate-delta applies to ALL hours worked (including the straight-time portion of OT hours), AND an additional 0.5 × (new − old) per OT hour is owed for the half-time recompute. Example: $20 → $22 over 180 hours with 20 OT hours = $360 straight + $20 OT premium = $380 total. The calculator shows both pieces in the result card and applies withholding to the total.

Source: 29 CFR §778.303 — retroactive pay increases

My employer paid the bonus late — does that trigger retro pay?

Maybe — if the bonus is non-discretionary (tied to performance, attendance, productivity, hours worked, etc.), it should have been included in the regular rate for the period the bonus covers. That retroactively increases the OT premium owed for any overtime hours in that period — the 29 CFR §778.209 bonus-apportionment rule. It's a separate calculation from rate-change retro pay; this calculator doesn't model it. The companion retro-pay article covers it as a top-five mistake (under "non-discretionary bonus apportioned back"), and the overtime article covers the §778.209 mechanic in full. Discretionary bonuses (true gifts, no expectation, paid for non-work reasons) don't trigger this.

How accurate are the California and New York state withholding estimates?

California is modeled at 6.6% (FTB's supplemental rate for regular supplemental wages, which is what retro pay is). The catch: FTB allows employers to elect the aggregate method instead, which most large CA payroll systems use — so the actual paycheck may differ by a few percentage points. New York is modeled at 11.7% state-only (DTF's supplemental rate). The catch: NYC residents owe an additional 4.25% NYC supplemental on top, and Yonkers residents owe 0.51% Yonkers — both NOT in the estimate. If you live in NYC, your actual state-plus-local withholding is closer to 16%. The methodology page explains both caveats in detail.

Source: California FTB DE 44 (Employer Withholding Guide) · NY DTF NYS-50-T-NYS (Withholding Methods)

How is FICA handled?

FICA (Social Security 6.2% + Medicare 1.45% = 7.65% total) applies to retro pay at the same rate as regular wages. The calculator shows the 7.65% line in the withholding breakdown so your net number matches your actual paycheck. Caveat: above the 2026 Social Security wage base ($184,500), the SS portion stops withholding — Medicare 1.45% continues. The calculator uses the full 7.65% for the typical sub-wage-base hourly worker; high-earners crossing the wage base will see slightly lower actual FICA than the 7.65% estimate.