When the National Labor Relations Board finds that an employer fired or penalized workers for union activity, the remedy is back pay: the wages those workers would have earned if the unlawful act had never happened. The idea is simple. The money puts the worker where they would have been, no better and no worse.
The arithmetic behind that idea is less simple. A back pay order subtracts what the worker earned elsewhere, adds interest, and turns on how hard the worker looked for a new job. The Cambridge Dictionary's business section defines back pay as money owed for work already performed, in arrears — which is exactly the frame a labor board uses, only stretched across months or years of a case. This explainer walks through how that calculation works and where it can go wrong for either side.
Back pay is not a penalty. It is a make-whole remedy, and that distinction shapes every step of the math. A worker cannot profit from the order, and an employer cannot shrink it by delay. The board's compliance machinery exists to hold both lines.
What does a back pay order actually cover?
The starting point is gross earnings the worker lost: the wages, and where they apply, the benefits and pension contributions that would have flowed from continued employment. The clock usually starts on the date of the unlawful act — the discharge, the layoff, the denial of a promotion — and runs to the date the employer offers reinstatement in good faith.
The calculation uses the rate the worker would have earned, not the rate they happened to leave at. If the job carried scheduled raises, a bonus structure, or a pension accrual, those enter the formula as forward-looking amounts. The board frames this as reconstruction: build the paycheck that would have existed, then adjust it.
Two qualifications matter. First, the employer's unlawful act must be the cause of the loss; a worker fired for union activity who would have been laid off anyway under a lawful reduction presents a different calculation. Second, the order binds the employer, not the union — although a union can owe back pay in its own unfair labor practice cases.
How do interim earnings change the number?
Here is the offset that surprises people. Money the worker earned during the back pay period — a new job, gig work, unemployment benefits under some formulas — is subtracted from the gross loss. The remedy replaces what was lost, not what was lost plus whatever the worker managed to earn on top.
The offset has limits that matter in practice. Earnings from a replacement job count, but so can earnings the employer argues the worker declined an offer of suitable equivalent work. The board distinguishes between income the worker actually received and income attributable to work the worker unreasonably refused. Only the second category is contested; the first is usually just arithmetic.
Employers hunt for the offset; workers' representatives document it honestly. A worker who earned more at a replacement job than at the old one may be owed nothing for that stretch of months, even though the discharge itself was unlawful. The remedy follows the wallet, not the wrong.
What is the two-year search rule for mitigation?
A worker has a duty to look for other work — to mitigate the loss. The board's long-standing framework gives that duty a shape: a worker generally must make a reasonable search for substantially equivalent employment, and the board's guidance has treated roughly a two-year window of diligent searching as the benchmark for how long that obligation runs before the burden shifts in earnest to the employer to prove the worker fell short.
What counts as a reasonable search is fact-specific. Applying for comparable jobs in the local market, keeping records of applications, accepting suitable work when it appears. A worker who sits out the market entirely can see the back pay period cut short. A worker who searched diligently but found only lower-paying work keeps the difference as part of the award, because the shortfall traces to the employer's act.
The search rule cuts both ways, and both sides should understand it as such. It protects employers from open-ended liability. It also protects workers, because the employer bears the burden of proving the worker did not search, not the other way around.
Where does interest come in?
Back pay awards carry interest, because a wage loss from three years ago is worth more than the same nominal sum paid today. The board adds interest from the date each lost payment would have been due, so the interest compounds across the period rather than accruing once at the end. The rate follows the board's prescribed formula, which tracks a tax underpayment rate and changes quarterly.
Interest is not a side issue. In a case that runs several years from discharge to decision to compliance, the interest component can be a meaningful share of the total. For long-running litigation, it is often the difference between an award that restores the worker and one that merely gestures at restoration.
What this means for workers and employers
For workers, the practical steps are documentation steps. Keep the applications, the rejection letters, the pay stubs from any interim job. The back pay calculation is built from paper, and the side with the better paper usually gets the better number.
For employers, the lesson is that delay has a price measured in interest, and that a documented, suitable job offer can cap liability. Reinstatement offered in good faith stops the clock; reinstatement offered on degraded terms does not.
Our analysis of how these cases move suggests the calculation is best understood as three clocks running at once: the earnings clock from the unlawful act, the mitigation clock from the duty to search, and the interest clock from each missed payday. The final number is where they intersect. For the broader process a case follows before any of this math begins, see how an unfair labor practice case moves through the NLRB, and for how board orders get enforced when an employer resists, see what a labor injunction means and when courts stop a strike.
The evidence for how these formulas operate comes from the board's own remedial framework, which is public but dense. What remains unknown in any given case is the number itself — that emerges only from the specific wage records, the interim earnings, and the search history on the table.
