What prejudgment interest is—and why it exists
Prejudgment interest is compensation for the delay in receiving money you were owed. The premise is that a dollar in hand today is worth more than the same dollar paid after a lawsuit concludes. If someone breaches a contract in January and a court awards the contract price two years later, the plaintiff has been deprived of the use of that money for two years. Prejudgment interest restores that lost time value, so the defendant does not effectively borrow the disputed sum interest-free by dragging out litigation.
It is distinct from post-judgment interest, which accrues on the judgment itself from the date it is entered until it is paid. Post-judgment interest is largely mechanical—one rate, one clear start date. Prejudgment interest is where the hard questions live: it looks backward over a period the parties frequently dispute, and its availability is governed by a patchwork of statutes and case law rather than a single clean rule.
Why it is harder than post-judgment interest
The core difficulty is that prejudgment interest usually attaches only to liquidated or readily ascertainable amounts—sums that were fixed, or calculable by a clear standard, before trial. An unpaid invoice of $40,000 is liquidated: the amount was knowable from day one. Damages for pain and suffering, emotional distress, or future harm are unliquidated—their value is not established until a jury decides it—and many states bar prejudgment interest on them entirely.
The rationale is one of fairness to the defendant. A party generally cannot be charged interest for failing to pay a sum whose amount it had no way to know. That single distinction explains most of the variation you will see across jurisdictions, and it is why contract and debt cases routinely carry prejudgment interest while personal-injury verdicts often do not (or do so under a narrower, claim-specific rule). Because the rules and carve-outs differ state by state, always check the controlling statute for your jurisdiction; our state-by-state index is a starting point, not a substitute for the statute itself.
Mandatory, discretionary, or barred
Even where a claim qualifies, availability falls into a few patterns. In some jurisdictions prejudgment interest is mandatory on liquidated claims—the court must add it once the conditions are met. In others it is discretionary: the judge may award it, weighing factors such as whether the delay was the plaintiff's fault, whether the amount was genuinely in dispute, and what equity requires. And in a meaningful set of cases it is simply unavailable, most often for unliquidated tort damages.
Discretion matters because it introduces uncertainty into what looks like a formula. Two plaintiffs with identical facts can walk away with different interest awards depending on the court's reading of the equities. When you model a case, treat a discretionary award as a range, not a certainty—and read the statute closely, because some "discretionary" regimes still fix the rate and the accrual date once the court decides to award interest at all.
How the rate is set—and the general-vs-tort split
There is no single national prejudgment rate. Each jurisdiction sets its own, and the mechanism varies. Some statutes fix a flat percentage that stays put until the legislature changes it. Others peg the rate to a moving market benchmark—a Treasury yield, a central-bank reference rate, or a published index—so it resets periodically. Because these numbers change, this guide will not quote a current figure; look up the live value on the relevant rate page instead. For a sense of how a benchmark-linked rate behaves, see the federal post-judgment rate, which is derived from Treasury yields, or the EU late-payment reference rate used across commercial claims in Europe.
A wrinkle that trips up many calculations: a number of states apply two different rates depending on the claim type—a general rate for contract and debt matters, and a separate, often higher, rate reserved for tort or personal-injury judgments. Using the general rate on a tort award (or vice versa) is a common and costly error. Confirm which rate your claim type triggers before you compute anything. For how we source and re-verify each rate, see our methodology.
When the clock starts: accrual dates
The accrual date—when interest begins running—can matter as much as the rate, because it fixes the length of the interest period. Common starting points include the date of the breach, the date the loss was sustained, the date a sum became due and payable, the date of a demand for payment, or the date the lawsuit was filed. Which one governs is set by statute or case law and can differ by claim type within the same state.
Small differences compound. Choosing the breach date over the filing date can add many months of interest to a long-running case. Where a claim involves a series of losses—say, missed monthly payments—interest may accrue separately on each installment from its own due date rather than in one lump from a single date. Nail down the correct accrual trigger first; the arithmetic that follows is only as accurate as that date.
Doing the math (and where to run it)
Once you have the three inputs—principal, rate, and the accrual-to-judgment period—the calculation is straightforward. Most simple prejudgment interest is computed as simple interest: principal × rate × time. For example, on a $50,000 liquidated debt at an illustrative 6% annual rate, one year of simple interest is 50,000 × 0.06 × 1 = $3,000; over two years and three months (2.25 years) it is 50,000 × 0.06 × 2.25 = $6,750. Some jurisdictions instead direct a daily rate (annual rate ÷ 365) multiplied by the exact number of days, which handles partial years precisely. A few compound the interest, but simple interest is the more common default—check your statute before assuming.
These figures are illustrative only; the operative rate is whatever the current statute or benchmark specifies. Rather than hand-calculate, let a tool apply the correct method and day count: the prejudgment interest calculator handles the pre-award period, and once judgment is entered you can carry the balance forward with the post-judgment interest calculator. Because prejudgment rules turn on jurisdiction, claim type, and dates, treat any result as an estimate. This guide is reference information, not legal advice—verify every rate, rule, and accrual date against the official statute for your jurisdiction before relying on it.