Calculate Prejudgment Interest Federal Court

Use this federal court prejudgment interest calculator to estimate interest on damages between an accrual date and an end date. Enter the principal, annual rate, date range, compounding approach, and day-count basis. This tool is designed for fast litigation planning, mediation prep, damages modeling, and settlement analysis.

Expert Guide: How to Calculate Prejudgment Interest in Federal Court

Prejudgment interest is the additional amount a court may award to compensate a prevailing party for the loss of use of money between the time the claim accrued and the time judgment is entered. In practical terms, it answers a simple damages question: if a plaintiff should have had money months or years earlier, how much value was lost because payment was delayed? For lawyers, experts, and business claimants, the answer can materially change the economics of a case, especially in contract, commercial, securities, fiduciary duty, and federal statutory disputes.

Although many people search for a single “federal court prejudgment interest rate,” the reality is more nuanced. Federal courts do not always use one universal method. Depending on the source of law, a court may apply federal common law, a federal statute, the law of the forum state, or a contractually agreed rate. The court may also decide whether to use simple interest or compounding, what start date controls, and whether an equitable adjustment is appropriate. That is why any calculator should be treated as a modeling tool, not a substitute for legal analysis.

Core idea: the basic formula is usually principal × rate × time for simple interest, or principal × (1 + rate/n)^n×time for compound interest. The real legal work is deciding which principal, which rate, which accrual date, and which compounding convention the court will accept.

What prejudgment interest means in federal litigation

Prejudgment interest is designed to make the prevailing party whole. If damages accrued on January 1, 2021 and judgment was not entered until January 1, 2024, the claimant did not merely lose the principal amount. The claimant also lost the opportunity to use, invest, or deploy that money during the interim. In many federal cases, awarding prejudgment interest is considered necessary to achieve full compensation. In others, it may be discretionary or limited by the governing statute.

In federal court, judges often ask several threshold questions:

• Is prejudgment interest mandatory, presumptively available, discretionary, or barred for this claim?
• Does federal law control, or does state substantive law apply through diversity or another choice-of-law framework?
• What date did damages become fixed, ascertainable, or due?
• Should the rate reflect a statutory benchmark, a contract rate, a prime rate, Treasury yields, or another equitable proxy?
• Should interest be simple or compounded?

How this calculator works

This calculator lets you estimate interest using a principal amount, an annual rate, a start date, an end date, and a compounding convention. Once you click the button, it calculates the elapsed number of days and applies either simple interest or one of several compound interest methods. You can also choose a day-count basis, such as Actual/365 or Actual/360, because the selected denominator affects the final figure.

That structure mirrors common litigation workflows. A damages expert may first run a simple interest model to establish a conservative floor, then compare it to monthly or quarterly compounding for settlement sensitivity testing. Trial counsel may also prepare multiple scenarios: one using the contractual rate, one using a Treasury-based rate, and one using a state statutory rate.

Simple interest versus compounding

The distinction between simple and compound interest is critical. Simple interest accrues only on the original principal. Compound interest accrues on principal plus previously accrued interest. Over long periods, compounding can have a significant effect, especially when the rate is high or the delay period spans several years.

Method	Formula Concept	Typical Use in Litigation Modeling	Effect on Award
Simple interest	Principal × rate × time	Conservative estimate; often used when case law favors simplicity or the court does not expressly authorize compounding	Lowest growth over time
Annual compounding	Principal grows once per year	Used where the governing law or equitable analysis permits compounding but not more frequent capitalization	Moderate increase
Quarterly or monthly compounding	Interest capitalized multiple times per year	Useful in commercial cases, finance disputes, and expert analyses based on market practices	Higher than annual
Daily compounding	Rate applied on a day-by-day basis	Often used for precision modeling; may be too aggressive if unsupported by authority	Highest growth among standard methods

The legal source matters more than the math

One of the biggest mistakes in prejudgment interest calculations is assuming that postjudgment interest law automatically supplies the prejudgment rate. That is not always true. For example, 28 U.S.C. § 1961 clearly governs postjudgment interest in federal cases and ties that rate to the weekly average 1-year constant maturity Treasury yield for the calendar week preceding judgment. But prejudgment interest often depends on the substantive law of the claim and the court’s equitable discretion. In diversity cases, state law frequently supplies the rule. In federal question cases, federal common law may control unless Congress has spoken directly.

That means the proper rate can vary widely. Some courts look to Treasury yields because they are objective and conservative. Others use state statutory rates. Others honor the contract rate if the agreement clearly addresses late payment or amounts due. In some cases, courts select a rate intended to fully compensate for the time value of money, including a prime rate or another market-based proxy.

Real rate data: why the chosen benchmark matters

A small percentage change can materially affect damages in larger disputes. Treasury-based rates, for example, moved sharply between the low-rate environment of 2021 and the higher-rate environment of 2023. That is one reason litigants frequently present competing prejudgment interest models.

Illustrative Benchmark	Approximate 1-Year Treasury Environment	Simple Interest on $100,000 for 1 Year	Observation
Low-rate period	0.10%	$100	Very small carrying cost; common in ultra-low-rate years
Moderate-rate period	2.50%	$2,500	Still relatively conservative compared with many contract rates
Higher-rate period	5.00%	$5,000	Meaningful increase in settlement value and damages presentation
Contractual default rate	9.00%	$9,000	May dramatically increase exposure if enforceable under governing law

These are simple one-year comparisons, but the same logic scales up in multi-year litigation. On a $1,000,000 principal over three years, the difference between a 0.10% rate and a 5.00% rate is not marginal. It can alter settlement posture, reserve analysis, and expert opinions by six figures.

Key inputs you must verify before using any estimate

1. Principal amount. Determine whether interest runs on the full claimed damages, only liquidated amounts, only past damages, or amounts net of offsets and credits.
2. Accrual date. The clock may start on the breach date, invoice due date, loss date, demand date, or another date when damages became ascertainable.
3. End date. Most models use the anticipated judgment date, but parties often model through mediation, summary judgment, or trial.
4. Annual rate. This may come from a contract, statute, Treasury yield, prime rate, or an equitable benchmark adopted by the court.
5. Simple or compound. A court may permit compounding, reject it, or remain silent unless properly briefed.
6. Day-count basis. Actual/365 and Actual/360 are both common modeling conventions. Even small denominator differences matter in high-value cases.

When federal courts may award prejudgment interest

Federal courts often award prejudgment interest when doing so is necessary to make the plaintiff whole and when the amount due is reasonably ascertainable. Commercial contract disputes, ERISA benefit litigation, maritime cases, securities cases, and certain federal statutory claims may all involve prejudgment interest analysis. In contrast, some statutes define damages in a way that narrows or precludes additional interest, and some categories of relief may already incorporate delay compensation.

In diversity cases, always check whether state law treats prejudgment interest as substantive. If so, the federal court may be required to follow the relevant state rule on entitlement, rate, and sometimes compounding. Lawyers frequently lose precision here by citing a federal postjudgment statute when the operative claim is actually governed by state contract law.

Example calculation

Assume a plaintiff is owed $250,000 under a commercial agreement, the damages accrued on March 1, 2022, and judgment is projected for March 1, 2025. If the court accepts a 6% simple annual rate with an Actual/365 convention, the estimated prejudgment interest is:

• Principal: $250,000
• Rate: 6.00%
• Time: 3.00 years
• Interest: approximately $45,000
• Total: approximately $295,000

If the same facts are modeled using monthly compounding instead, the estimated interest increases. That difference may become central in settlement discussions, expert reports, and motions for entry of judgment.

Common litigation errors

• Using the wrong accrual date and overstating or understating the damages period.
• Assuming prejudgment and postjudgment rates are identical.
• Applying a contract rate without confirming enforceability or scope.
• Ignoring compounding when case law supports it, or assuming compounding where the court is unlikely to allow it.
• Calculating interest on future damages that have not yet accrued.
• Failing to net out payments, credits, offsets, or partial performance.

Best practices for lawyers and experts

Prepare at least three scenarios. A disciplined federal court damages presentation usually includes a conservative model, a moderate model, and an advocate’s model. For example, counsel may show simple interest at a Treasury-like rate, simple interest at the state statutory rate, and monthly compounding at the contract rate. Then tie each scenario to specific authority and explain why the court should adopt the preferred approach.

You should also preserve the arithmetic in a transparent format. Judges appreciate a table showing the principal, date range, exact day count, annual rate, legal authority, and resulting calculation. If the claim involves multiple invoices or separate categories of damages, break out each tranche rather than applying a single blended date. Precision increases credibility.

Authoritative sources to review

Cornell Law School Legal Information Institute: 28 U.S.C. § 1961
U.S. Department of the Treasury: Interest Rate Data
United States Courts: Federal Court Management Statistics

Final takeaway

If you need to calculate prejudgment interest in federal court, the arithmetic itself is usually straightforward. The hard part is selecting the legally correct assumptions. Start with the claim source, identify whether federal or state law governs, confirm the accrual date, determine the proper benchmark rate, and verify whether the court is likely to use simple or compound interest. Then use a calculator like the one above to model the numbers clearly and consistently.

For motion practice, mediation statements, expert reports, and reserve analysis, a good prejudgment interest calculation can materially improve the quality of the damages presentation. Just remember that the strongest calculation is not the most aggressive one. It is the one that connects every input to the governing law and the facts the court is most likely to accept.

Educational use only. This page provides a financial estimate and general legal information, not legal advice. Always confirm the governing authority, local precedent, and claim-specific rules before filing or relying on a prejudgment interest number in federal court.