Prejudgment & Post-Judgment Interest Calculator
Calculate interest on a legal claim or judgment from a number of days or from actual dates: simple or compound prejudgment interest from the loss to judgment, post-judgment interest to payment, a 365, 360 or actual-day count, costs added to the judgment, several partial payments applied interest-first, the daily per-diem amount and a dated interest schedule.
How it works
Simple interest = principal × rate × days ÷ 365 (or ÷ 360, or ÷ 365/366 by calendar year); compound = balance × [(1 + rate/n)^(n × years) − 1]; judgment = principal + unpaid prejudgment interest + costs; each payment pays accrued interest first, then principal; total = judgment − payments + post-judgment interest; per diem = balance × post-judgment rate ÷ 365
Interest on a legal claim compensates the winning party for the time value of money it should have had. Prejudgment interest runs from the loss or breach to the entry of judgment, at a rate set by statute or the contract. It is usually simple interest, often at rates well above market. Post-judgment interest runs from judgment until payment. It is mandatory almost everywhere, and in federal court it is compounded annually at the Treasury rate. Enter the periods as a number of days, or switch to dates and the calculator counts the days, including leap years if you choose the actual/actual day count. Court costs or fees awarded in the judgment are added to it and earn post-judgment interest. Partial payments, on the judgment date or any other date, are applied first to accrued interest and then to principal under the US Rule. The dated schedule shows the interest for each stretch and the balance after every payment. The per-diem figure is what each extra day adds, which matters in settlement letters. Whether prejudgment interest is available at all, and from what date, is often contested in tort cases.
Worked example
An $85,000 claim accruing simple interest at 10% for 600 days before judgment earns $13,972.60 of prejudgment interest ($23.29 a day), so judgment is entered at $98,972.60. If it is paid 200 days later at 10% post-judgment interest, another $5,423.16 accrues ($27.12 a day), for a total of $104,395.76. If the debtor had paid $20,000 on the judgment date, post-judgment interest would run on $78,972.60 instead, and the total after 200 days would be $83,299.87.
Sources: 28 U.S.C. § 1961 — Interest on federal judgments (Cornell LII) · US Courts — Post-judgment interest rate · Texas OCCC — Judgment interest rates
Frequently asked questions
Which rate applies to my case?
The forum state’s statute (e.g. California 10% on contracts and 7% on most other claims, New York 9%, Massachusetts 12%; Texas ties it to the prime rate within a 5–15% band) unless the contract sets a lawful rate. Federal courts use the federal post-judgment rate but state prejudgment rules in diversity cases. Several states index their rate, so check the current statute.
How are partial payments applied?
Under the usual US Rule a payment first pays the interest accrued to that date and only the rest reduces principal. Unpaid simple interest is not added to principal before judgment. Enter each payment with its date and the calculator recalculates the interest after it.
Should I use a 365-day or 360-day year?
Statutory interest is normally computed on a 365-day year. A 360-day year gives slightly more interest and appears in some loan contracts. The actual/actual option divides by 366 for days that fall in a leap year.
Is interest taxable?
Generally yes. Interest on a judgment or settlement is taxable income even when the underlying damages (for personal physical injury) are not.
How do I calculate the per diem?
Multiply the amount the interest runs on by the annual rate and divide by 365. On $100,000 at 10% that is $27.40 a day. Settlement letters often state this so the payoff can be updated for each day of delay.
Related calculators
General-information arithmetic based on common US practices; statutes, court rules, fee schedules and credit rules differ by jurisdiction and change over time. This is not legal advice — confirm with a licensed attorney or the court.