Definition
Compound interest is interest calculated not only on the original principal of a loan or debt, but also on previously accumulated interest that has been added to that principal. Each time interest is compounded, the unpaid interest from a prior period is folded into the balance, creating a new, larger base on which future interest accrues. The result is exponential growth of the debt or investment over time, as opposed to simple interest, which is calculated only on the original principal throughout the life of the obligation.
In legal contexts, compound interest most often arises as a remedy question: when a court or statute awards interest on a judgment, debt, or tax liability, the choice between simple and compound interest can dramatically affect the amount owed.
Common Language
Modern common usage (Wiktionary): Interest, as on a loan or a bank account, that is calculated on the total on the principal plus accumulated unpaid interest.
Historical common usage (Webster's 1913): Not separately defined; treated as a mathematical and commercial concept rather than a legal one.
The common and legal meanings here are substantively identical in how they describe the mechanism. The legal significance lies not in a different definition but in a different set of questions the law asks about it: Is compound interest permissible at all? When must it be expressly authorized? Who bears the burden of proving entitlement to it? Researchers should understand that the dictionary definition is the easy part — the legal complexity lives in enforceability and remedy doctrine, not in the definition itself.
Common Confusion
Compound interest is routinely confused with simple interest, and the distinction matters enormously in remedies practice. Simple interest is computed on the original principal only; compound interest is computed on principal plus accrued interest. Courts and statutes that are silent on which applies have historically defaulted to simple interest, meaning a litigant who assumes compound interest applies without statutory or contractual authority may find the award reduced substantially. Separately, the term is sometimes loosely conflated with usury analysis, but usury doctrine governs the rate of interest, not the compounding mechanism — a compound interest arrangement at a lawful rate is not inherently usurious.
Why It Matters in Research
The critical navigational point for corpus researchers is that compound interest occupies a disfavored position at common law. Bouvier states plainly that it "is not, in general, allowed." This baseline presumption against compound interest pervades historical sources and persists in many modern contexts, though it has been substantially eroded by contract, statute, and equitable doctrine.
When reading historical cases, researchers must be alert to the default rule: courts denied compound interest unless there was an express agreement between the parties, a statutory mandate, or a recognized equitable exception. This means that silence in a historical opinion on the type of interest awarded is not neutral — it likely means simple interest was applied. Modern sources, by contrast, increasingly permit or even require compound interest in specific contexts (tax underpayment and overpayment calculations, for example, are now often compounded by statute at the federal level).
Jurisdictional variation is significant here. Some states have codified when compound interest is available; others still rely on the common law presumption against it. Researchers examining state court judgments across different eras should not assume consistent treatment.
In the Law Mind corpus, compound interest surfaces most densely in tax law materials (particularly overpayment and underpayment interest, where federal statutes prescribe compounding), in commercial lending and mortgage materials, and in equity cases involving breach of fiduciary duty or unjust enrichment where courts sometimes awarded compound interest as an equitable remedy to strip a wrongdoer of gain. These equitable compound interest awards are doctrinally distinct from contractual compound interest and should be tracked separately.
The encyclopedia entry on Interest on Overpayments (tax_120) is a natural next stop for researchers tracing compound interest in tax contexts, where the statutory compounding rules operate quite differently from common law defaults.
Historical Dictionary Support
The three historical dictionary entries are in close agreement on the mechanical definition. Black's (both editions) and Bouvier define compound interest identically in substance: interest on a sum of money is added to the principal, and that combined total then bears further interest, making the former interest "a sort of secondary principal" in the phrasing Black's uses. That phrase — secondary principal — is analytically useful because it captures the structural logic of compounding: what was once interest is reclassified as principal for purposes of the next calculation cycle.
Bouvier's entry is the most legally pointed of the three, adding the normative judgment that compound interest is "not, in general, allowed." Black's (2nd Ed.) follows the same definition and supports it with case citations, including Camp v. Bates, 11 Conn. 487, and U.S. Mortgage Co. v. Sperry, 26 Fed. 730, which give researchers entry points into the nineteenth-century common law treatment.
What the historical dictionaries collectively miss is any treatment of the statutory transformation of compound interest law, particularly in tax and commercial lending contexts, which is where most modern compound interest disputes arise. Researchers relying solely on these sources would have an accurate picture of the historical common law baseline but an incomplete picture of modern practice.
Jurisdictional Note
The common law presumption against compound interest absent express agreement or statutory authority remains the baseline in many U.S. jurisdictions, but it is substantially modified by statute in others and in specific subject-matter areas. Federal tax law mandates compounding on both underpayments and overpayments. State consumer lending statutes often regulate compounding frequency as part of usury and disclosure regimes. Researchers should identify the governing statutory framework before assuming the common law default applies.
Encyclopedia Cross-Reference
The Law Mind Tax Encyclopedia: Interest on Overpayments (tax_120) — for compound interest in the federal tax context, where statutory compounding rules displace the common law default.