Definition
Simple interest is interest calculated solely on the original principal amount, without any interest being added to the principal and then earning further interest. The formula is straightforward: interest accrues as a fixed percentage of the original sum for each period, and those accumulations do not themselves generate additional interest.
In contrast to compound interest, where unpaid interest is periodically folded into the principal base, simple interest keeps the calculation flat. A loan of $1,000 at 5% simple interest for three years produces $150 in interest (5% × $1,000 × 3 years), never more, regardless of how long the obligation runs or whether interim interest goes unpaid.
In legal contexts, simple interest appears most often in:
- Judgment interest and post-judgment calculations, where statutes frequently specify simple interest on unpaid judgments
- Prejudgment interest awards, where courts must determine the applicable rate and method
- Contract disputes, where the agreed rate of return on a debt is characterized as simple or compound
- Statutory interest obligations, where legislatures default to simple interest unless compound interest is expressly authorized
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Common Language
Modern common usage (Wiktionary): Interest paid only on the principal.
Historical common usage (Webster's 1913): Not separately defined; subsumed under general definitions of interest as a charge for the use of money.
The common and legal definitions are essentially identical here — both describe interest computed only on principal. The legal significance lies not in a gap between common and legal meaning, but in the legal consequences that attach to the characterization: courts and legislatures treat simple versus compound interest as a binary legal choice with measurable monetary consequences, and the default rule in most legal settings favors simple interest absent express contrary authority.
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Common Confusion
Simple interest is frequently confused with compound interest in legal documents, and the distinction matters substantially over time. A judgment bearing 6% simple interest for ten years produces a predictable, linear accumulation. The same judgment at 6% compounded annually produces a meaningfully larger sum. Parties drafting settlement agreements, promissory notes, or consent decrees who fail to specify the method may find courts defaulting to simple interest even when compound was intended — or vice versa in jurisdictions where compound interest is disfavored without explicit authorization. The word "interest" alone, without qualification, does not legally resolve which method applies.
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Why It Matters in Research
The primary research trap with simple interest is assuming the default rule is uniform. It is not. Most American jurisdictions default to simple interest for judgment and statutory interest, but the authority for that default varies — some states embed it in a general interest statute, others in civil procedure rules, others in case law alone. A researcher working across jurisdictions must locate the specific authorizing provision rather than assuming simple interest is the universal baseline.
Historical legal materials through the nineteenth and early twentieth centuries often treat simple interest as so obviously the default that they address it only obliquely, in the course of explaining compound interest as the departure requiring special justification. This means that if you are researching pre-twentieth-century sources, you may find very little direct discussion of simple interest as a standalone concept — it was assumed.
For prejudgment interest research, the characterization as simple or compound intersects with questions of whether interest is compensatory or punitive, and whether statutory caps or rate schedules govern. These doctrines are distinct and should be researched separately.
The encyclopedia cross-reference to fee simple (property_2) warrants a brief caution: the word "simple" in "fee simple" is entirely unrelated to interest calculations. It derives from the Latin sine conditione and means unconditional or absolute. Researchers new to property law should not allow that usage to create confusion in a finance or contracts research context.
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Historical Dictionary Support
Black's Law Dictionary defines simple interest as "interest computed merely upon the principal sum due, and not compounded." This is accurate and has remained stable across editions — simple interest is one of the few legal terms that has not meaningfully shifted in definition over time. Black's entry is deliberately brief because the concept is not contested; its legal significance arises in application rather than definition.
The historical dictionaries uniformly treat this as a derivative concept, defined by contrast to compound interest rather than on its own terms. This is editorially consistent but means historical sources provide limited positive guidance. No major treatise has substantially expanded on the definition itself; the doctrinal work appears in case law applying the concept to particular statutory schemes and contractual contexts.
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Jurisdictional Note
Most U.S. jurisdictions apply simple interest to statutory judgment rates and prejudgment interest unless compound interest is expressly authorized by contract or statute. Federal courts applying state law follow state interest rules, while federal judgment interest is governed by 28 U.S.C. § 1961, which specifies simple interest calculated from the date of judgment. International commercial contracts and some civil law jurisdictions may apply different default rules.
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