Word Differences
Principal vs. Principle: Person, Sum or Rule?
Principal is usually a person, a sum of money or “main.” Principle is always a rule or belief. Here’s how to keep them apart.
By Vocabaura Editorial · Published 13 August 2026 · 3 min read
Quick answer
Principal means main or most important, and also names a head of school or an original sum of money. Principle is a noun only, meaning a rule, standard or fundamental truth.
This pair matters in professional writing, where both words appear regularly, a principal investigator works on a project, while a principle governs how it’s run. Getting it wrong reads as careless in exactly the contexts where care counts.
Quick Answer
Principle is always a noun and always abstract: a rule, a belief, a law of nature. Principal is usually an adjective meaning chief or main, and as a noun refers to a person in charge or the original amount of a loan.
Principal (adjective)
The principal reason for the delay was weather.
Principal (person)
The principal met with the new teachers.
Principal (money)
Your payments cover interest before principal.
Principle (rule)
She refused on principle.
Principle (law)
The principle of conservation of energy.
Quick check
Which word completes this sentence?
“The ______ reason for the delay was weather.”
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Common Mistakes
- “A man of principals” should be “a man of principles.”
- “Principle investigator” should be “principal investigator.”
- “In principal” when you mean “in principle” (that is, in theory).
- Treating principle as an adjective, it never is.