Principal vs Principle: What Is the Difference?
Principal means main or most important, and is also the head of a school or a sum of money. Principle means a basic rule, belief or truth. The principal reason for the policy is a principle of fairness.
Updated 21 October 2026 · EssayWriting.com editorial team
- Principal means main, or the head of a school or a loan sum.
- Principle is a rule or belief and is always a noun.
- Trick: a principle is a rule (both end in -le).
Principal vs principle: the difference at a glance
| Word | Part of speech | Meaning | Example |
|---|---|---|---|
| principal | adjective | main, most important | The principal aim of this study is… |
| principal | noun | head of a school; original sum of a loan | The principal met parents. Interest on the principal. |
| principle | noun only | rule, belief, law | The principle of informed consent. |
How to use principal
Use principal as an adjective (main) or for a person or sum of money in charge or at the centre.
- The principal findings are summarised in Table 2.
- The school principal approved the trip.
- Repay the principal plus interest.
How to use principle
Principle is always a noun meaning a rule or belief.
- The principles of academic integrity.
- In principle, the plan works.
- She refused on principle.
- Archimedes’ principle.
How to remember the difference
The principal is your pal (the head of school). A principle is a rule (both end in le).
Tricky cases and exceptions
"In principle" means in theory. "Principled" means acting according to moral rules. "Principal component analysis" uses principal (main components), not principle. A principal investigator leads a research project.
Principal vs principle in academic writing
In research papers, "principal" appears in "principal investigator", "principal component analysis" and "the principal aim". "Principle" appears when discussing ethics, theory and laws of nature. Proofread both carefully, since a spell checker will not flag the wrong one.
Practice quiz
Choose the correct option, then open each answer to check your reasoning.
1. The ___ goal of this essay is to compare two theories. (principal / principle)
principal: main.
2. Equality is a core ___ of democracy. (principal / principle)
principle: a belief.
3. ___ component analysis reduced the variables. (Principal / Principle)
Principal.
4. In ___, the method is sound. (principal / principle)
principle: in theory.
Related guides
Frequently asked questions
Is it "in principle" or "in principal"?
In principle.
Is it principal investigator or principle investigator?
Principal investigator.
Can principle be an adjective?
No. Only principal is an adjective.
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