IOU vs Promissory Note (2026): 7 Differences That Matter When Someone Owes You Money

Here are the seven differences that matter most.

1. Legal enforceability

A promissory note satisfies the requirements of Article 3 of the Uniform Commercial Code (UCC), which all 50 states have adopted in some form. To qualify under UCC § 3-104, the instrument must be an unconditional promise to pay a fixed amount of money, payable on demand or at a definite time, to a specific payee or to bearer. When those elements are present, the note is a negotiable instrument — a court can enforce it without requiring extensive proof of the underlying agreement.

An IOU carries no such statutory status. Courts in most states treat it as evidence of a debt, not a contract in itself. The creditor typically has to prove the original transaction, the terms, and the amount owed. That extra burden is manageable for small claims, but it becomes a real problem when the debtor disputes the terms.

2. Required elements

A valid promissory note must include: the principal amount, the interest rate (or a clear statement that no interest applies), the repayment schedule or due date, the identities of both parties, and a signature from the borrower. Some states layer additional requirements on top of the UCC baseline — such as specifying the place of payment — so checking local commercial law before finalizing the instrument is worthwhile.

An IOU has no prescribed form. "I owe John Smith $500, signed Jane Doe" is technically an IOU. That flexibility is also its weakness: ambiguous terms invite disputes. If the amount, repayment date, and interest rate are not spelled out, a court may refuse to read them in.

3. Interest clauses

Promissory notes can carry interest — and that interest is enforceable. Most states set a legal maximum rate through usury statutes. New York, for instance, caps personal loans between individuals at 16% per year under General Obligations Law § 5-501. California caps at 10% on non-exempt personal loans under Article XV of its Constitution. Exceed those caps and a court can void the interest portion of the note entirely, sometimes the entire instrument.

IOUs can reference interest, but an informal document rarely includes the detail needed to enforce it. If you just write "plus interest," you will likely get the default statutory rate — or nothing at all — rather than the rate you intended.

4. Notarization and witnessing

Neither an IOU nor a promissory note requires notarization to be valid in most U.S. jurisdictions. However, notarization matters for enforcement. A notarized promissory note is harder to challenge on the grounds of fraud or forged signature. Some states — Louisiana being the most notable exception, where certain commercial instruments must follow Civil Code formalities — have specific requirements, but for standard personal and business loans, a signature is enough.

If you want maximum protection, notarize. If you cannot get to a notary, two disinterested witnesses signing alongside the borrower substantially strengthens any document, IOU or otherwise.

5. Transferability

A properly drafted promissory note can be transferred to a third party. Under UCC Article 3, a "holder in due course" who takes the note in good faith and for value acquires enforceable rights even if the original borrower had defenses against the original lender. This matters for businesses that factor receivables, sell loan portfolios, or use notes as collateral.

An IOU is not a negotiable instrument. Assigning it to someone else may work contractually, but the assignee takes it subject to all defenses the borrower could raise against the original creditor — the holder-in-due-course protection disappears.

6. Court admissibility and burden of proof

In small claims court — the venue where most personal debt disputes land — both documents can be introduced as evidence. The difference is what happens next. A signed promissory note that complies with Article 3 shifts the burden: once the creditor produces the note and proves the borrower's signature, the borrower must affirmatively prove why it should not be enforced (payment, fraud, illegality, etc.).

With an IOU, the creditor typically carries the burden of proving the full transaction: the loan was made, the terms were agreed to, and the borrower has not paid. If your IOU is vague — no repayment date, no stated amount beyond a round number, no mention of interest — you may win anyway in small claims, but the path is harder and more contested.

7. What happens if the borrower disappears

A promissory note functions as a standalone document. If you need to sue, you file it with the court, prove the signature, and the note speaks for itself on the material terms. Collecting on a judgment is another matter (wages, bank accounts, liens on property), but the liability piece is clean.

With an IOU, if the borrower disputes the underlying transaction — claims it was a gift, argues the amount was different, says it was already repaid — you are back to testimonial evidence and whatever supporting records you kept: texts, bank transfers, emails. None of that is impossible to prove, but it costs more time and turns a clear case into a credibility contest.

Which one should you use?

For anything over $500, or any loan between people who are not immediate family members living under the same roof, use a promissory note. The additional formality takes ten minutes and can save months of litigation headaches.

IOUs still have a place. For very small amounts — splitting a dinner tab, covering a friend's parking meter — an IOU creates a paper trail without turning a minor transaction into a legal procedure. Some people also use them as a first step: the IOU confirms the debt exists while both parties arrange time to sign a proper note.

A well-drafted promissory note covers the elements courts look for: principal, rate, schedule, and a clear signature block for the borrower. Getting those terms right the first time is far cheaper than reconstructing them later from memory in front of a judge.

The statute of limitations question

Even a perfect promissory note expires. Each state sets its own limitations period for written contracts. In Texas, a promissory note that qualifies as a negotiable instrument is subject to a six-year limit under Business & Commerce Code § 3.118; if the note does not qualify as a negotiable instrument, the four-year period under Civil Practice & Remedies Code § 16.004 applies instead. In New York, the limit is six years for written contracts under CPLR § 213. In California, the limit is four years for written contracts under Code of Civil Procedure § 337. Miss the deadline and you lose the right to sue, regardless of how well-documented the debt is.

IOUs, as oral or informal agreements, often fall under shorter limitations periods for unwritten contracts — typically two to four years depending on the state. One more reason the more formal instrument protects you better over time.

A note on cosigners and collateral

Promissory notes accommodate secured and cosigned arrangements in ways IOUs do not. A secured note can attach collateral — a vehicle, equipment, a deposit — and if properly perfected under UCC Article 9 (for personal property), gives the lender priority over other creditors in the event of default. A cosigner on a promissory note creates joint and several liability, meaning you can pursue either party for the full amount.

An IOU with a reference to collateral or a cosigner is not unenforceable, but the legal machinery for actually claiming that collateral or pursuing the cosigner requires additional documentation. Start with the right instrument and you avoid retrofitting security interests later.

Bottom line

The practical gap between an IOU and a promissory note is not about formality for its own sake. It is about what happens when the relationship goes wrong. Courts enforce promissory notes as instruments. Courts weigh IOUs as evidence. For any loan you actually expect to be repaid, the note is the right tool.