Disappointed describes the person who feels let down — "I was disappointed by the exam results". Disappointing describes the thing doing the letting down — "the exam results were disappointing". Feelings take -ed; causes take -ing.
The core difference
This pair belongs to a whole family of -ed/-ing adjective pairs in English: bored/boring, interested/interesting, frightened/frightening, confused/confusing. The pattern is consistent across the family. The -ed form says how someone feels. The -ing form says what produces that feeling.
Using disappointed
Only something capable of feeling can be disappointed — a person, a group of people, sometimes an animal.
- My parents were disappointed when I dropped out.
- She looked disappointed but said nothing.
- I'm disappointed in him, and disappointed with the outcome.
Note the prepositions: disappointed in a person's character, disappointed with or by a result, disappointed at a piece of news, disappointed that something happened.
Using disappointing
Things, events and performances are disappointing:
- The film was disappointing after all the reviews.
- A disappointing turnout — fewer than thirty people came.
- It is disappointing that no one wrote back.
The test
Ask which side of the feeling the subject is on. If the subject is having the feeling, use disappointed. If the subject is causing it in someone else, use disappointing. Something that cannot feel at all — a match, a meal, a quarter's sales — can only be the cause, so it is always disappointing. People can be either: "the fans were disappointed" (they felt it) but "the new striker has been disappointing" (he caused it).
A second version of the test: try substituting "let down" for -ed and "underwhelming" for -ing. "I was let down" ✓ / "the sales were underwhelming" ✓.
The mistake that changes your meaning
Saying "I am disappointing" is grammatical, but it means other people are let down by me — a confession, not a complaint. Learners who mean "I am unhappy about this" and write "I am very disappointing" accidentally apologise for themselves. The same trap catches "I am boring" (you send others to sleep) versus "I am bored".
The edge case worth knowing
The -ed form can attach to something that isn't a person but expresses a person's feeling: a disappointed sigh, a disappointed expression, disappointed hopes. A sigh cannot feel, but it belongs to someone who does, so the -ed form is correct and idiomatic. Similarly, disappointed of survives in formal phrases about expectations that came to nothing.
Both in one sentence
The clearest way to fix the pattern in your head is to write it out both ways: "The disappointing result left the disappointed players sitting on the pitch." The result caused it; the players felt it.
Frequently Asked Questions
Should I say "I'm disappointed" or "I'm disappointing"?
"I'm disappointed" if you feel let down. "I'm disappointing" means you let other people down — a very different statement, and almost never what someone intends to say.
Is it "disappointed in" or "disappointed with"?
Both are correct but idiomatic in different places: disappointed in a person (their behaviour or character), disappointed with a thing or result. "Disappointed by" works for either.
Can a person be described as disappointing?
Yes, but it judges them from outside: "a disappointing signing for the club" means they failed to meet expectations. It is not a description of their feelings.