Complex describes something made of many interconnected parts, where the connections matter: "a complex ecosystem." Complicated describes something difficult to understand or handle, often unnecessarily so: "a complicated tax return." Complex is largely neutral and structural; complicated implies effort or frustration for whoever has to deal with it.
The core difference
Complex is a description of structure. Complicated is a description of experience. A thing is complex because of what it is made of; it is complicated because of what it does to you.
Be aware, though, that in ordinary conversation these words overlap heavily, and using one where a specialist would use the other is not an error. The distinction below is a useful analytical one, drawn on in systems thinking, engineering and design — it is not a rule of grammar, and no reader will think you illiterate for saying "a complex form."
Using complex
Reach for complex when the parts are genuinely interdependent and cannot be understood in isolation.
- Migration is a complex phenomenon with economic, political and personal drivers.
- She has a complex relationship with her home town.
- The protein folds into a complex three-dimensional shape.
Using complicated
Reach for complicated when the difficulty is the point — when something has more steps, conditions or exceptions than it needs.
- The booking process is unnecessarily complicated.
- "It's complicated" — the standard English evasion about a relationship.
- Recovery was slower than expected because the surgery was complicated by an infection.
The test
Ask whether the difficulty could be designed away. If a clever redesign could make the thing simple without changing what it fundamentally is, it is complicated — a tax code, an assembly manual, a form. If stripping out the intricacy would destroy the thing itself, it is complex — a rainforest, a language, a city.
A second, quicker check: complicated usually invites a sigh. If the word carries an implied "and it shouldn't be," complicated is your word.
The edge case
Complicated has a live verb behind it, and complex does not. You can complicate matters, and a wound or a pregnancy can be complicated by something else — a medical usage that means "made worse by an additional problem." There is no verb to complex.
Complex also works as a noun in ways complicated cannot: a sports complex, a housing complex, an inferiority complex, a chemical complex. Only the adjectives are in competition.
One last note on stress: as an adjective, complex takes stress on either syllable depending on dialect, but the noun is always COM-plex.
Frequently Asked Questions
Is it wrong to say 'complicated' when I mean many parts?
No. In everyday English the two words are used interchangeably and nobody will misunderstand you. The distinction only earns its keep in technical writing, where calling a system complex rather than complicated signals that its difficulty is irreducible.
Which word should I use in a job application?
Complex, almost always. "I managed a complex project" reads as intellectually demanding; "I managed a complicated project" hints that the project was a mess. Complex flatters, complicated complains.