They are near-synonyms and interchangeable in most sentences. The slight lean: reliable emphasises consistent performance — it works the same way every time. Dependable emphasises being there when needed, and is used of people slightly more readily than of machines. A reliable engine starts every time; a dependable friend turns up.
The core difference
Small enough that no reader will object either way, but there is a tendency worth knowing.
- reliable — performs consistently: a reliable car, reliable data, a reliable witness.
- dependable — can be counted on when it matters: a dependable colleague.
Where they separate
Reliable extends comfortably to information and sources — a reliable estimate, a reliable account — where dependable sounds odd. That is the clearest gap between them: reliability can describe accuracy, dependability really only describes availability.
The roots
Rely comes through Old French from Latin religare, to bind fast. Depend comes from dependere, to hang from. One binds, the other hangs — which fits the modern difference better than it has any right to.
Choosing
For data, sources or measurements, use reliable. For people and their showing up, either works and dependable is slightly warmer.
Frequently asked questions
Are reliable and dependable interchangeable?
In most sentences, yes. The exception is information: a reliable source is standard, while a dependable source sounds wrong.
Which is better for describing a person?
Either. Dependable is slightly warmer and emphasises being there when needed; reliable emphasises consistency.
Can data be dependable?
It is unusual. Reliable is the standard word for data, estimates and sources, because it covers accuracy as well as consistency.