Ability is what someone can already do: "she has the ability to read music". Capability is the capacity to do it, whether or not it is being used — often about potential, systems or organisations: "the plant has the capability to double output". Ability leans personal and demonstrated; capability leans latent and technical.
The core difference
Both describe what is possible, but they point at different moments. Ability is proven and usually personal. Capability is potential, and it attaches as easily to a machine, a system or a company as to a person.
- He has the ability to speak four languages. — he does it
- The software has the capability to export to PDF. — the potential is built in, used or not
The test
Ask whether you are describing something demonstrated or something latent. If the person has actually shown it, ability is the natural word. If you are describing what could be done given the right circumstances, capability fits better.
A second clue is the subject. People have abilities. Systems, organisations, equipment and armed forces have capabilities — which is why the word is so common in technical, military and corporate writing.
Why capability sounds corporate
Capability has been worn thin by business writing, where it often appears simply because it is longer: "our capability to deliver" where "we can deliver" would do. If the sentence is about a person doing something they already do, ability is almost always the clearer choice, and often no noun is needed at all.
The related words
- capacity — how much, rather than whether: seating capacity, capacity to endure
- capable — the adjective for both senses: "capable of running the department"
- aptitude — a natural tendency to become good at something, before any training
Frequently asked questions
Are ability and capability interchangeable?
Often, but not always. Ability suits a person doing something they already do; capability suits latent potential and non-human subjects like systems, equipment or organisations.
Which word for a machine or software?
Capability. Machines and systems have capabilities; people have abilities.
What is the difference between capability and capacity?
Capability is whether something can be done; capacity is how much can be done or held — seating capacity, production capacity.