My cat purrs loudly for very long stretches — does purring tire her out?

There is no good reason to think it is tiring, and the mechanism is the reason why.

The exact process is still not fully settled, but the leading account is that purring involves rapid oscillation of the muscles controlling the larynx, which modulates airflow during both inhalation and exhalation. Critically, it appears to be an almost passive addition to breathing that is happening anyway, rather than a separate effortful act.

That matches the observed behaviour. Cats purr while falling asleep, while asleep, while eating, while being handled — none of which are compatible with an activity that demands real exertion. An animal does not do something tiring continuously while dozing.

So the honest answer is: as far as anyone can tell, it costs very little, and the fact that cats can sustain it for long periods is itself decent evidence.

One important nuance for anybody reading this and drawing conclusions about their own cat: purring is not only a sign of contentment.

Cats also purr when stressed, injured, unwell, giving birth, and sometimes when dying. This surprises people badly, and it matters because a purring cat at a veterinary practice is not necessarily a comfortable one. We see it constantly.

The usual interpretation is that purring is a self-soothing behaviour as well as a social one — there is a well-known hypothesis that the frequencies involved may have some restorative effect, though that is a hypothesis rather than an established fact.

So read it in context. Purring while relaxed, kneading, half-asleep on your lap, is contentment. Purring while hiding, hunched, off food or reluctant to move is a reason to phone somebody.

On your cat being loud specifically: volume varies enormously between individuals and it does not mean anything in particular.

It is largely anatomy and individual variation, in the same way some people have carrying voices. A very loud purr from a relaxed cat is not a signal of anything beyond a relaxed cat.

The more interesting observation in your question is that she is not generally affectionate but purrs hard when she does seek attention. That pattern is common and it is worth taking at face value — cats that solicit contact on their own terms often engage very intensely when they do. Letting her set the timing is usually what maintains it.

One thing genuinely worth noting: a change in purring pattern can be information.

A cat that suddenly purrs far more than usual, or one that stops purring in situations where she always did, is showing a change in behaviour — and in cats, behaviour changes are often the first and sometimes the only sign that something is wrong. They conceal discomfort extremely well.

Not a reason to worry about a loud purr. A reason to notice if it changes.