Three that actually help, and when to use each
When anxiety rises, your breath gets fast and shallow before you notice it. That is useful to know, because it also means the breath is one of the few levers you can pull on purpose. There is no single "best" breathing exercise. There are three that cover almost every situation, and the trick is matching the pattern to the moment.
All three take about a minute. None of them needs a mat, an app, or a quiet room.
When anxiety spikes, you want the fastest lever. Breathe in for a count of four, hold gently for two, and breathe out for six. The long exhale is the part that matters: your heart naturally slows on the out-breath, so making the exhale longer than the inhale spends more of each cycle calming you down. Two or three rounds is usually enough to take the edge off a racing feeling.
When you are not panicking but you are tense and braced, box breathing gives your mind something even to hold on to. Breathe in for four, hold for four, out for four, hold for four. The equal counts make it easy to remember and hard to rush. This is the one to use before something you are dreading, because it settles you without dropping your alertness.
When your mind will not slow down, try two quick inhales through the nose, the second one short and on top of the first, then one long slow exhale through the mouth. That double inhale is what your body does on its own when it is overwhelmed, and doing it deliberately tends to release a held feeling in the chest. One or two of these is often enough.
The exhale should be longer than the inhale, and nothing should be forced. If you strain or get light-headed, shorten the counts. A gentler pattern you will actually use beats a perfect one you abandon. Anxiety is not a discipline problem, so this is not a test to pass.
Breathing exercises lower the volume of anxious moments. They do not treat an anxiety disorder, and they are not a substitute for real help if anxiety is running your life rather than visiting it. Think of them as turning the dial down so you can think clearly, not as making the problem disappear.
If you would rather not count in your head, Peakora has a breathing ring that paces the pattern for you: it expands on the inhale, holds, and contracts on the exhale, so you follow the shape instead of the numbers. It is free, and it takes about a minute to try.