Why am I out of breath swimming when I can run?

Almost always because you are holding your breath with your face in the water rather than exhaling continuously. That retains carbon dioxide, and the air hunger that follows arrives within a few strokes.

Should I breathe out through my nose or mouth?

Either, or both — what matters is that it is continuous and complete while your face is in. Many swimmers find nose-only exhale more comfortable and less likely to let water in.

Is hypoxic training useful?

Not enough to justify the risk. Breath-hold swimming can cause blackout without warning, particularly after hyperventilating, and drowning is the failure mode. Train the exhale instead.

Out of breath swimming: the exhale is where it goes wrong

Twenty-five metres and you are hanging off the wall. You can run for half an hour. In the water your chest is bursting by the third breath and it makes no sense.

Do this now

Fix the exhale, in the shallow end

3 minutes

Stand in chest-deep water. Put your face in and breathe out steadily through the nose and mouth, all the way, for three or four seconds.
Lift the face just clear, take one quick breath in, and put it straight back in. The in-breath should be the short part.
Twenty repetitions. The rule: air is leaving your body whenever your face is in the water.
Then swim one length thinking only about a continuous exhale, not about the stroke.
Never practise breath holding alone, and never underwater. That rule has no exceptions.

Run it with a timer

The problem is retained carbon dioxide

New swimmers hold their breath with their face in the water and try to exhale and inhale in the brief window when the mouth is clear. There is not enough time, so CO₂ accumulates breath by breath. The resulting air hunger feels like a lack of oxygen and is actually an excess of carbon dioxide — which is why the fix is a long continuous exhale underwater and a short sharp inhale above it, the reverse of what most people do instinctively.

Why the water makes it harder

Hydrostatic pressure on the chest makes inhalation slightly more work, and horizontal immersion shifts blood centrally. Add a face in cold water, which triggers the diving response, and the first lengths of any session are genuinely harder than the equivalent effort on land. It settles within a few minutes, which is why the first hundred metres should be slow.

Why not to do hypoxic training

Deliberate breath-hold swimming and repeated hypoxic sets carry a real risk of shallow-water blackout: hyperventilating beforehand lowers CO₂ and removes the urge to breathe, so loss of consciousness can arrive without warning while the face is in the water. Several national bodies and lifeguard organisations advise against prolonged breath-hold training for exactly this reason. There is no training benefit that justifies it.

What changes it, over a week

Every session, first five minutes
Standing exhale drill before you swim. The exhale is the skill; the stroke is downstream of it.
Every length
Continuous exhale with the face in, short inhale when it clears. Bilateral breathing every three strokes if you can.
First 100 metres
Deliberately slow. The immersion response settles and the session is different afterwards.
Never
No breath-hold sets, no underwater distance swimming, no hyperventilating before a hold, and never alone.

When this is not a breathing pattern

See a clinician — promptly, and before assuming any of this applies to you — if any of these describe you:

Any episode of feeling faint or losing awareness in water — stop swimming and get assessed before returning
Coughing, wheeze or breathlessness after swimming, which can indicate exercise-induced asthma or chlorine-related airway irritation
Chest pain or palpitations in the water
Breathlessness on exertion out of the water too
Any cardiac condition or epilepsy — swim with someone who knows, and take medical advice about open water

This page is education, not medical advice, and no part of it is a diagnosis. Full disclaimer.

Questions people ask

Why am I out of breath swimming when I can run?: Almost always because you are holding your breath with your face in the water rather than exhaling continuously. That retains carbon dioxide, and the air hunger that follows arrives within a few strokes.
Should I breathe out through my nose or mouth?: Either, or both — what matters is that it is continuous and complete while your face is in. Many swimmers find nose-only exhale more comfortable and less likely to let water in.
Is hypoxic training useful?: Not enough to justify the risk. Breath-hold swimming can cause blackout without warning, particularly after hyperventilating, and drowning is the failure mode. Train the exhale instead.

What this page is built on

— Craig, 1976, JAMA, and subsequent work — hyperventilation, breath-hold swimming and underwater blackout
— US Centers for Disease Control and lifesaving-body guidance advising against prolonged breath-hold training in pools
— Held & Pendergast, 2014, European Journal of Applied Physiology — respiratory mechanics during immersion

Named by author and year. The DOIs live in the journal.

Put a number on it

Most of what is on this page tracks one measurement: the Control Pause. It takes sixty seconds, needs no account, and it is the thing that tells you whether any of this is working.