Most written breathing instructions specify one or the other, usually without saying why, and usually as though it were the important part.
What is actually different is narrower than that, and knowing which part is mechanical makes the instructions easier to follow — and easier to ignore where they do not matter.
The nasal airway is much narrower than the mouth, so air moving through it meets more resistance. That is the core difference and most of the rest follows from it.
More resistance means a slower breath for the same effort. It is harder to take a fast, large breath through the nose than through the mouth, which is why nasal breathing tends to pace itself and mouth breathing does not.
The nose also warms and humidifies the air on the way in, which the mouth does much less of. That is a description of the plumbing, not an argument for either one.
Because the resistance does some of the pacing for you. An instruction to take four seconds to breathe in is much easier to follow through a narrow airway that will not let you rush it than through a wide one that will.
This is why the in-breath is the half that is almost always specified. If you are following a four-count and you finish in two, the count has stopped being a guide — and through the mouth that is easy to do without noticing.
4-7-8 is the pattern on this site that names it: out through the mouth, lips slightly parted. The exhale there is the longest phase of the three, and letting it out through a wider opening makes an eight-count exhale a controlled release rather than a squeeze.
Box breathing, resonance and extended exhale do not specify the exhale at all. In those the out-breath is either the same length as the in-breath or only moderately longer, so there is nothing that needs the extra room.
A timer cannot tell which you are doing. It marks the phases and their lengths, and everything about how you move the air is yours.
If your nose is blocked, breathe through your mouth and keep the counts — a pattern followed through the mouth is far closer to the intent than a pattern abandoned because one instruction could not be met.
And if a four-count through the nose leaves you short of air, the count is too long rather than the route being wrong. Set a longer pace where the pattern allows one.
The timer below is the app, running in this page. It opens on 4-7-8 — in through the nose for four, hold for seven, out through the mouth for eight.
The pattern In 4 s · Hold 7 s · Out 8 s
Most patterns specify the in-breath through the nose, because the narrower airway paces it for you. The out-breath is only specified where it is unusually long, as in 4-7-8.
Its exhale is eight counts, the longest phase of the three. A wider opening makes that a controlled release rather than something squeezed out.
Breathe through your mouth and keep the counts. Following the pattern matters more than following the route.
No. It marks the phases and their lengths; how you move the air is entirely yours.
It is a wider, faster route, which makes a slow count harder to hold to. That is a practical difference for following a pattern, and this page is not the place to look for anything more than that.
On your phone it runs with sound and haptics, and it works with no connection at all.