CO2 tolerance test

One full breath in, then out as slowly as you can. It gives you a rough sense of how calmly you handle a rising urge to breathe, with no breath-hold. It takes about a minute.

Take three calm breaths in and out through your nose. When you're ready, tap Start.

Breathe in through your nose, as full as is comfortable.

Tap anywhere when you're full, and start breathing out.

Now breathe out through your nose, as slowly as you can.

Keep it slow and steady. Tap Stop when you run out of air, need to swallow, have to breathe in, or your out-breath stops.

That was very short. Did you tap Stop by accident?

The timer stopped at three minutes. Did you forget to tap Stop?

Notice your breath now. Where do you feel it?

There's no validated scale for this test. It's a personal benchmark, and your trend over weeks says more than today's number.

What the number points to: CO2 tolerance and why breathlessness is rarely about oxygen.

Sit or lie down and breathe normally through your nose. The test takes about a minute.

About 1 minute

Breathe out slowly and stop when you run out. Don't force it, and don't hold your breath at the end. Never in water or while driving.

Turn on sound to do this with your eyes closed.

Space to stop · Esc to cancel

What is the CO2 tolerance test?

The CO2 tolerance test times how long you can breathe out, slowly and steadily, after one full breath in. As you breathe out, carbon dioxide builds up and the urge to breathe grows. The longer you can stay slow and calm through that, the higher your time.

It was popularised by the breathing coach Brian Mackenzie and his company, Shift, and became widely known through podcasts. It has no breath-hold, which makes it a gentler cousin of the BOLT score test.

How to do the CO2 tolerance test

Sit or lie down and breathe normally through your nose for a minute.
Take three calm breaths in and out through your nose.
Take one full breath in through your nose, as full as is comfortable.
Start the timer and breathe out through your nose as slowly and steadily as you can. (If your nose is blocked, breathe out through pursed lips and do it the same way each time.) On this page, you tap anywhere as the out-breath begins, and that starts the timer.
Stop the timer when you run out of air, need to swallow, or have to breathe in.

Keep the exhale smooth: a thin, steady stream, without stops and starts. Don’t hold your breath at the end to add time. If you feel dizzy, stop and breathe normally.

What does your time mean?

Treat it as a personal benchmark. You’ll find charts online that grade the test from “poor” under 20 seconds to “elite” over 80, with labels about stress and anxiety attached. Those bands trace back to coaching material and have no published research behind them, so this page doesn’t grade your time.

What the time does reflect is a mix of things. It’s shaped by how sensitive you are to carbon dioxide, how much air you took in, and how well you can control a slow exhale while the urge to breathe grows. Practising the test itself probably raises it too. That’s why the useful comparison is with your own earlier times, taken the same way.

The limits of the test

No validated scale. The popular bands have no published basis.
It measures breath control as much as CO2 tolerance. Lung size, technique, and practice all change the result.
It varies from day to day, with sleep, stress, and the time of day.
A better time doesn’t always mean lower sensitivity. In one study, two weeks of daily breath-holds lengthened people’s holds by almost half (opens in new tab) while their sensitivity to carbon dioxide stayed the same. Getting more comfortable with the urge is a real change, though a different one.

CO2 tolerance test or BOLT?

	CO2 tolerance test	BOLT score
What you do	One full breath in, then out as slowly as you can	A normal breath out, then pinch your nose until the first urge to breathe
Breath-hold?	None	A short, comfortable one
Takes	About 1 minute	About 2 minutes
What it’s said to reflect	CO2 tolerance and breath control	How sensitive you are to carbon dioxide

Both are rough benchmarks. Pick one and stay with it, so your trend means something. If breath-holds are uncomfortable for you, this test is the one to use.

What is CO2 tolerance?

CO2 tolerance is how much carbon dioxide your body lets build up before it pushes you to breathe. Your urge to breathe runs mainly on carbon dioxide, so a low tolerance can feel like breathlessness even when your oxygen is normal. Anxiety and habitual over-breathing tend to go with an urge that arrives early. The CO2 tolerance article explains the physiology.

How to improve your time

Breathe through your nose, at rest and on easy walks.
Slow down for five to ten minutes a day, to around six breaths a minute. You can use the free coherent breathing timer to set your rhythm.
Lengthen the exhale a little in your everyday practice, so the out-breath runs longer than the in-breath.
Test once a week at the same time of day and watch the direction over a month or two.

Is the CO2 tolerance test safe?

For nearly everyone, yes. There’s no breath-hold, and you stop when you run out of air.

Safety

Don't force the exhale, and don't hold your breath at the end.
Never test in water, near water, or while driving.
If you feel dizzy or unwell, stop and breathe normally.

More on this in breathwork safety and contraindications.

Note on AI use

I researched and drafted this page with AI assistance, then reviewed and edited it myself.

Frequently Asked Questions About the CO2 Tolerance Test

What is a good CO2 tolerance test score?

There's no validated answer. The bands online ("poor" under 20 seconds, "elite" over 80) come from coaching material, not research. Use your own trend: the same test, the same time of day, once a week.

Why does my time change?

Because it depends on your state that day (sleep, stress, food, the time of day) and on how full your breath in was and how smoothly you breathed out. A few seconds either way is normal.

CO2 tolerance test or BOLT: which should I use?

Either works as a personal benchmark. The CO2 tolerance test has no breath-hold, so it suits people who find holds uncomfortable. BOLT is older and more widely used. Pick one and stay with it.

How often should I test?

Once a week is plenty. Daily results swing with sleep and stress, so a weekly test at the same time of day shows the direction more clearly.

Is this the CO2 tolerance test from the Huberman podcast?

It's the same slow-exhale test, popularised by Brian Mackenzie and discussed on health podcasts, Huberman Lab among them. Details vary slightly between versions (how many breaths first, nose or mouth), so use the same steps each time.