CO2 Baseline Test
Hold your breath and stop at the first urge to breathe, not at your limit. The time you get is your CO2 baseline: the point where discomfort starts. It sizes an equanimity table and it moves with training in a way a maximum attempt does not. Dry land only.
Press start when you are ready
Start from a preset
Two named protocols use this timer with a different instruction and their own published bands.
- CO2 tolerance test — times a slow controlled exhale after a full inhale, with the Huberman Lab bands
- BOLT score test — the hold after a normal exhale, with Patrick McKeown's published bands
What counts as the first signal
A breath hold has two parts. In the first you are comfortable and nothing much happens. Then carbon dioxide rises far enough that your body asks for a breath: an involuntary swallow, a twitch in the throat, or the first contraction of the diaphragm. That moment is the signal. Everything after it is the struggle phase.
Stop the timer at the first one you notice. Not the strongest, not the third — the first. Most people are somewhere between 20 and 60 seconds when they start, and the number is meant to feel unimpressive.
How to use this test
- Sit or lie down on dry land, away from water. Same room, same position, same time of day if you can manage it.
- Breathe calmly and normally for a couple of minutes. Do not use rapid or repeated deep breaths: they lower carbon dioxide and delay the very signal you are measuring.
- Take one comfortable full breath, low to high, and start the hold.
- Stay still and put your attention on the sensations rather than the clock. Do not anticipate the signal; wait for it.
- Stop at the first urge you notice, and end the hold there. This test does not run toward a maximum.
There is no published norm for this number and it is a subjective marker, so a target would be misleading. Compare it only with your own earlier readings. Once a week under the same conditions is enough to see a trend without turning it into another maximal test.
Why not just test your maximum
A maximum attempt measures how long you can tolerate the struggle phase, which depends heavily on how motivated you feel that day. Two tests a week apart can differ by 30 seconds for reasons that have nothing to do with fitness. The first signal is quieter and steadier, so a change in it is more likely to be a real change in you.
It is also the safer of the two to run alone, because you stop before the part where things get hard. If you want a maximum for CO2 or O2 tables, the breath hold timer does that separately. Two named versions of this measurement have their own published bands: the BOLT score test and the CO2 tolerance test, which times an exhale rather than a hold.
What to do with the number
An equanimity table repeats the same hold ten times, sized from this baseline rather than from a maximum. The point is not to go further each round. It is to arrive at the same discomfort ten times and stay relaxed through it, which is the skill that transfers to a dive.
Feed it straight into the equanimity table generator, which reads whatever you saved here. Retest every few weeks, under the same conditions — same time of day, same position, same rest beforehand — or you are comparing noise.
Getting a clean measurement
- Breathe normally for two minutes first. The 30-second preparation phase is for settling, not for loading up on air.
- One full breath, not a forced one. Fill your lungs comfortably and start the hold.
- Do not test after coffee, a hard workout, or a meal. All three move the number.
- Test once per session. Repeat attempts on the same day drift longer because CO2 is already elevated, which flatters the result.