Same shape, different stopping rule
Both tests measure how long you can hold your breath after a normal exhale. The difference is when you stop the clock. BOLT stops at the first definite urge to breathe — the first involuntary twitch, swallow, or throat spasm. The CO₂ tolerance test stops when the hold becomes actively uncomfortable, which is later.
Why the stopping rule matters
BOLT is more repeatable across days because the "first urge" signal is chemoreceptor-driven — it does not depend much on mental state. The "point of discomfort" is easier to override with will, which makes the CO₂ test more variable session-to-session.
Which one to trust
For training decisions and long-term tracking, BOLT. Auralize uses your BOLT to scale box breathing interval and to gate the CO₂ Capacity Builder program specifically because of the stricter rule. For a quick home introduction, the CO₂ tolerance test is a fine starting point.
The mapping
Expect your CO₂ tolerance test score to run 5–10 seconds longer than your BOLT, in the same direction. As you train, both rise. If you switch between them, do not compare the raw numbers — compare each to its own history.