Your BOLT score is the time from the end of a normal exhale until the first real urge to breathe — the first involuntary swallow or diaphragm twitch. It is a quick, repeatable read on your CO₂ tolerance and your everyday breathing habit. What it is not is a fitness test, a performance predictor, or a number with a scientifically fixed “ideal”. Track your own trend and it tells you something honest. Read it as a verdict on your health and it tells you a story.
Key takeaways
- BOLT measures comfort, not effort: it ends at the first urge to breathe, not at your limit.
- The hypercapnic ventilatory response is how strongly your breathing reacts to rising CO₂, and a short BOLT genuinely indexes a sensitive one (Stanley et al., 1975).
- It does not predict athletic performance. In 49 highly-trained speed skaters, BOLT correlated with nothing — VO₂max, peak power or exercise-test output (Kowalski et al., 2024).
- The famous 40-second ideal has no controlled study behind it. Use your own trend across weeks instead.
What BOLT actually measures — and how it differs from a max hold
BOLT stands for Body Oxygen Level Test, which is already a misnomer: it has almost nothing to do with oxygen. It is a comfort test, and it stops at the first signal, not the last.
That single detail is what separates it from a baseline hold. The two are easy to confuse, so it is worth being precise:
| Baseline hold | BOLT score | |
|---|---|---|
| Start | One comfortable inhale, ~80 % full | End of a normal, relaxed exhale |
| Effort | A relaxed maximum — you push into the struggle phase | No effort — you stop at the first urge |
| End point | The last moment before you must breathe | The first involuntary contraction or swallow |
| Typical range | 30–120 s untrained | 10–40 s |
| What it reflects | Total breath-hold capacity | CO₂ sensitivity + breathing habit |
A baseline is a ceiling; BOLT is a threshold. Because BOLT starts on an empty-ish, relaxed exhale and ends at the first twinge, it strips out willpower, technique and lung volume, and leaves you with something closer to a raw reading of how twitchy your CO₂ alarm is. That is why the two measurements are complementary rather than redundant — one tells you how far you can go, the other how soon your body starts complaining.
How to measure it correctly
BOLT only means anything if you take it calmly and the same way every time.
- Sit and settle. Breathe normally through your nose for a couple of minutes. No warm-up holds, no deep breaths.
- Take a normal breath in, then a normal breath out. Not a big one. This is the step everyone gets wrong — a deep exhale inflates the number and ruins comparability.
- Pinch your nose and start the clock.
- Stop at the first definite urge to breathe — the first swallow, the first involuntary movement of the diaphragm or throat. Not when it gets hard. At the first sign.
- Release and breathe normally. If your first breath is a gasp, you held too long — the number is invalid, so rest and redo it.
Measure it in the morning, before coffee, before your day loads you up. One reading is noise; a weekly reading over a month is a signal.
Reading your score
Read your BOLT score
Enter the seconds you measured — from the end of a normal exhale to the first real urge to breathe (not your maximum hold). The scale places you and tells you what it does, and does not, mean.
What the score does not say: in 49 highly trained athletes, BOLT showed no significant correlation with VO₂max, sprint power or time to exhaustion (Kowalski et al. 2024). It reflects your CO₂ sensitivity and breathing habit — not your endurance ceiling.
Broadly, a low score (under ~20 s) suggests a sensitive CO₂ response and often a habit of light, upper-chest over-breathing — your body reacts early to a small rise in carbon dioxide. A higher score suggests a calmer response and a slower, more diaphragmatic breathing pattern. Both directions are trainable, and that is genuinely useful: the number moves when your CO₂ tolerance and breathing habit change, so it works as a personal progress marker.
The physiology behind that is real. Classic respiratory work by Stanley and colleagues showed that breath-hold time is inversely related to the hypercapnic ventilatory response — in plain terms, people who react strongly to rising CO₂ break their hold sooner (Thorax, 1975, PMID 1145539). So a short hold really does index a more sensitive CO₂ chemoreflex. That is a legitimate physiological signal. It is just a signal about chemosensitivity — not about your fitness, your endurance or your health.
What the evidence does not say
Here is where honesty matters, because the internet has turned BOLT into something the data never claimed.
BOLT does not predict athletic performance. The most direct test of this was published in 2024: Kowalski and colleagues measured BOLT in 49 highly-trained speed skaters and compared it against a full battery of performance markers. There was no meaningful relationship with anything — not VO₂max, not Wingate peak power, not cardiopulmonary exercise-test output. Correlations ranged from r(47) = −0.172 to 0.013, with p-values from 0.248 to 0.984 (Frontiers in Physiology, 2024, PMID 39290618). In trained athletes, your BOLT score tells you nothing about how fast or fit you are.
The “ideal 40 seconds” is coaching lore, not a study result. The 20 s / 40 s thresholds you see everywhere trace back to breathing coaches — Patrick McKeown and the Oxygen Advantage method — reinterpreting a general observation from exercise-physiology textbooks and turning it into targets. They are popular, memorable and clinically useful as rough handrails. But there is no controlled trial that validated 40 seconds as an optimum, established normal ranges in the general population, or showed that hitting a target number produces a specific health outcome. The methodology that would be needed — a population sample, a standardised measurement protocol, and an outcome measured over time — has not been published for BOLT. When you see “40 s means elite health”, treat it as a rule of thumb someone invented, not a finding someone measured.
A number is not a diagnosis. Your BOLT can be dragged down by a cold, poor sleep, anxiety on the morning of the test, or simply exhaling too hard. It is sensitive to how you took it. For example, a slightly forced exhale instead of a passive one can cost several seconds on its own — enough to invent a “decline” that never happened. That makes it excellent for tracking yourself against yourself, and useless for comparing yourself against a stranger’s benchmark.
How to actually improve it
If your score is low and you want to move it, the lever is the same one that trains every breath-hold skill: deliberate, gradual exposure to a rising CO₂ signal.
- Run structured CO₂ work. The classic tool is a set of CO₂ tables — fixed holds with shrinking rests — which teach your nervous system to stay calm while carbon dioxide climbs. That is exactly the sensitivity BOLT is reading, so it is the most direct way to nudge the number.
- Fix the everyday habit. Slow, quiet, nasal, diaphragmatic breathing during ordinary life does more for a chronically low BOLT than any single session. The test is partly a mirror of how you breathe when you are not thinking about it.
- Establish your ceiling too. Pair BOLT with the baseline test every couple of weeks. One tracks your comfort threshold, the other your maximum — together they show whether progress is coming from tolerance, technique, or both.
Expect movement over weeks, not days, and expect it to plateau. A rising trend that then levels off is exactly what a working CO₂ tolerance looks like. The measured timescale supports that: two weeks of daily dry holds lengthened the struggle phase by an average of 59.7 % (Bourdas & Geladas, 2024, PMID 37797907), which is the same tolerance BOLT is sampling.
Sources
- Kowalski, T. et al. (2024). Body Oxygen Level Test (BOLT) is not associated with exercise performance in highly-trained individuals. Frontiers in Physiology 15:1430837. PMID 39290618
- Stanley, N. N. et al. (1975). Evaluation of breath holding in hypercapnia as a simple clinical test of respiratory chemosensitivity. Thorax 30(3):337–343. PMID 1145539
- Bourdas, D. I. & Geladas, N. D. (2024). Respiratory Physiology & Neurobiology 319:104168. PMID 37797907
Every source above links to its abstract, and each link was opened and re-read against this text during the last editorial pass. Where a study measured trained athletes rather than beginners, for example, the article says so instead of generalising.
Written by Jan Luther — keen amateur freediver, and the developer behind Apnea Trainer since 2010. About this blog explains how sources are chosen; the imprint has the contact details for corrections. All articles
Frequently asked
What is a good BOLT score?
There is no validated ideal. The widely repeated 40-second target comes from coaching literature, not from a controlled study. What is useful is your own trend over weeks, measured the same way each time.
Does a low BOLT score mean I am unfit?
No. Kowalski et al. (2024) measured BOLT in 49 highly-trained speed skaters and found no meaningful relationship with VO₂max, Wingate peak power or cardiopulmonary exercise-test output. It indexes CO₂ chemosensitivity, not fitness.
How is BOLT different from a maximum breath hold?
BOLT starts after a normal, passive exhale and ends at the first involuntary urge — the first swallow or diaphragm twitch. A maximum hold starts on a full inhale and runs well past that point. BOLT is a comfort measurement, not an effort measurement.
Can I improve my BOLT score?
Yes, the same way you improve any CO₂ tolerance — consistent dry table work and calm, slow everyday breathing. Two weeks of daily dry holds lengthened the struggle phase by 59.7 percent on average in Bourdas and Geladas (2024).
Apnea Trainer is a breath-hold training app for iPhone, Apple Watch and Android. It guides you through timed breathing cycles and builds progressive tables around your personal bests. On Apple Watch the whole session runs on your wrist — live heart rate and haptic cues for every phase.
Keep reading
Find Your Breath-Hold Baseline: A Free Interactive Test (No Water Needed)
Before you train, measure. A guided dry test — paced breathing, one relaxed max hold — and an honest reading of what your result means.
CO₂ Tables Explained: The Science of Training Your Urge to Breathe
The urge to breathe is a CO₂ alarm, not an oxygen alarm. Here is how CO₂ tables train that alarm — and how to build one from your current max.