What this solves
"My pulse-oximeter read 78 — am I dying?" Most trekkers don't know what a normal SpO₂ should be at 4,400 m on day 2 vs day 5. This tool gives you a curve, not a single number, so you can tell whether your body is on track — or whether you are the unlucky 10 % who need to descend.
Predict your SpO₂ curve
Predicted SpO₂ curve (acclimatised profile)
How to read the result
Acclimatised baseline at your target altitude is the curve's right-hand endpoint. Expect roughly 85 % at 3,500 m, 80 % at 4,400 m, 75 % at 5,000 m after a week of acclimatisation.
A reading below 75 % at rest at any altitude after day 2 is a yellow flag — re-check in 30 min, hydrate, stop ascending. A reading < 70 % or any drop > 10 % from your own baseline is a "stop, evaluate, consider descent" trigger.
Finger pulse-oximeters are not reliable in the cold or with peripheral vasoconstriction. Warm your hand for 2 min before trusting the number.
Limitations
FAQ
A: Not yet. Day 2 at 4,200 m, 79 % is on the low side of normal but inside the curve. Re-test after 30 min sitting still, hydrated. If it's still < 78 %, that's your stop signal.
A: Wrist optical sensors are 4-6 % off at altitude compared to a fingertip pulse-oximeter. Use a dedicated finger cuff (Beurer, Contec, Yuwell) for decisions; the watch is good for trends.
A: Hypoxic ventilatory response is genetically variable. Some people breathe harder and faster at altitude — they're the lucky ones. The slow responders are not unfit, just wired differently.