What's a Normal Oxygen Saturation, and Can You Trust a Pulse Oximeter?

Peripheral oxygen saturation (SpO₂) is the share of the haemoglobin in your blood that’s carrying oxygen, as a pulse oximeter measures it through your skin. For a healthy adult at sea level it’s usually 95% to 100%. Below 90% is a medical emergency. In between, the reading needs checking, and part of the reason is that oximeters can be wrong.

This article is general information, not medical advice. If you’re short of breath, confused, or have blue lips, get urgent care whatever an oximeter says.

What the Numbers Mean

The World Health Organization’s Pulse Oximetry Training Manual gives:

SpO₂	Meaning
95% to 100%	Normal
94% or less	Find out why: the person may be hypoxic
Under 90%	A clinical emergency that needs treatment

People with some chronic lung diseases live with lower targets their doctors set, and SpO₂ is naturally lower at altitude: a few percentage points lower is common at a few thousand metres.

How an Oximeter Works, and Where It Goes Wrong

An oximeter shines red and infrared light through your finger. Oxygenated and deoxygenated haemoglobin absorb those colours differently, and the device compares the pulsing part of the signal to estimate saturation. Readings can be wrong when:

• Your hands are cold or circulation is poor, so the pulse signal is weak.
• You move, which wrist and ring sensors suffer from most.
• Nail polish, artificial nails, or bright light interfere with the light.
• Carbon monoxide binds haemoglobin and reads as oxygen, so SpO₂ looks normal in poisoning.
• Skin pigmentation. In a 2020 study of hospital patients, Sjoding and colleagues found that among readings of 92% to 96%, the true arterial saturation was below 88% almost three times as often in Black patients as in white patients. The US Food and Drug Administration (FDA) has since proposed stricter testing of oximeters across skin tones, in draft guidance from January 2025.

Oximeters are least accurate at low saturations, exactly where accuracy matters most.

How to Take a Reading You Can Trust

1. Sit and rest for five minutes, with warm hands.
2. Remove nail polish from the finger, and rest your hand still at heart level.
3. Leave the oximeter on for at least 30 seconds, until the number is steady, and read the steady value, not the first one.
4. Note the reading and the trend: a value that’s falling over hours or days matters more than a single number.

Watches and rings estimate SpO₂ mostly during sleep, when you’re still. Brief dips at night can be a sign of sleep apnea, but a watch can’t diagnose it; see What Is the Apnea–Hypopnea Index?

Following It in Kaizora

Peripheral Oxygen Saturation is on the Vitals page, logged by hand or imported from Apple Health. The WHO’s ranges come as default reference ranges, so a value of 94% or less is flagged as below normal and one under 90% as hypoxemia, on the chart and on Home. Each value records its device, so readings from a watch and from a fingertip oximeter can be told apart on the chart. See Reference Ranges.

The Short Version

• A normal SpO₂ is 95% to 100%; at 94% or less find out why; under 90% is an emergency.
• Cold hands, movement, nail polish, and darker skin can make readings wrong, often too high.
• Take a reading at rest, with warm hands, and read the steady value.

Sources

• World Health Organization, Pulse Oximetry Training Manual (2011).
• Sjoding MW et al., “Racial bias in pulse oximetry measurement,” New England Journal of Medicine 383, no. 25 (2020): 2477–2478. doi:10.1056/NEJMc2029240
• US Food and Drug Administration, Pulse Oximeters.