Blood Oxygen Level Guide

Blood oxygen level (SpO2) is the percentage of the hemoglobin in your blood that is carrying oxygen. A pulse oximeter estimates it by shining red (about 660 nm) and infrared (about 940 nm) light through a fingertip and comparing how much of each the pulsing blood absorbs.

The underlying formula (Beer-Lambert Law basis):

SpO2 = (Ratio of oxygenated Hb / Total Hb) × 100

In practice, pulse oximeters use a calibration curve derived from this ratio (R):

R = (AC660 / DC660) / (AC940 / DC940)

Where AC = pulsatile (arterial) signal and DC = baseline signal at each wavelength. SpO2 is then looked up from a stored R-curve. You never calculate this by hand, since the oximeter does it in the clip on your finger.

What the numbers mean at sea level:

Inside the below-normal band, the low end matters more than it looks. If you are unwell and the reading sits at 92% or below at rest, get seen the same day rather than waiting to see if it passes.

Altitude moves the scale, within limits

Air at altitude holds less oxygen per breath, so a healthy person saturates lower there. At 8,000 ft a reading of 92% is ordinary, where at sea level it would be worth a second look. So this page moves every band down by the typical drop for your altitude, which is how an altitude doctor reads a number: against what that height produces.

It only does that up to 10,000 ft (about 3,000 m), and one line never moves: below 80% is an emergency wherever you are.
Higher up, healthy people commonly read in the 80s. In one field study at 3,275 m, people who stayed well averaged 87.7% and people who developed altitude sickness 85.4%, so a number on its own cannot tell acclimatising from getting ill. Up there symptoms decide: headache, nausea, breathlessness at rest, a cough, confusion. The altitude sickness risk calculator covers that side.

Other normal variation:

• During deep sleep, SpO2 may briefly dip to 90 to 92% even in healthy people
• People with COPD (chronic obstructive pulmonary disease) and some other lung conditions are often given a lower target, usually 88 to 92%, because more oxygen can raise their carbon dioxide. The drop calculator checks a reading against that target and against your own usual number

Accuracy notes:

• Nail polish (especially dark colors) and acrylic nails can cause falsely low readings: use a fingertip without polish or try the earlobe
• Poor circulation (cold hands) reduces accuracy significantly: warm up before measuring
• Movement ruins the trace. Sit still for a full 30 seconds and read the number once it stops changing
• Oximeters read too high more often on darker skin. A 2020 New England Journal of Medicine study found hidden low oxygen about three times as often in Black patients as in white patients, so a reassuring number does not outrank how you feel
• Carbon monoxide poisoning produces falsely normal SpO2 readings: oximeters cannot distinguish COHb from O2Hb
• For clinical accuracy, arterial blood gas (ABG) testing is the gold standard. The SpO2 to PaO2 calculator shows roughly what a blood gas would find behind your reading