Pulse oximeter accuracy calculator

How the range is worked out

ARMS is the root mean square difference between the oximeter (SpO2) and arterial blood measured on a laboratory co-oximeter (SaO2), across paired samples from 70 to 100%. The formula comes from ISO 80601-2-61. If the errors are bell-shaped with no offset, about 68% of readings fall within one ARMS of the arterial value and about 95% within 1.96. The calculator applies those two multipliers and stops the top of the range at 100%.

ARMS folds the average offset and the scatter into one figure. A device that reads 1 point high every time and a device that scatters by 1 point can state the same number. The range is centred on the reading because the stated figure does not say which way a device leans.

Where to find the accuracy figure

Look in the specifications table of the manual, under SpO2 accuracy. It is usually written as ±2% or ±2 digits for 70 to 100%, and that number is the ARMS to enter. Most manuals give no figure below 70%, so the calculator has nothing to go on for readings that low. If you are choosing a device, the overnight oximeter picker compares recording oximeters by feature.

What the FDA asks manufacturers to show

The FDA's 2013 guidance for prescription oximeters lists an ARMS of 3.0 or less for fingertip (transmittance) sensors and 3.5 or less for reflectance sensors. Testing is on 10 or more healthy subjects, with 200 or more paired samples and at least 2 darkly pigmented subjects or 15% of the group, whichever is larger.

On 7 January 2025 the FDA issued a draft guidance that would replace it. The draft recommends 150 or more participants spread across the Monk Skin Tone scale, measured skin colour (individual typology angle), and evidence that accuracy does not differ by pigmentation. It is a draft, and the 2013 document applies until a final version is published.

Both documents are written for oximeters intended for medical purposes. An oximeter sold for general wellness or sport is outside them, so the accuracy figure in its manual is the manufacturer's own claim.

Skin pigmentation

Sjoding and colleagues (NEJM, 2020) compared oximeter readings with arterial samples taken within 10 minutes in hospital patients. When the oximeter read 92 to 96%, arterial saturation was below 88% in 11.7% of Black patients and 3.6% of White patients in the University of Michigan cohort.

Bickler and colleagues (Anesthesiology, 2005) tested three finger oximeters on 21 volunteers in a laboratory. At an arterial saturation of 60 to 70%, the oximeters read 3.56 points high on average in the 11 darkly pigmented subjects and 0.37 points high in the 10 lightly pigmented subjects.

These are findings in hospital patients and laboratory volunteers, on the oximeters studied. They do not give a correction to apply to one person's reading.

Getting a reading you can compare

The stated ARMS assumes a warm, still hand. Before comparing two readings:

• Warm the hand and sit still for a minute.
• Use a finger without polish or an artificial nail.
• Use the same device on the same finger each time.
• Wait for the number to hold steady for several seconds before reading it.

An overnight recording gives thousands of samples from one sensor position, which a spot check cannot. Load a night into the free session viewer to see the trace and the time spent under 90%, or put two recordings on one chart with compare nights. The guide to pulse oximeter accuracy covers how to tell a sensor artifact from a repeated dip.

Questions

What does ARMS mean on a pulse oximeter?

ARMS is the accuracy root mean square: the root mean square difference between the oximeter reading (SpO2) and arterial blood measured on a laboratory co-oximeter (SaO2), across paired samples from 70 to 100%. It combines the average offset and the scatter in one number.

What does plus or minus 2% accuracy mean?

An ARMS of 2 means about two thirds of readings fall within 2 points of the arterial value and about 95% within 4 points, under test conditions. A reading of 94% is then consistent with an arterial value from about 90 to 98%.

Where do I find the ARMS for my oximeter?

Look in the specifications table of the manual, under SpO2 accuracy. It is usually written as ±2% or ±2 digits for the 70 to 100% range. Most manuals give no figure below 70%.

What accuracy does the FDA ask for?

The FDA 2013 guidance for prescription oximeters lists an ARMS of 3.0 or less for fingertip (transmittance) sensors and 3.5 or less for reflectance sensors, tested on at least 10 healthy subjects and 200 paired samples. A draft guidance issued on 7 January 2025 would replace it and recommends at least 150 participants across the Monk Skin Tone scale.

Does skin tone change the reading?

Published studies found that it can. In Sjoding and colleagues (NEJM, 2020), among hospital patients whose oximeter read 92 to 96%, arterial saturation was below 88% in 11.7% of Black patients and 3.6% of White patients in one cohort.

Is a 1 or 2 point change between two readings real?

It can be, but a change that size is inside the stated error of most finger oximeters, so one pair of readings cannot settle it. Take both readings warm and still on the same finger, and look at whether the difference repeats.

Does the FDA guidance cover my oximeter?

Both FDA guidance documents are written for oximeters intended for medical purposes. An oximeter sold for general wellness or sport is outside them, so the accuracy figure in its manual is the manufacturer's own claim.

Sources

1. FDA. Pulse Oximeters for Medical Purposes: Non-Clinical and Clinical Performance Testing, Labeling, and Premarket Submission Recommendations. Draft guidance, 7 January 2025. Docket FDA-2023-N-4976.
2. FDA. Pulse Oximeters: Premarket Notification Submissions [510(k)s]. Guidance, 4 March 2013.
3. FDA. Pulse Oximeters (factors that affect accuracy).
4. Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial Bias in Pulse Oximetry Measurement. N Engl J Med. 2020;383:2477-2478.
5. Bickler PE, Feiner JR, Severinghaus JW. Effects of skin pigmentation on pulse oximeter accuracy at low saturation. Anesthesiology. 2005;102:715-719.
6. ISO 80601-2-61. Particular requirements for basic safety and essential performance of pulse oximeter equipment (defines the ARMS formula).

Related tools and guides

• ODI and T90 calculator: work out a desaturation index from a count of dips and the hours recorded.
• Oximetry results explainer: what each number on an overnight report measures.
• Oxygen level by age: published SpO2 reference ranges by age band.
• Using a pulse oximeter overnight: how to set up a recording.