> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sonohealth.com/llms.txt
> Use this file to discover all available pages before exploring further.

# How Does a Pulse Oximeter Work?

> A clear explanation of how a fingertip pulse oximeter measures blood oxygen and pulse with light, what SpO2 and PI mean, and the limits of the technology.

A fingertip pulse oximeter gives you two numbers in seconds without a single needle. Understanding how it does that helps you trust good readings — and recognize when one might be off.

<Note>
  A pulse oximeter like the SonoHealth fingertip [Pulse Oximeter](/pulse-oximeter/overview) is for quick home checks. It estimates oxygen saturation; it doesn't diagnose any condition. Always treat symptoms, not just the number.
</Note>

## How does it measure oxygen with light?

The clip shines two wavelengths of light (red and infrared) through your fingertip to a sensor on the other side. Oxygen-rich blood and oxygen-poor blood absorb these wavelengths differently, so by comparing how much light passes through, the device calculates the percentage of hemoglobin carrying oxygen — your **SpO2**.

## What is SpO2?

SpO2 is your blood oxygen saturation, shown as a percentage. For most healthy people a [normal reading is 95–100%](/pulse-oximeter/normal-oxygen-levels). It's an estimate of how well oxygen is being carried in your blood at that moment.

## What is the pulse reading (PR)?

Because the device detects the pulsing of blood with each heartbeat, it also reports your **pulse rate** (PR or BPM). A normal adult resting pulse is roughly 60–100 BPM. An unusually high, low, or irregular pulse — especially with symptoms — is worth raising with your doctor.

## What is perfusion index?

Some oximeters also show a **perfusion index (PI)**, which reflects how strong the pulse signal is at the fingertip. A low PI often means cold hands or poor placement rather than a real problem, so it's a handy quality check on the reading.

## What affects accuracy?

Because it relies on light and a steady pulse, readings can be thrown off by motion, cold hands, poor circulation, dark nail polish, or very bright ambient light. For reliable results, warm the hand, remove polish, hold still, and re-check — see the full [accuracy guide](/pulse-oximeter/pulse-oximeter-accuracy-guide).

<Warning>
  Severe shortness of breath, bluish lips, confusion, or trouble staying awake is an emergency — call 911 even if the number looks borderline.
</Warning>

***

**Related:** [Normal Oxygen Levels](/pulse-oximeter/normal-oxygen-levels) · [Accuracy Guide](/pulse-oximeter/pulse-oximeter-accuracy-guide) · [Oximetry With COPD or Asthma](/pulse-oximeter/copd-asthma-monitoring) · [Pulse Oximeter Overview](/pulse-oximeter/overview)