There’s a surprising amount of signal locked in the humble heart rate monitor.

At Empirical Health, we’ve analyzed more than 3,025,980,708 heart rate measurements from Apple Watch. While the raw sensor data is surprisingly accurate—studies show that Apple Watch’s heart rate sensor is accurate to within 2.3 bpm—what’s more important is that the raw data can be transformed into 10 distinct biometrics that help you monitor your overall heart health.

The rest of this article explains the accuracy of the Apple Watch’s heart rate monitor, heart health biomarkers derived from it, how to use them to monitor your heart health, and details on how the Apple Watch’s heart rate sensor works.

Apple Watch’s heart rate monitor. Screenshot from Empirical Health.

Accuracy of Apple Watch heart rate monitor

Accuracy of Apple Watch heart rate monitor vs Garmin, Fitbit, and Polar. Source: European Journal of Sports Science, 2023.

The Apple Watch’s heart rate monitor was accurate to within 2.3 bpm in a 2023 study of 60 healthy participants. Among patients with cardiovascular disease, a 2019 study found an average error of 6-7 bpm. Earlier studies on the Apple Watch Series 4, which used the second generation of sensors, found higher error. A 2019 study found a correlation of 70%.

In general, if you’re using the latest generation of Apple Watch’s heart rate monitor (Apple Watch Series 6 and later), it’s accurate.

Metrics derived from Apple Watch’s heart rate monitor

When we think about heart health, we can divide metrics into three buckets: measuring the health of the heart’s “pump”, “electrical system”, and “power supply”. The pump is the heart’s ability to pump blood, and the Apple Watch measures 7 metrics of overall cardiovascular fitness. The electrical system controls the heart’s rhythm, and Apple Watch measures 3 heart rhythm metrics. Blockages in the power supply (coronary arteries) are a heart attack — Apple Watch doesn’t measure this, but a blood test can. Last, heart rate variability (HRV) is a window into your nervous system and many internal organs.

Here’s the breakdown of each of the metrics and why it matters:

Category	Metric	Why it matters to your health
Cardiovascular Fitness	Resting Heart Rate (RHR)	For every 10 bpm decrease in RHR, mortality goes down by 10-20%.
	VO2 Max (Cardio Fitness)	Every 1 mL/kg/min increase in VO2 max reduces risk of death by 9 percent.
	Cardio Fitness Notifications	Alerts when cardio fitness is low for your age/gender, which may indicate increased health risks.
	Cardio Recovery (Heart Rate Recovery)	How quickly heart rate drops after exercise. Faster recovery suggests better cardiovascular fitness. Poor recovery can indicate heart problems.
	Walking Heart Rate	Average heart rate during walking. Elevated walking heart rate may indicate poor fitnes ](https://www.empirical.health/metrics/heart-rate-zones)
Heart Rhythm	High/Low Heart Rate Notifications	Alerts for unusually high (>120 bpm) or low (<40 bpm) heart rates when inactive, which can indicate arrhythmias or other cardiac issues.
	Irregular Rhythm Notifications	Detects potential atrial fibrillation, a common heart rhythm disorder responsible for 1 in 4 strokes.
	AFib History	Tracks the percentage of time your heart spends in atrial fibrillation (for diagnosed patients only).
Heart Attack	N/A	Apple Watch does not measure heart attack risk; for that you need a blood test and a heart attack risk calculator.
Nervous system	Heart Rate Variability (HRV)	Higher HRV generally indicates a better functioning autonomic nervous system.

How to use Apple Watch as a part of a comprehensive heart health program

A comprehensive heart health program requires tracking metrics of the heart’s pump, electrical system, and power supply (heart attack risk). While Apple Watch measures the first two, to measure heart attack risk, you need the results from a blood test plus a heart attack risk calculator. Empirical Health includes heart attack risk calculactor, as well as a comprehensive health score that combines your Apple Watch metrics with results from a blood test.

Comprehensive heart health scores in Empirical Health, including heart attack risk derived from a blood test.

Don’t have recent blood test results? You can order a blood test diretly through the Empirical app.

How Apple Watch’s heart rate sensor works

Apple Watch’s optical heart rate sensor uses an array of photodiodes to whine light through your skin. There are three wavelengths of light used: green (525 nm), infrared (850-940 nm), and red (660 nm). The green and infrared light are used to measure your pulse, and the red light is used to measure your blood oxygen saturation.

Three generations of Apple Watch’s heart rate sensor

Apple has revised the optical heart rate sensor three times since 2015. The first optical heart rate sensor debuted in 2015 with the original Apple Watch. The second generation was introduced in the Series 4 (2018) and is currently used in the Apple Watch SE2. The third generation was introduced in the Series 6 (2020), alongside the oxygen sensor, and is now used in the Apple Watch Series 10 and Apple Watch Ultra 2.

Generation	Year Introduced	Apple Watch Models	Key Features
First gen optical heart rate sensor	2015	Apple Watch Series 1-3, SE	Basic heart rate monitoring
Second gen optical heart rate sensor	2018	Apple Watch Series 4, 5, SE2	Improved accuracy, lower power consumption
Third gen optical heart rate sensor	2020	Apple Watch Series 6-10, Ultra/Ultra 2	Blood oxygen monitoring capability
ECG Sensor	2018	Apple Watch Series 4 and later	FDA-cleared Class II medical device for electrocardiogram readings

Optical heart rate sensor vs ECG sensor

The Apple Watch has two heart rate sensors: the optical heart rate sensor and the ECG sensor. The optical sensor uses light to measure your heart rate, whereas the ECG measures the electrical signals generated by your heart. A doctor can use the ECG to diagnose heart rhythm disorders, like atrial fibrillation, atrial flutter, supraventricular tachycardia, and more.

Closing thoughts

Wearables have evolved from simple fitness trackers to sophisticated heart monitoring devices. While they’re not a replacement for medical devices, the fact that you wear them every day makes them a valuable tool to take your heart health into your own hands.