How accurate are iOS apps that measure heart rate?
Very interesting question! I wasn't even aware that these kind of apps exist. In order to answer your question, I decided to put one of them to the test against a chest strap heart rate monitor. (I was using a Garmin brand chest monitor similar to this one.) I was using both heart rate monitors at the same time when I performed my tests and compared their values.

I tried out Instant Heart Rate Monitor, and it turns out that it is remarkably accurate... when it worked.

The main problem was that it only worked about 1 in 30 times. Every other time it would say it could either not detect a finger, or it started detecting the pulse, but then never finished the process.

When it did work, though, the results were surprisingly similar to my other heart rate monitor. After performing 5 (non-scientific) tests, it was off by an average of only 1.26%. Had it worked more consistently, I would have tried it out more than 5 times.

A few notes:


While it is beeping and gathering info, the heart rate it shows is usually way off from your actual heart rate. Sometimes it's almost exactly half your real heart rate.
The final heart rate it outputs is usually very close to your actual heart rate.


Therefore, you should only trust the last heart rate value it outputs (i.e. the average).