An HRV score out of 100 is a millisecond value in disguise. Elite HRV says it computes RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , applies a natural log, and expands that number onto a 0 to 100 scale set from over six million readings; its average user scores about 59. Welltory also derives its HRV Score from lnRMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD . Each app’s scale is its own, so a 70 in one app is not a 70 in another.
How the score is made
Elite HRV’s help article gives the steps. The app records the gaps between beats from a chest strap or finger sensor, removes obvious artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact , and computes RMSSD, the root mean square of successive differences. It then takes the natural log, because “the ln(RMSSD) value typically ranges from 0 to 6.5” and behaves in a more linear way, and stretches that onto a scale “where everyone fits in a 0 to 100 range”, built from its database of readings. The exact stretch is not given. The score updates on a rolling 15-second basis during a reading and the final number uses all the beats recorded.
Welltory’s help page says its HRV Score is “calculated from lnRMSSD” and that “a score near 100% reflects an ideal state”. It gives no formula either.
What the log does
The natural log turns multiplication into addition. Each doubling of RMSSD adds about 0.69 to lnRMSSD:
| RMSSD | lnRMSSD |
|---|---|
| 10 ms | 2.3 |
| 20 ms | 3.0 |
| 40 ms | 3.7 |
| 80 ms | 4.4 |
| 160 ms | 5.1 |
So a score built on the log treats a rise from 20 to 40 ms the same as a rise from 40 to 80 ms. That is deliberate: a review of HRV norms notes that researchers report time-domain measures as natural logarithms to bring their distribution closer to normal. The lnRMSSD entry and the lesson on RMSSD, SDNN and lnRMSSD explain it step by step.
Why scores from different apps do not match
Elite HRV’s help page on why its score differs from other apps says those apps apply “a seemingly random multiplier” to the natural log of RMSSD, so many people end up scoring well over 100 elsewhere. There is no standard. Each app chooses its stretch, its reading length, its posture instructions and its artifact rules. A score also cannot be set against the millisecond values from a ring or a watch, which are measured overnight with different windows. The HRV checker refuses to compare app scores with research values for this reason, and explains what your device measures instead.
How to use a score
- Read the RMSSD or lnRMSSD next to it. Most apps show one of them. That is the number you can look up in the normal HRV by age tables and carry to another app.
- Measure the same way each time. Same time of day, same posture, same breathing, same length. The lesson on taking a good reading has the routine.
- Watch the 7-day trend. One reading of 48 against a usual 62 says more than any colour.
When to talk to a doctor
A score is a wellness number. Talk to a doctor if you have palpitations, dizziness, fainting, chest pain or breathlessness, whatever the app shows.