HRV calculator
Heart rate variability (HRV) differs a lot from person to person, so the useful comparison is with your own baseline. Paste your recent daily readings to find your baseline and normal range, then see where today falls.
ms
Your baseline from readings. Your normal range is to ms.
Today is below your normal range. It is percent under your baseline.
Today is within your normal range. It is percent from your baseline.
Today is above your normal range. It is percent over your baseline.
How the baseline is worked out
HRV readings are skewed: a few very high days pull a plain average up. So, as most HRV research does, the calculator takes the natural log of each reading first. Your baseline is the average of those logs turned back into milliseconds, which is the geometric mean. Your normal range is one standard deviation either side of it, on the same log scale. About two thirds of your days should fall inside it.
The CV (coefficient of variation) of the log values shows how much your readings bounce from day to day. A rising CV over weeks can mean your routine or recovery is less stable.
A single low day means little. Poor sleep, alcohol, a late meal, stress, illness and hard training can all lower HRV. Look for several low days in a row, or a drifting baseline, rather than reacting to one number. This is not medical advice.
SDNN and RMSSD are not the same number
Devices measure HRV in different ways, so their numbers do not compare.
- Apple Health has long stored HRV as SDNN, the standard deviation of the time between beats. Apple Watch records it from short readings taken at different times of day.
- Oura, Whoop and Garmin report RMSSD, the root mean square of the differences between successive beats, measured during sleep.
RMSSD and SDNN are different formulas, and they are measured at different times and for different lengths of time. A 40 ms SDNN from Apple Watch and a 60 ms RMSSD from a ring can come from the same person on the same day. Build your baseline from one device and one metric only, and do not switch halfway through.
iOS 27 adds a separate RMSSD type to Apple Health. Marrow 1.6.2 added RMSSD as its own metric next to SDNN, and the two are never merged.
Questions
What is a normal HRV?
There is no single normal value. HRV depends on age, fitness, genetics, the device and the metric. Many healthy adults sit somewhere between 20 and 100 ms. Your own baseline is far more useful than a population average.
Is a higher HRV always better?
Generally a higher HRV relative to your own baseline points to good recovery. But a sudden jump well above normal can also follow illness or heavy fatigue, so read it alongside how you feel and your resting heart rate.
Why is my Apple Watch HRV different from my Oura or Whoop HRV?
Apple Watch reports SDNN from short daytime and nighttime readings. Oura and Whoop report RMSSD measured during sleep. They are different measures taken at different times, so the numbers are not comparable.
How many days do I need for an HRV baseline?
A week gives a rough start. Two to eight weeks of daily readings from the same device and time of day gives a more stable baseline.
Why did my HRV drop?
Common reasons are short or poor sleep, alcohol, illness, stress and hard training. If it stays low for many days without a clear reason, consider rest, and talk to a doctor if you feel unwell.
Your readings, already collected
Marrow reads HRV, sleep and resting heart rate from Apple Health, so your daily readings are there without copying them by hand. You can also export any Apple Health metric as CSV or JSON.
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