HRV Calculator
Compute RMSSD, ln(RMSSD), and SDNN from your raw RR intervals — the beat-to-beat data behind every HRV reading.
How to use it
- Export your RR/IBI intervals in milliseconds from a chest strap or HRV app.
- Paste them into the calculator.
- Read your RMSSD, ln(RMSSD), and SDNN, computed instantly.
What is RMSSD and how is it calculated?
RMSSD is the root mean square of successive differences between consecutive RR intervals — the time gaps between heartbeats, in milliseconds. It is the most widely used short-term heart rate variability metric and primarily reflects parasympathetic (vagal) activity. Many apps report ln(RMSSD) to make day-to-day numbers easier to compare.
SDNN is the standard deviation of the RR intervals over the measurement window, capturing overall variability. A stable reading typically uses a consistent 1–5 minute window taken at the same time and posture each day.
What is a good HRV?
There is no universal 'good' number — HRV varies widely by age, genetics, fitness, and measurement method. What matters most is your own baseline and how today compares to your recent trend, not where a single reading lands against anyone else's. Track it under consistent conditions (same time, same posture) so the comparison is fair.
Frequently asked questions
How do I calculate HRV from RR intervals?
RMSSD is the square root of the mean of the squared differences between successive RR intervals. Paste your intervals in milliseconds and this calculator returns RMSSD, ln(RMSSD), and SDNN.
What is the difference between RMSSD and SDNN?
RMSSD captures short-term, beat-to-beat variability and mostly reflects parasympathetic (vagal) activity, which is why it is the common choice for daily readiness. SDNN is the standard deviation of all the intervals in the window and reflects overall variability from both branches of the autonomic nervous system.
Why does HRV drop after a hard workout?
Intense exercise shifts the autonomic nervous system toward sympathetic dominance and temporarily suppresses parasympathetic activity, lowering HRV. As you recover, HRV typically rebounds toward or above baseline.
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Educational, performance-oriented content for athletes — not medical advice. Thresholds and reference ranges come from group data and vary between individuals.