Heart Rate Variability Calculator
Heart rate variability (HRV) measures the variation in time between heartbeats and is a powerful, non-invasive window into the autonomic nervous system's balance between recovery (parasympathetic) and stress (sympathetic) states. Athletes and coaches use HRV to guide training load decisions: a suppressed HRV relative to an individual's baseline suggests inadequate recovery and argues for reduced training intensity. This calculator computes two standard time-domain HRV metrics: RMSSD (root mean square of successive differences, the most popular metric for athlete monitoring) and SDNN (standard deviation of all NN intervals, reflecting total autonomic variability). Enter your RR interval data in milliseconds, separated by commas, to get your metrics instantly.
HRV formulas
Mean RR = SUM(RR) / N
SDNN (ms) = SQRT( SUM((RR(i) - Mean RR)^2) / (N - 1) )
RMSSD (ms) = SQRT( SUM((RR(i+1) - RR(i))^2) / (N - 1) )
Mean HR (bpm) = 60000 / Mean RR
Using HRV for training decisions
- Establish a 2-week baseline by measuring HRV under identical conditions each morning.
- If daily HRV is more than 1 standard deviation below your baseline, reduce training intensity.
- If HRV is at or above baseline, training as planned is appropriate.
- Trend changes over weeks are more meaningful than single-day readings.
Frequently asked questions
What is heart rate variability (HRV)?
Heart rate variability is the variation in the time between successive heartbeats (RR intervals). Higher HRV generally indicates better autonomic nervous system balance, greater cardiovascular fitness, and better recovery readiness. Lower HRV can signal fatigue, illness, or overtraining.
What is RMSSD?
RMSSD (root mean square of successive differences) is the most widely used HRV metric in sport and health monitoring. It reflects parasympathetic (rest-and-digest) nervous system activity. It is calculated as the square root of the mean of squared differences between successive RR intervals.
What is SDNN?
SDNN (standard deviation of NN intervals) reflects overall autonomic variability, capturing both sympathetic and parasympathetic contributions. It is used in clinical research and is one of the oldest HRV metrics. A higher SDNN generally correlates with better cardiovascular health.
What are normal HRV values?
RMSSD norms vary by age and fitness. For adults, RMSSD values of 20 to 50 ms are average; above 50 ms is good; athletes often see values of 60 to 100 ms or higher. Values decrease with age. Your personal baseline is more important than population norms.
When should I measure HRV?
HRV should be measured under consistent conditions: ideally in the morning before getting out of bed, at a consistent time. A 2 to 5-minute recording period is standard for meaningful RMSSD and SDNN values. Consumer devices typically record during a 1 to 3-minute session.
Official sources
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology: Heart Rate Variability Standards of Measurement (1996).
- American College of Sports Medicine: ACSM Physical Activity Guidelines.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.