📡HRV Baseline Statistics Calculator

Seven-day mean, SD and today z-score

Enter your last seven morning RMSSD readings and today reading to get the mean, the sample standard deviation and the z-score of today value. It makes no recovery or overtraining judgement and produces no readiness score — only the statistics.

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The z-score is (today reading minus the seven-day mean) divided by the sample standard deviation, which divides by n-1. The mean and standard deviation are shown to one decimal and the z-score to two, and the inside-or-outside statement is decided at the same precision, so the numbers and the wording always agree.

This is not a medical assessment. HRV moves with the time of day, body position, breathing, caffeine, alcohol, the device and the analysis method. Chest-strap and wrist readings are not comparable, so readings only mean something when the conditions are held constant. Chest pain, palpitations, dizziness or shortness of breath are reasons to see a clinician rather than to study a number. Written as of October 2026.

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Reading HRV as distance from your own baseline

Morning RMSSD bounces around from day to day, so a single reading says very little; it is normally compared with a recent personal average instead. This calculator takes seven recent readings, works out the mean and the sample standard deviation, and reports how many standard deviations today reading sits from that mean as a z-score. It also shows the usual variation band, the mean widened by one standard deviation in each direction, and states plainly whether today value falls inside or outside it.

The arithmetic is simple. The mean is the average of the seven readings and the standard deviation uses the sample divisor of n-1. The z-score subtracts the mean from today reading and divides by that standard deviation. If all seven readings are identical the standard deviation is zero and no z-score exists, which the calculator says instead of printing a number. Written as of October 2026.

This tool is not a medical assessment. It does not decide whether you are recovered or overtrained and it produces no fatigue or readiness score. HRV shifts with measurement time, posture, breathing, caffeine, alcohol and the device used, so readings must be taken the same way to be comparable. Chest pain, palpitations, dizziness or breathlessness are reasons to see a clinician rather than to keep measuring.

FAQ

My z-score is -2. Should I skip training?

This page does not answer that. The z-score only states how many standard deviations today reading sits from your own seven-day mean; it carries no training instruction.

Why divide by n-1 for the standard deviation?

Seven days is a sample, not the whole of anything. When a sample is used to estimate spread, the n-1 divisor gives the sample standard deviation.

Can I keep the series after changing devices?

Better not to. Chest straps, wrist devices and apps compute the figure differently, so the level itself shifts. After a device change, collect seven fresh days and rebuild the baseline.