HRV Checker: RMSSD vs SDNN

Quick answer: there is no “good” HRV, and the charts that rank yours by age are usually comparing two different measurements. A ring reports RMSSD measured overnight; an Apple Watch reports SDNN from short samples taken while you happen to be still; the published norms come from five-minute or 24-hour recordings — and the review that collects those norms states plainly that they are not interchangeable. This page picks the one published figure that matches your device and shows you that, with the spread it came with.

How to use it

Type the number your device shows and pick the device. If you have a figure from a few weeks ago, put that in too — comparing you with yourself is the comparison every source here recommends, including the ring makers.

Why the metric matters more than the number

RMSSD measures the beat-to-beat scatter. It settles quickly, which is why wearables that sample at night use it. SDNN is the standard deviation of the intervals, and it grows with the length of the recording — so an SDNN from a two-minute sample and an SDNN from 24 hours are not the same animal, let alone the same as an RMSSD.

Apple’s own documentation is explicit: “Apple Watch calculates HRV using the standard deviation of normal-to-normal measurements (SDNN)”, captured opportunistically while you are still, by default every four hours. Read that number against a ring’s night-time RMSSD chart and you are comparing a photograph with a thermometer.

What is actually published

For short (about five-minute) recordings in healthy adults, pooled across dozens of studies: SDNN 50 ms (SD 16), with study means spread from 32 to 93, and RMSSD 42 ms (SD 15), spread 19 to 75. The width of those ranges is the finding. A single figure inside a range that wide tells you very little about a person.

For night-time ring readings, the nearest published comparison is the ring maker’s own membership: Oura reports an average of 41 ms across all members, 39.1 ms for women (median 36) and 42.8 ms for men (median 35), and says the sleeping average falls with age. That is a population of people who bought a ring — not a sample of humanity, and not a target. Oura says so itself: “there’s not an ‘optimal’ HRV for men, women, or even a certain age group.”

The comparison that is worth making

Your own baseline, built over a few weeks, and then the trend. HRV moves with sleep, alcohol, illness, training load, hydration and stress; one night says almost nothing. A change of under about 10% from your own figure is inside the noise every source describes. A sustained change is worth looking at alongside what else changed in your life — and, if it persists and you feel unwell, worth a doctor’s attention rather than an app’s.

Worked examples

38 ms on an Oura Ring. Three below Oura’s member average of 41 — a difference well inside daily variation, and a comparison with ring owners rather than with a population.

38 ms on an Apple Watch. That is an SDNN, so the ring chart does not apply. Against pooled five-minute SDNN norms (50 ms, study means 32–93) it sits inside the spread — and the watch’s sample is shorter than five minutes, which lowers SDNN by itself.

28 ms RMSSD from a five-minute app recording. Below the pooled mean of 42, inside the 19–75 spread of study means.

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