HRV Calculator

Why There Is No Age Chart Here

Every "normal HRV by age" chart you are likely to meet was built from one specific recording setup, and your watch does not match any of them:

• Umetani 1998 — 24-hour Holter ECG, 260 healthy people aged 10 to 99.
• Tegegne 2020 (Lifelines) — 10-second resting ECGs from 84,772 people.
• van den Berg 2018 — 10-second ECGs again, 13,943 of them, ages 11 days to 91 years.

Your wearable does none of those. Most read optical pulse for hours while you sleep, which is a fourth recording condition that none of the tables covers.

This is not a technicality. The review those charts usually lean on says it outright: 24-hour, short-term and ultra-short-term normative values are not interchangeable (Shaffer and Ginsberg 2017). You can see the mismatch in the data itself. Over 24 hours, women under 30 measured lower than men and the gap disappeared after 50 (Umetani); in 10-second recordings, the difference between men and women was minimal (van den Berg). Same organ, different recording window, different answer about who counts as normal.

So this tool runs the one comparison that survives: your nights against your nights. That is also where the people living with these numbers keep landing. A reply in an r/whoop thread about age charts puts it flatly: "never be compared to anyone or any chart you find online... There is no standard number for age and sex."

Practical version: judge tonight by whether it sits inside your own band, and treat any external table as trivia. If a low stretch lines up with a big training week, your training load for those days explains more than any chart will. For how to read a run of nights rather than one, see the HRV guide for runners.

What Your Device Actually Measures

Two devices can put very different numbers on the same night, because they are not reporting the same statistic over the same window. Here is what each maker's own documentation says:

Read across the middle two columns and the problem is obvious: three of these brands look at three different slices of the same night. Apple then reports a different statistic on top of that. There is no valid conversion between SDNN and RMSSD, so nothing in this table lets you translate one device's number into another's — which is exactly why this tool only ever compares you with your own nights.

Garmin says the same thing in cooler language on its own page: differences in the timing and duration of the measurement "make it a challenge to make apples-to-apple comparisons between HRV measurements produced with alternative protocols and different devices". Runners hit this in the wild. One r/whoop poster reported readings in the 200-300 range from WHOOP after every previous tracker he had owned put him at 60-90.

Two things follow for how you feed this tool:

• Use night values, not daytime spot checks. Against a chest-strap reference sensor, overnight optical readings tracked closely (intraclass correlation 0.93 for LnRMSSD), though the watch read systematically higher — a gap that seemed largest in people whose HRV is low to begin with (Nuuttila et al. 2021). Under free-living conditions the same technology was judged "a poor surrogate for HRV", with daytime readings the worst (Lam et al. 2020, 10 people).
• One device, one metric, one kind of reading. If you switch watches, change apps, or move from a morning reading to an overnight one, start the series over from the switch. Apple Health users especially: pick one nightly figure from one app and stick to it, since Apple stores discrete samples rather than a single nightly value.

Device-specific background lives in the WHOOP guide and the Garmin readiness guide.

One Bad Night Keeps Pulling Your Average

One freak night — a late race, a stomach bug, three glasses of wine — does not distort just one day. In a plain 7-day average it stays in the mix for a full week, dragging every reading that follows. Runners have been complaining about this for years, and WHOOP's own staff have confirmed there is no way to do it there.

Asked directly whether a bad night could be excluded, a WHOOP staff member answered in the official WHOOP community (February 2026): "At this time, individual nights can't yet be excluded from HRV baseline calculations." A second staff reply pointed people at time instead, saying baselines should normalise on their own within two to three weeks, depending on the person.

This tool does the arithmetic those users were doing by hand. It converts your nights to natural logs, because HRV is right-skewed rather than symmetric — in 417 healthy young Japanese men, RMSSD averaged 37.5 ms with a clearly right-skewed distribution (Kobayashi et al. 2012). It then measures each night against the median of your series rather than the average, so one wild value cannot inflate the very spread used to catch it. Anything past a robust z of 2.5 is flagged, dropped from your baseline, and given a countdown: how many more days that night would keep pulling a plain 7-day average.

What to do with a flagged night:

• Do not re-plan around it. Note the likely cause, let the countdown run out, then read the trend. Keep your easy days genuinely easy in the meantime — heart rate zones are a more reliable brake than a single number.
• Do not delete it on the device. WHOOP's own advice is to leave the data alone and let the baseline settle, and deleting only costs you a night of history here.
• Watch what stacks up behind it. A run of low nights with poor sleep is a different story from one bad Saturday — see sleep and recovery and the overtraining guide.

Be clear about what these thresholds are. The 2.5 cut-off, the 7-day washout and the plus-or-minus one standard deviation band are engineering choices, not research findings. Seven days in particular is a convention: in athlete monitoring the 7-day rolling average of Ln rMSSD is the standard smoothing window, and the case comparison that applied it to overreaching followed exactly two elite triathletes (Plews et al. 2012).

Race Week: Why a Drop Is Normal

HRV sagging in the last week before a race is common enough that runners named it. Runners call it the "taper tantrum" — the phrase turns up on r/AdvancedRunning: sleep gets patchy, the routine changes, nerves arrive, and the number falls while your legs are actually getting fresher. The top-voted answer in an r/ultrarunning thread on exactly this worry opens — "Before a race ignore it." Tell this tool a race is within seven days and it reads a low night in that light instead of as a warning.

The opposite case is the one people misread. In a randomised study of 21 trained male triathletes, the group pushed into functional overreaching ran 9.0% ± 2.1% worse on a maximal running test — while their weekly average Ln RMSSD rose rather than fell (Le Meur et al. 2013). A number climbing in the middle of a brutal block, with legs that feel dead, is not automatically good news. The finding is from trained triathletes, not recreational runners, but it is enough to stop you treating "higher" as "better".

That same study also settled how much one reading is worth: sampling HRV once a week detected nothing, and only the weekly average of daily recordings picked up the change. It matches the reproducibility data — day-to-day agreement for short resting HRV recordings came out at only about 0.5 to 0.6 (intraclass correlation) across 417 men (Kobayashi et al. 2012). That figure is for a few quiet minutes sitting still rather than a full night, but it is the clearest published number on how much a single reading wanders on its own.

So for race week: read the number as context, not a verdict, and let how you feel and your planned recovery drive the taper. Nothing in a single race-week reading tells you how you will run on Sunday.