HRV training
calculator

Enter a week of morning HRV readings (rMSSD, in milliseconds) and today's reading. The calculator builds your personal baseline, works out your decision band — the smallest worthwhile change — and tells you whether today lands inside it. The band is your own, not an age chart; and a number above it is a flag, not a gold star.

What this calculator answers

The question is "should today be hard or easy?", and the honest answer isn't a single HRV number — it's whether your reading has moved outside your own normal range. HRV has no universal zones. A fit adult can sit near 30 ms rMSSD while another sits near 120 ms, both healthy, so an "HRV by age" chart tells you almost nothing about you (Plews 2012). What works is your personal baseline and the smallest change worth acting on.

So the calculator does two things: it builds your baseline from a week of morning readings, and it draws a band around it — the smallest worthwhile change. Inside the band, train as planned. Below it, go easy. Above it usually means freshness, with one catch covered further down.

How the math works

HRV here is rMSSD — the root mean square of successive differences of your heartbeats, in milliseconds — the parasympathetic (recovery-side) metric that short morning readings are built to measure (Task Force 1996). Because rMSSD is right-skewed, the math runs on its natural log, ln rMSSD (Plews 2013). Your baseline is the 7-day rolling average of that; the band is half a standard deviation either side of it — the smallest worthwhile change (SWC), Hopkins' small-effect threshold (Hopkins 2000), applied to HRV by Plews and Buchheit.

One number that matters more than it looks: the decision quantity is the 7-day average, not this morning alone. A single reading bounces around far more than the SWC, so you act when the smoothed line crosses the band, not on one low morning. This is the part the red/amber/green apps flatten.

Worked example

A recreational runner logs seven mornings of rMSSD: 62, 58, 65, 60, 55, 63, 59 ms. rMSSD is a millisecond value, so there is no imperial-versus-metric version — a millisecond is a millisecond everywhere.

• Baseline: mean of ln rMSSD = 4.097759, which back-transforms to 60.2 ms (the geometric mean of the week).
• Day-to-day SD: 0.055952 in ln units, so the SWC = 0.5 × 0.055952 = 0.027976.
• Normal band: 4.069783 to 4.125735 in ln units — that is 58.5 to 61.9 ms. The coefficient of variation is 1.37%, a stable baseline.
• This morning, 44 ms: ln 3.784190, below the band — about −27% of the 60.2 ms baseline. The rule says easy or rest today.
• Contrast: a 60 ms morning lands inside the band (train as planned); a 68 ms morning lands above it (about +13%).

When this calculator is wrong

There are no universal HRV zones: the only usable threshold is your own smallest worthwhile change, and a lower number is not automatically worse. The consumer tools fail in two opposite ways — some print "normal HRV by age and sex" tables, others fire a red light off a fixed rule like "10% below your 7-day average." Both pretend HRV has population zones. It doesn't: between healthy people, resting HRV varies several-fold — one fit adult near 30 ms, another near 120 ms — so a group norm carries almost no signal about you (Plews 2012). The threshold the HRV-guided-training trials used is individual: the SWC, 0.5 × the SD of your own ln rMSSD. Timed that way, HRV guidance held its own — Vesterinen (2016) had the guided group improve 3,000 m speed 2.1% while doing fewer hard sessions (13.2 vs 17.7) than the fixed-schedule group.

• Higher isn't always better. At high training loads ln rMSSD can fall while vagal tone rises — parasympathetic saturation. In world-champion rowers, 3 of 4 showed rising parasympathetic activity despite falling ln rMSSD (Plews 2013/2017). So a reading above your band is flagged, not blessed, and a dropping number under heavy load can be a good response, not a warning.
• The counter-case is the healthy person tracking a stable baseline. There the ordinary reading holds: a rolling average sagging well below your band alongside poor sleep, heavy legs, or a raised resting heart rate is a genuine recover-today signal, and a baseline trending up over months tracks improving aerobic fitness. The individualised caution earns its keep against the fixed chart and the single-morning light — not against the trend.
• Measurement noise dwarfs the band. Posture, a late night, caffeine, alcohol, or a different device can move a single reading more than training does. One week fixes the baseline only loosely; the more data behind the rolling average, the more the band means.

What to do with the result

Use it to time hard days, not to grade yourself. Take the reading the same way every morning — on waking, lying or seated consistently, same strap or app — and watch the 7-day average. While it sits inside your band, run your plan: put the hard sessions where they were scheduled. When it drops below the band for more than a single morning, move the hard session and do easy aerobic work instead; when it's inside or above and the rest of the picture (sleep, legs, mood) is fine, that's a green light for intensity.

HRV is one input. Pair it with the resting heart rate calculator — a morning resting heart rate jumping above its own baseline is a second recovery flag — and the sleep debt calculator, since short sleep is one of the fastest ways to push HRV down. When your band says "go," the heart rate zones calculator and MAF calculator set the ceilings for the easy days and the intensity days alike.

Common questions

What is a good HRV number?

There isn't one, and that's the honest answer. Resting HRV varies several-fold between healthy people — a fit adult can sit near 30 ms rMSSD and another near 120 ms — so "good" only means good relative to your own baseline. A rising or stable personal baseline is the signal; a single number compared to a chart or a friend is noise.

Should I change training based on one low HRV reading?

No. Single readings are noisy — posture, sleep, caffeine and the device all move them more than training does. The decision quantity is your 7-day rolling average against your smallest worthwhile change (baseline ± 0.5 × SD of ln rMSSD). Act when the smoothed line crosses the band, not on one morning.

Is a higher HRV always better?

Usually, but not always. At high training loads ln rMSSD can fall even as parasympathetic (recovery) activity rises — parasympathetic saturation — which is why 3 of 4 world-champion rowers in Plews' data showed rising vagal activity despite falling ln rMSSD. So a number above your band is usually freshness, but a dropping number under heavy load isn't automatically a red flag.

How many days of readings do I need?

Seven is the working minimum for a rolling baseline, which is why this tool takes a week. Two readings will compute a band, but it's under-powered — the more consistent daily readings you feed it, the more stable the baseline and the more the band means. Take the reading the same way each morning so day-to-day differences reflect recovery, not method.

What is rMSSD, and why not SDNN or a "readiness score"?

rMSSD is the root mean square of successive differences between heartbeats — the time-domain metric that best reflects parasympathetic activity in a short at-rest recording (Task Force 1996), and the one most morning straps report. It's used in its natural-log form (ln rMSSD) because the raw value is skewed. A proprietary 0–100 "readiness score" is usually a rescaling of the same thing with the math hidden; this page shows the band so you can reproduce it.

Does HRV-guided training actually work?

The trials say it matches or beats a fixed plan. Kiviniemi (2007) was first to guide daily training by morning HRV; Vesterinen (2016) then ran 40 recreational runners and found the HRV-guided group improved 3,000 m running speed 2.1% versus 1.1% (non-significant) for the predefined group — while doing fewer hard sessions (13.2 vs 17.7). Placing hard days when you're ready beats forcing them on a calendar.