A heart rate of 150 beats per minute means nothing by itself. For one runner it's a hard tempo effort, for another it's a warm-up jog. What makes the number useful is context: how it compares to your own ceiling, and what training effect sitting there for 30 minutes actually produces. That's what heart rate zones are for, turning a raw bpm reading into something you can act on mid-run.

How zones get calculated

Zirv splits your heart rate range into five zones, each one a percentage band of your estimated maximum heart rate:

Zone 1: below 60% of max
Zone 2: 60 to 70%
Zone 3: 70 to 80%
Zone 4: 80 to 90%
Zone 5: 90% and above

To build those bands, Zirv estimates your max heart rate with the Tanaka formula: 208 - 0.7 x age. A 30 year old gets an estimated max of 187 bpm, and the five zones are carved out as percentages of that number.

Worth being honest about upfront: this is a population average, not a measurement of your actual max heart rate. It was derived from studying a wide range of people and fitting a formula to the trend, and any individual can sit meaningfully above or below what the formula predicts for their age. If you know your real max from an actual max-effort test, that number will always beat an age-based estimate. But for the vast majority of runners who have never deliberately tested a true max, the formula gives a reasonable starting point to build training zones around.

What each zone is actually for

The value of splitting the range into five bands is that each one corresponds to a different training stimulus, at the methodology level:

Zone 1 is recovery-level effort, low enough that it mostly supports recovery between harder days rather than building fitness on its own.
Zone 2 is the conversational aerobic base zone, easy enough to hold a conversation, and where a large share of endurance training volume typically lives.
Zone 3 is steady aerobic work, harder than easy running but still sustainable for a long stretch.
Zone 4 is threshold territory, an effort you can hold for a sustained but limited period before it becomes unsustainable.
Zone 5 is short, maximal effort, the kind of intensity you can only hold briefly.

The practical use of this is not picking a favorite zone, it's zone discipline: keeping easy runs actually easy, zone 1 to 2, instead of drifting up into zone 3 out of habit, so the hard days can actually be hard. That contrast between easy and hard is a big part of what makes a training week work as a whole, the same logic behind pairing hybrid training days deliberately rather than running at the same moderate effort every time.

Why the number on screen does not flicker

Heart rate is noisy in real time. It moves a few beats up and down constantly, and if a display recalculated the zone on every reading, you'd see it flip back and forth whenever your heart rate hovered near a boundary. Zirv smooths live readings and applies hysteresis to the displayed zone: it only switches to a new zone after your heart rate has moved roughly 2% of max heart rate past the boundary, not the instant it technically crosses the line. That gap is what keeps the display stable instead of flapping between two zones every few seconds.

That said, the smoothing is not there to hide real changes. If your heart rate jumps two or more zones at once, going from an easy jog straight into a hard sprint, the display switches immediately instead of waiting for the usual buffer. The delay exists to filter out noise at a boundary, not to make the display slow to react to something real.

Zirv also filters out readings that clearly are not real: anything outside 30 to 220 bpm gets rejected as sensor noise rather than shown as your current heart rate, and a reading older than 5 minutes stops being treated as current. Both are guardrails against a flaky sensor connection producing a number that would otherwise throw off the whole display.

The feedback loop after a run

Mid-run zone awareness is useful, but the bigger picture shows up after you finish. Zirv shows how long you spent in each zone for the run, a zone distribution rather than a single average number. That distribution is the real feedback loop: it tells you whether an "easy" run actually stayed easy, or whether a planned tempo effort mostly happened in zone 3 instead of zone 4. Over a training block, that's a more honest picture of what you actually did than pace alone, since pace varies with terrain and weather in a way that effort relative to your own max heart rate does not.

If you'd rather get some of that awareness live instead of only after the fact, zone-change announcements are part of what your tracker should say mid-run, currently available on iOS.

The takeaway

A raw heart rate number is just noise until it's placed against your own ceiling. Five percentage bands built from an age-based max estimate turn it into zones with an actual training meaning, from recovery to maximal effort. Smoothing and hysteresis keep the live display from flickering at boundaries without dulling its response to a real change, and the zone distribution after a run is where the number actually pays off, showing you what effort you truly put in rather than what pace alone would suggest.