Your heart rate zones
for the Norwegian 4×4

Zone boundaries update as you type. Nothing is sent anywhere — the arithmetic runs in your browser.

The arithmetic, in full

Two steps. First an estimate of your maximum heart rate, then the zones underneath it.

Step 1 — the maximum

The Tanaka formula, from a 2001 meta-analysis of 351 studies, replaced the older 220 − age. It is flatter, and it is noticeably kinder to people over fifty:

At 25 the two formulas differ by about 4 bpm. At 60 they differ by 6 bpm in the other direction — 220 − age under-reads an older athlete's maximum, which pushes every zone down with it.

If Apple Health holds a VO₂ Max estimate, the app tries a second route to the same number:

It keeps that answer only when it lands within ±12% of the age estimate. Outside that band the two disagree too much for either to be trusted, so age wins. In practice the VO₂ Max route only survives the check for a narrow range of fitness levels — which is the point of having the check.

Step 2 — the zones

Zones are cut from your heart rate reserve — the room between resting and maximum — not from a raw percentage of the maximum:

That is the Karvonen method. Zone 1 ends at 60% of the reserve, zone 2 at 70%, zone 3 at 80%, zone 4 at 90%, and zone 5 runs to your maximum.

What counts as “in zone” during an interval

The interval target is zone 4 and up. The app counts every second at or above the bottom of zone 4 — 80% of your reserve — with no upper limit, because a four-minute interval that drifts into zone 5 is a good interval, not a failed one.

For most people that threshold lands somewhere around 85–88% of maximum heart rate. The original Norwegian trials prescribe 90–95% of maximum, which is a little higher. The gap is deliberate: a fixed percentage of maximum quietly demands more of someone with a low resting heart rate than of someone with a high one, and the first minute of an interval is spent climbing anyway. Reaching the top of zone 4 — 90% of reserve — puts you squarely inside the classic band.

The one thing that does not work is guessing. Four minutes at what feels like hard is routinely twenty beats short, and twenty beats is the difference between the session that raises VO₂ Max and the one that only makes you tired.

Getting the inputs right

• Resting heart rate matters more than age here. A wrong resting rate shifts every boundary; a wrong age shifts them by a beat or two. Apple Health averages yours over time — use that number, not a reading taken while sitting at a desk.
• Age formulas are estimates. Individual maximum heart rates scatter roughly ±10 bpm around any formula. If you have ever seen a genuine maximum on a watch during an all-out effort, put it in the fourth box.
• Recalculate when your resting rate moves. Eight weeks of 4×4 will lower it, and lower resting means a wider reserve and higher zone boundaries. The app re-reads it from Health; this page does not remember anything between visits.

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