The two methods
The simple one takes a percentage of your maximum heart rate and stops there:
The Karvonen method takes the same percentage of your heart rate reserve — the room between your resting rate and your maximum — and adds your resting rate back on:
The second one is doing something the first cannot: it treats the beats you spend just staying alive as a floor rather than as training.
The same person, both ways
A 40-year-old with a resting heart rate of 55. Tanaka puts the maximum at 180, so the reserve is 125 beats.
| Intensity | % of maximum | Karvonen (% of reserve) | Gap |
|---|---|---|---|
| 60% | 108 bpm | 130 bpm | 22 bpm |
| 70% | 126 bpm | 143 bpm | 17 bpm |
| 80% | 144 bpm | 155 bpm | 11 bpm |
| 90% | 162 bpm | 168 bpm | 6 bpm |
| 100% | 180 bpm | 180 bpm | — |
The two methods converge at the top and diverge at the bottom. That matters, because the easy end is where an easy session gets prescribed — and 108 bpm is a brisk walk, not a warm-up.
Where it bites: two people, one age
Take two forty-year-olds. Both have a maximum near 180. One is trained and rests at 45; the other is starting out and rests at 75.
| Trained · resting 45 | Untrained · resting 75 | |
|---|---|---|
| Heart rate reserve | 135 beats | 105 beats |
| 80% of maximum | 144 bpm | 144 bpm |
| 80% of maximum, as % of reserve | 73% | 66% |
| 80% of reserve (Karvonen) | 153 bpm | 159 bpm |
By percentage of maximum, both are told to run at 144. But that single number sits at 73% of one person's reserve and 66% of the other's — the untrained runner is being asked for less relative effort, which is the opposite of what a zone is supposed to guarantee. Karvonen puts them at 153 and 159, and the harder number goes to the person with less room to work in.
So what about “90–95% of maximum”?
The Norwegian 4×4 trials describe the interval target as 90–95% of maximum heart rate, and that phrasing has been copied into every article since. It is a real number from real research — it is just stated in the other currency.
Converted for the 40-year-old above, 90% of maximum is 162 bpm, which is 86% of their reserve. NTRVL starts counting an interval as in-zone at 80% of reserve — 155 bpm — and keeps counting all the way up. So the app opens the gate slightly earlier than the literature does, then rewards you for climbing past it.
That is a deliberate choice, and worth stating plainly rather than hiding: the first minute of a four-minute interval is spent getting your heart rate up, and a threshold that only starts counting at 90% of maximum throws away work that was genuinely hard. The top of zone 4 — 90% of reserve — puts you inside the classic band with room to spare.
The catch nobody mentions
Both methods rest on an estimated maximum, and individual maximums scatter roughly ±10 bpm around any age formula. A percentage of a guess is still a guess.
What the reserve method adds is a second real measurement — your resting heart rate — which you actually have, which your watch tracks for free, and which changes as you get fitter. That last part is the quiet advantage: after eight weeks of intervals your resting rate drops, your reserve widens, and your zones move up on their own. A percentage of maximum would have you running the same numbers in week eight as in week one.
What NTRVL does
Karvonen reserve zones over a Tanaka maximum, recalculated from whatever Apple Health currently knows about your resting heart rate — and, if you have measured your maximum for real, an override that replaces the estimate entirely. During an interval the app tells you which side of the line you are on, live, and afterwards what share of the four minutes landed there.
Download NTRVL — freeFirst session free and complete. No trial to cancel.