“Run it in zone 2” is now one of the most common instructions in endurance training, and one of the least precise. On the usual five-zone scale, zone 2 is the band between 60 and 70 % - but 60 to 70 % of what is where two honest calculators part ways.
Two methods, two ranges
The percentage-of-maximum method takes 60–70 % of your maximum heart rate. The heart rate reserve method, from Karvonen’s 1957 study, takes 60–70 % of the gap between your resting and maximum heart rate and adds the resting rate back. Because the reserve method starts from your resting pulse, it always gives higher numbers. Here is zone 2 by age for a runner with a resting heart rate of 60, maximum estimated with the Tanaka formula:
| Age | Est. max HR | Zone 2, % of max | Zone 2, reserve |
|---|---|---|---|
| 20 | 194 | 117–135 | 141–153 |
| 30 | 187 | 113–130 | 137–148 |
| 40 | 180 | 108–126 | 132–144 |
| 50 | 173 | 104–121 | 128–139 |
| 60 | 166 | 100–116 | 124–134 |
| 70 | 159 | 96–111 | 120–129 |
A 40-year-old gets 108–126 bpm one way and 132–144 bpm the other. Neither is wrong; they describe the same intention with different arithmetic. What matters is using one method consistently - the one your plan or coach assumed.
The weak link is the maximum
Both methods need a maximum heart rate, and an age formula can be more than ten beats off for an individual. That error moves every zone. If you have a measured maximum from a stress test or a hard field test, enter it in the heart rate zone calculator and it replaces the estimate everywhere.
Checking it on the road
Zone 2 should feel easy and conversational. If the number says zone 2 but you cannot finish a sentence, trust your breathing and slow down; if the number says zone 3 on a hot day while the effort feels easy, heat and dehydration are raising your pulse. Lab tests that locate your first lactate or ventilatory threshold give the most precise zone 2, but for most runners a consistent method plus the talk test is enough.
If you take medication that affects heart rate, such as beta-blockers, formula zones do not apply to you - agree your limits with your doctor.