Heart rate training zones are one of the most useful frameworks in fitness — and one of the most misunderstood. I've seen people spend years doing the same moderate-intensity cardio and wonder why their fitness isn't improving. The answer is almost always zone distribution: too much time in the middle, not enough at the extremes. Here's what the zones actually mean and how to train them.
| Zone | % of Max HR | Feel | Primary Fuel | Purpose |
|---|---|---|---|---|
| Zone 1 | 50–60% | Very easy, could sing | Fat | Recovery, warm-up |
| Zone 2 | 60–70% | Easy, full sentences | Fat (mostly) | Aerobic base, fat burn |
| Zone 3 | 70–80% | Moderate, short sentences | Mixed | Aerobic endurance |
| Zone 4 | 80–90% | Hard, few words | Carbohydrates | Lactate threshold |
| Zone 5 | 90–100% | Maximum, can't speak | Carbohydrates | VO2 max, speed |
The standard formula is 220 − age. A 40-year-old has an estimated max HR of 180 bpm. This is a useful starting point, but it's an average — individual max HR can vary by ±10–12 bpm from the formula. If you've done an all-out sprint or a very hard interval session and your heart rate monitor showed a peak reading, that's closer to your true max.
The Karvonen method goes further by incorporating resting heart rate, giving you Heart Rate Reserve (HRR). Instead of calculating zones off raw max HR, you calculate them off the range between resting and max, which produces more personalised zone boundaries.
Karvonen formula: Target HR = ((Max HR − Resting HR) × Intensity %) + Resting HR
Example: 40-year-old with resting HR of 55, max HR of 180. For Zone 2 lower bound (60%): ((180−55) × 0.60) + 55 = 75 + 55 = 130 bpm. For Zone 2 upper bound (70%): ((180−55) × 0.70) + 55 = 87.5 + 55 = 143 bpm. Their Zone 2 is 130–143 bpm.
Zone 2 training has become one of the most talked-about topics in fitness, and for good reason. It's where the aerobic system develops — where your body builds mitochondria (the energy factories in muscle cells), improves fat oxidation efficiency, and creates the cardiovascular base on which all other fitness depends.
The problem is that Zone 2 feels too easy for most people. It's a pace where you could comfortably hold a conversation — which feels almost guilty when you're at the gym surrounded by people working harder. But this is the zone where elite endurance athletes spend roughly 70–80% of their training volume. "Polarised training" — lots of Zone 2, some Zone 4–5, and relatively little Zone 3 — is what most endurance research supports for long-term performance gains.
A good Zone 2 test: you should be able to speak in full sentences without gasping. If you can only say a few words before needing a breath, you've gone into Zone 3.
Zone 4 is where you push into uncomfortable territory — you're working hard enough that lactate is building in your muscles faster than it clears. Training at this intensity raises your lactate threshold, meaning you can sustain higher intensities before fatigue sets in. One 20–40 minute Zone 4 interval session per week (after a solid Zone 2 base is established) produces significant fitness gains.
Zone 5 is maximum effort — short, brutal intervals of 30 seconds to 4 minutes with full recovery between them. This is where VO2 max (the maximum rate of oxygen consumption) is developed. Very few athletes need more than one Zone 5 session per week, and many get results from one every 10–14 days. Doing too much Zone 5 without sufficient Zone 2 base leads to overtraining and stagnation.
"Grey zone" training — spending most of your time in Zone 3 — is the most common pattern among recreational exercisers. It feels productive (you're breathing hard, you're sweating) but it's too intense to provide the aerobic base benefits of Zone 2 and not intense enough to drive the adaptations of Zone 4–5. Many people would improve faster if they slowed down their easy days and worked harder on their hard days.
Enter your age and resting heart rate. We calculate all 5 zones using both the standard and Karvonen methods.
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