If you've been using your Apple Watch to guide Zone 2 training, there's a good chance you've been training in the wrong zone — and you wouldn't know it.
This isn't a dig at Apple. The Watch is a remarkable piece of technology. But there's a fundamental flaw in how it calculates your heart rate zones, and for anyone serious about Zone 2 training, it matters.
Apple Watch estimates your maximum heart rate (MHR) using the classic formula: 220 minus your age.
So if you're 45, it assumes your max heart rate is 175 bpm. Your Zone 2, calculated at 60–70% of that, would be roughly 105–122 bpm.
The problem: The 220-minus-age formula was derived from group data in the 1970s. It describes an average trend across large populations — not your individual physiology.
Studies show that for any given age, actual maximum heart rates vary by ±10–20 bpm from the formula's prediction. That's a huge range. A 45-year-old might have a true max heart rate anywhere from 155 to 195 bpm.
Apply that error to Zone 2 calculations and your "Zone 2" window on the Apple Watch could be off by 10–15+ beats per minute in either direction.
If your Zone 2 ceiling is set too high, you end up training in Zone 3 — the "grey zone" that's too hard to be truly aerobic but too easy to build VO2max. It feels productive. You're sweating. Your heart is pumping. But you're not getting the mitochondrial and fat-burning adaptations that make Zone 2 so powerful.
This is one reason people say "I've been doing lots of cardio and nothing's changing." They're not doing Zone 2. They're grinding in Zone 3 and wondering why the benefits aren't showing up.
Technically, yes — you can manually set a custom maximum heart rate in the Health app. But most people don't know what their true MHR is, and going out to find it (a genuine all-out effort to exhaustion) is neither safe nor enjoyable for most people over 40.
And even if you nail your MHR, Zone 2 still isn't purely about percentages. Factors like your current fitness, sleep quality, stress, and recovery status all affect where your aerobic threshold sits on any given day.
Real Zone 2 isn't a fixed heart rate number. It's a physiological state: primarily aerobic, below your first ventilatory threshold, where lactate remains low and your body is efficiently burning fat for fuel.
The gold standard for finding it? A lab lactate test. Accurate. Expensive. Not available to most people.
The practical alternative? Learning to identify the zone through a combination of heart rate data, perceived exertion, and — crucially — feedback over time.
Zone2AI takes a fundamentally different approach. Instead of locking you into a generic heart rate range, Zone2AI's AI coach learns how you respond to training. After each workout, it incorporates your feedback — how the effort felt, how your body responded — and continuously refines your personal Zone 2 window.
The result: your Zone 2 targets get smarter and more accurate the more you use the app. It's like having a coach who actually knows you, not one running the same playbook for every 45-year-old they've ever met.
Your Apple Watch zones are off by 10-15 bpm. Zone2AI replaces the formula with your real physiology — free to start.
Fix Your Zones Free →If you have an Apple Watch, the fastest fix is Zone2AI — an app built specifically to replace Apple's generic formula with your actual physiology. It calculates your real Zone 2 using your resting HR, max HR from your workout history, and refines it after every session based on how the effort felt. Free to start, works natively on Apple Watch.
If you want to calibrate manually in the meantime:
There's a second error that hits treadmill users specifically: Apple Watch has no idea you're running on an incline.
If you do Zone 2 on a treadmill at 6 km/h and 10% incline — which is one of the most effective Zone 2 protocols for beginners — your Apple Watch calculates your effort as if you're running flat. The incline isn't factored into its power or calorie estimates. Your heart is working significantly harder, but the Watch thinks you're jogging gently on a track.
This creates two problems. First, your calories burned are wildly underreported. Second, your effort profile doesn't match what your Watch expects for a "Zone 2 workout" — which can cause the zone calculation to drift even further from reality.
Zone2AI accounts for incline. Before each treadmill session, you enter your incline, and the app factors it into your effort calculation so your Zone 2 range reflects what you're actually doing to your body — not just your speed on a flat surface.
This question comes up constantly among beginners: "I'm running slowly, feel fine, breathing normally — but my Apple Watch says I'm in Zone 4 or 5. What's going on?"
Your Watch is wrong. Here's why: if the formula underestimates your true max heart rate — which it does for most people — it sets all your zones artificially low. So when you're genuinely in Zone 2, the Watch thinks that heart rate is "high" relative to its incorrect maximum, and flags it as Zone 4 or 5.
The rule: If you can speak in full, comfortable sentences, you're in Zone 1 or 2 — regardless of what any watch says. Your mouth knows more than the algorithm.
This is why accurate zone calibration matters so much. If your Watch tells you you're in Zone 5 when you're genuinely in Zone 2, you'll either panic and slow down (undertraining) or trust the label and feel confused when you never progress.
Your Apple Watch is a great fitness tool. Its Zone 2 calculation is not.
If Zone 2 training is part of your longevity plan — and if you're over 40, it should be — you need personalized heart rate zones, not a 50-year-old population average. The difference between training in your actual Zone 2 versus your watch's Zone 2 can mean the difference between seeing results in 3 months or wondering why nothing's changing after 6.
Zone2AI brings AI-powered coaching to your Apple Watch and gives you the personalized Zone 2 guidance your Watch can't. Start your first session free.
Download Zone2AI →