Most people who exercise have one speed: as hard as they can for as long as they can. Heart rate training replaces that with a smarter approach: different intensities produce different physiological adaptations, and knowing your zones lets you train with specific goals instead of just exhausting yourself.
Maximum Heart Rate — Where It All Starts
Every heart rate zone is calculated as a percentage of your maximum heart rate (MHR). The most common estimation formula is: MHR = 220 − age. A 35-year-old has an estimated MHR of 185 bpm. More accurate formulas exist (like Tanaka: 208 − (0.7 × age)) but the simple version is close enough for most training purposes.
💡 For more precision: Add your resting heart rate to the calculation using the Karvonen formula: Target HR = ((MHR − Resting HR) × Intensity%) + Resting HR. Someone with a resting HR of 60 bpm will have different zone numbers than someone with a resting HR of 50 bpm, even at the same age.
The Five Training Zones
- Zone 1 (50–60% MHR): Very light. Recovery, warm-up, cool-down. Could hold a conversation easily. Burns primarily fat.
- Zone 2 (60–70% MHR): Light. The fat-burning zone. Could talk in full sentences. Builds aerobic base. The zone most recreational exercisers undervalue.
- Zone 3 (70–80% MHR): Moderate. Cardio zone. Conversation is effortful. Improves cardiovascular efficiency.
- Zone 4 (80–90% MHR): Hard. Threshold zone. Can only speak a few words. Builds speed and lactate threshold.
- Zone 5 (90–100% MHR): Maximum effort. Unsustainable beyond 1–2 minutes. Builds peak power and VO2 max.
The "Fat Burning Zone" Myth
Zone 2 burns the highest percentage of calories from fat — which is why it's called the fat-burning zone. But Zone 4–5 burns more total calories per minute. The right zone depends on your goal. For long-duration endurance and sustainable weight loss: more Zone 2. For performance and maximum calorie burn in limited time: Zone 4. Research increasingly supports that most recreational athletes benefit from spending 80% of training in Zones 1–2 and 20% in Zones 4–5.
How to Check Your Zone During Exercise
A chest strap heart rate monitor is the most accurate option. Wrist-based monitors (Apple Watch, Garmin, Fitbit) are convenient and within 5–10% for most activities. Perceived exertion correlates well with zones once you're familiar: Zone 2 feels like you could sustain the effort for hours, Zone 4 feels like you're pushing hard but could hold it for 20–30 minutes.