The fat-burning zone is a specific heart rate range where fat provides a disproportionately high percentage of energy during exercise. Training in this zone for sustained periods builds your aerobic base and maximises fat oxidation efficiency.
This calculator gives you your personalised fat-burning zone plus all 5 training zones, with the option to use the more accurate Karvonen formula if you know your resting heart rate.
The Science of the Fat-Burning Zone
At 60–70% max HR (Zone 2), your body relies predominantly on fat oxidation for fuel — approximately 80% of energy from fat, 20% from carbohydrates. At higher intensities, the ratio shifts toward carbohydrates. However, total fat calories burned per session is often higher at moderate intensity (70–80%) because total energy expenditure is greater, even though fat percentage is lower.
Zone 2 Training Benefits Beyond Fat Burning
Zone 2 cardio (conversational pace where you can speak in full sentences) builds mitochondrial density — the number and efficiency of cellular energy factories. This improves fat oxidation capacity, aerobic efficiency, and recovery. Elite endurance athletes spend 70–80% of training time in Zone 2. It's the foundation of all other training zones.
HIIT vs Zone 2 for Fat Loss
Both have a place. Zone 2 builds the aerobic base that makes all exercise more efficient and burns fat during the session. HIIT produces a significant 'afterburn' effect (EPOC — excess post-exercise oxygen consumption) and burns more total calories in less time. The optimal approach for fat loss combines both: 2–3 Zone 2 sessions plus 1–2 HIIT sessions per week.
How to Measure Heart Rate During Exercise
A chest strap heart rate monitor (Polar, Garmin) gives the most accurate readings — within 1–2 bpm of an ECG. Wrist-based monitors (smartwatches, fitness trackers) are less accurate during high-intensity exercise due to movement artefact — fine for Zone 2, potentially 10–20 bpm off during Zone 4–5. For precise zone training, invest in a chest strap.
💡 Expert Tips
- ✓Zone 2 should feel like a 'comfortably uncomfortable' pace — you should be able to hold a conversation but not sing.
- ✓Add at least 20–30 minutes of Zone 2 cardio 3× per week before adding higher-intensity work — build the aerobic base first.
- ✓Track heart rate during Zone 2 sessions to ensure you stay in zone — it's surprisingly easy to drift into Zone 3, which changes the physiological stimulus.
- ✓As your fitness improves, you'll be able to go faster at the same heart rate — this is a direct sign of improved fat-burning efficiency.
- ✓Fasted Zone 2 cardio (before breakfast) slightly increases fat oxidation — though total daily fat balance matters more than timing.
📊 Heart Rate Training Zones and Their Benefits
| Zone | % Max HR | RPE | Primary Fuel | Key Benefit |
|---|---|---|---|---|
| Zone 1 | 50–60% | Very easy | Fat (90%) | Recovery, warm-up |
| Zone 2 | 60–70% | Easy | Fat (80%) | Fat burning, aerobic base |
| Zone 3 | 70–80% | Moderate | Mixed (50/50) | Aerobic endurance |
| Zone 4 | 80–90% | Hard | Carbs (85%) | Lactate threshold, speed |
| Zone 5 | 90–100% | Max | Carbs (95%) | VO₂ max, peak power |
Frequently Asked Questions
What is the fat-burning heart rate zone?+
Does Zone 2 cardio burn more fat than HIIT?+
How do I know if I'm in my fat-burning zone?+
Should beginners start with Zone 2 training?+
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