Find your 5 personalised training zones — from fat-burning Zone 2 to peak-effort Zone 5 — using the science-backed Karvonen formula.
Training zones divide the range between your resting heart rate and maximum heart rate into bands, each of which produces a different physiological response. By training in the right zone for your goal, you can maximise fat burning, build aerobic capacity, or develop peak power — rather than just working out at a vague "medium effort" every time.
Most exercise physiologists use a 5-zone model, ranging from Zone 1 (very light recovery activity) to Zone 5 (all-out effort). Each zone uses a different primary fuel source and causes different adaptations in the cardiovascular and muscular systems.
The Karvonen formula is more accurate than simple percentage-of-max-HR calculations because it accounts for your fitness level through resting heart rate (RHR). A well-trained athlete with a low RHR has a larger heart rate reserve (HRR), which shifts their zones differently to an untrained person with the same maximum HR.
The formula: Target HR = ((Max HR − RHR) × % intensity) + RHR
Maximum HR is estimated as 220 − age (the most widely used formula). Your HRR is Max HR minus your resting HR. Applying the target percentage to the HRR, then adding back resting HR, gives you a personalised zone range.
Zone 2 (roughly 60–70% of max HR) is often called the "fat-burning zone" because at this intensity, the primary fuel source shifts toward fat oxidation. While higher-intensity exercise burns more total calories per minute, Zone 2 maximises the percentage of those calories coming from fat.
For keto and low-carb athletes, Zone 2 training is particularly powerful. After fat adaptation (typically 4–12 weeks on a ketogenic diet), the body becomes far more efficient at burning fat at all intensities. Fat-adapted athletes can sustain Zone 2 intensity for hours using body fat as fuel, with minimal glycogen depletion.
Aim for 60–80% of your total training volume in Zone 2 (the "80/20 rule" from elite endurance training) for the best long-term aerobic development.
While Zone 2 is optimal for fat oxidation during exercise, Zone 4–5 training (high-intensity interval training, or HIIT) produces greater EPOC — excess post-exercise oxygen consumption — meaning your metabolic rate stays elevated for hours after training, burning additional calories.
For fat loss, the research suggests a hybrid approach: 2–3 Zone 2 sessions per week for aerobic base and fat oxidation, plus 1–2 HIIT sessions for metabolic rate elevation and Zone 5 adaptation. This combination, paired with a ketogenic diet for optimal insulin control, is a powerful protocol for body composition improvement.
During the first 2–4 weeks on a ketogenic diet, some people notice that their heart rate feels higher at the same effort level — this is normal during the "keto adaptation" phase as the body transitions its metabolic machinery from glucose to fat. Electrolyte depletion (especially sodium, potassium, and magnesium) can also contribute to elevated HR in the early weeks.
After full fat adaptation, many endurance athletes report improved Zone 2 endurance, lower heart rate at submaximal efforts, and better recovery. If you're starting keto alongside a new training programme, be patient with the first 4–6 weeks and consider electrolyte supplementation.
Check our food database for electrolyte-rich keto foods: leafy greens, avocado, salmon, and nuts are excellent sources. Use our Keto Macro Calculator to dial in your daily intake.