What is the fat-burning zone?
The 'fat-burning zone' is the heart rate range (typically 60-70% of HRmax) where the body uses a higher PROPORTION of calories from fat vs carbohydrates. At rest you burn ~85% from fat; during peak exercise ~15% from fat. The 60-70% zone is the sweet spot where fat oxidation is high AND you can sustain it for 30-60+ minutes. Important caveat: total fat burned per session can be higher in cardio (70-80%) and HIIT zones because total calories burned are much higher.
HRmax formulas - Tanaka vs Fox
Fox/Haskell (1971): HRmax = 220 - age. The classic formula, derived from small studies, widely taught but inaccurate for adults over 40 (underestimates HRmax). Tanaka, Monahan, Seals (2001): HRmax = 208 - 0.7 × age. Based on meta-analysis of 351 studies with 18,712 subjects across all adult ages - significantly more accurate. The calculator above offers both; Tanaka is the default and recommended.
The Karvonen heart-rate-reserve method
Adding your resting HR (RHR) lets the calculator use the Karvonen formula: Target HR = ((HRmax - RHR) × intensity%) + RHR. This uses your heart-rate RESERVE (the gap between rest and max) and gives more individualized zones - especially important for trained athletes with low RHR. A fit person with RHR 50 needs higher absolute target HRs to reach the same effort intensity as someone with RHR 75. Measure RHR first thing in the morning before getting out of bed, averaged over 3-5 days.
Which zone for which goal?
Weight loss / fat oxidation: Zone 2 (60-70%) for 45-60 min, 3-5x/week. Endurance / aerobic capacity: Zone 3-4 (70-85%) for 30-45 min intervals. Speed / lactate threshold: Zone 4 (80-90%) for 15-30 min. VO2max / peak power: Zone 5 (90-100%) for 1-5 min intervals with recovery. Beginners should start in Zone 1-2 and build a base over 8-12 weeks before adding higher-intensity work.
Limitations of HR-based zones
Heart rate can drift up during long efforts even at constant pace ('cardiac drift'), so target HRs may be lower late in long sessions. Caffeine, dehydration, heat, illness, sleep, and medications (especially beta blockers) all shift HR response. For wearable accuracy, chest straps are more reliable than wrist-based optical sensors. For elite training, use direct lactate or VO2 testing - HR zones are a useful approximation, not a replacement.
Frequently asked questions
Not necessarily. The 60-70% HRmax zone maximizes the PROPORTION of calories from fat, but higher-intensity training burns MORE total calories AND more fat in absolute terms. For weight loss, choose the intensity you can sustain consistently - 30 min of Zone 2 plus 10 min of HIIT often outperforms pure Zone 2 for time-efficient fat loss.
Tanaka (208 - 0.7 × age, 2001) is significantly more accurate based on a meta-analysis of 18,712 subjects, especially for adults over 40 where '220 - age' underestimates HRmax by 5-15 bpm. Use Tanaka by default. For precise zones, a graded exercise test under medical supervision is the only gold standard.
Karvonen uses your heart-rate RESERVE (HRmax - HRrest) instead of just HRmax. Formula: Target HR = ((HRmax - HRrest) × intensity%) + HRrest. This gives more individualized zones - especially for trained athletes with low resting HR who need higher absolute target HRs to reach the same effort intensity. Measure resting HR first thing in the morning.
Sources
- Age-predicted maximal heart rate revisited— Tanaka, Monahan, Seals, Journal of the American College of Cardiology, 2001;37(1):153-156
- Prediction of energy expenditure from heart rate monitoring during submaximal exercise— Keytel et al., Journal of Sports Sciences, 2005;23(3):289-297
- Physical Activity Guidelines for Americans (2nd edition)— US Department of Health and Human Services
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