About

Lipid oxidation (fat burning) is not linear. It peaks at a specific metabolic intensity known as Zone 2. Below this threshold, caloric expenditure is too low to be significant. Above it, the body recruits Type II muscle fibers and switches fuel sources from fatty acids to glycogen (carbohydrates) due to oxygen availability constraints. Precision is critical. The standard age-based formula is often inaccurate for individuals with varying fitness levels. This tool uses the Karvonen method, which incorporates Resting Heart Rate (RHR) to determine Heart Rate Reserve (HRR), ensuring the target zone aligns with your actual cardiovascular capacity.

Formulas

The Karvonen formula calculates target heart rate (THR) by utilizing the Heart Rate Reserve (HRR):

HRR = HR_max - HR_rest

THR = (HRR × Intensity) + HR_rest

We estimate Max HR using the Tanaka formula for better accuracy than 220-Age:

HR_max = 208 - (0.7 × Age)

Reference Data

Intensity Zone	% of HRR	Energy Source	Primary Benefit
Zone 1 (Recovery)	50% - 60%	Fat	Blood flow, recovery
Zone 2 (Fat Burn)	60% - 70%	Fat (Maximal)	Mitochondrial efficiency
Zone 3 (Aerobic)	70% - 80%	Fat & Glycogen	Cardiovascular endurance
Zone 4 (Threshold)	80% - 90%	Glycogen	Lactate tolerance
Zone 5 (VO2 Max)	90% - 100%	Glycogen / CP	Speed, Power

Frequently Asked Questions

Resting Heart Rate (RHR) is a proxy for cardiovascular fitness. A lower RHR indicates a stronger heart. Including it creates a "reserve" range that scales the intensity to your specific fitness level, preventing the target from being too easy for fit individuals or too hard for beginners.

Physiological adaptation for lipid oxidation typically requires sustained effort. Duration is more important than intensity in Zone 2. Sessions should generally last 45 to 90 minutes to exhaust immediate glycogen stores and maximize fat utilization.

Crossing the upper threshold (Ventilatory Threshold 1) shifts metabolism toward glycolysis. While you burn more calories per minute, the percentage of fat burned decreases, and fatigue accumulation increases, potentially hindering recovery for subsequent workouts.

Immediately after waking up, before getting out of bed or consuming caffeine. Take the average over 3-5 days for the most accurate baseline.