Leave blank for standard calculation. Enter for Karvonen precision.
Karvonen Method (Best Accuracy)
Standard (220 – Age)
Max Heart Rate (MHR):0 bpm
Heart Rate Reserve:0 bpm
Target Intensity:60% – 70%
Your Fat Burning Zone
0 – 0 bpm
function calculateFatBurn() {
var ageInput = document.getElementById('fb_age').value;
var rhrInput = document.getElementById('fb_rhr').value;
var method = document.getElementById('fb_formula').value;
// Validation
if (!ageInput || ageInput 0) {
// Karvonen Formula: Target = ((MHR – RHR) * %Intensity) + RHR
var hrr = mhr – rhr;
// Fat burning zone is typically Zone 2 (60-70% intensity)
lowerBpm = (hrr * 0.60) + rhr;
upperBpm = (hrr * 0.70) + rhr;
hrrDisplay = Math.round(hrr) + " bpm";
} else {
// Standard Percentage Formula: MHR * %Intensity
// Note: If user selected Karvonen but didn't enter RHR, we fallback to standard
lowerBpm = mhr * 0.60;
upperBpm = mhr * 0.70;
if (method === 'karvonen' && (isNaN(rhr) || rhr <= 0)) {
hrrDisplay = "Enter RHR for Reserve data";
}
}
// Update UI
document.getElementById('res_mhr').innerHTML = Math.round(mhr) + " bpm";
document.getElementById('res_hrr').innerHTML = hrrDisplay;
document.getElementById('res_zone').innerHTML = Math.round(lowerBpm) + " – " + Math.round(upperBpm) + " bpm";
document.getElementById('fb_results').style.display = "block";
}
Understanding Your Fat Burning Heart Rate
Calculating your specific fat burning heart rate is one of the most effective ways to optimize your cardio sessions for weight loss. Unlike high-intensity training which burns glycogen (sugar), training in the "fat burning zone" relies primarily on oxidizing fat stores for energy. This calculator helps you define that precise physiological window.
What is the Fat Burning Zone?
The fat burning zone is generally defined as an aerobic exercise intensity between 60% and 70% of your maximum heart rate. At this specific intensity, your body works hard enough to require significant energy but not so hard that it switches purely to glucose for fuel. This allows your metabolic system to utilize free fatty acids as the primary fuel source.
The Formulas: Standard vs. Karvonen
This calculator offers two methods to determine your optimal beats per minute (BPM):
Standard Method: This simply takes percentages of your theoretical maximum heart rate (220 minus your age). It is a good baseline estimate for beginners.
Karvonen Method: This is the gold standard for accuracy. It incorporates your Resting Heart Rate (RHR) to calculate your Heart Rate Reserve. By accounting for your current fitness level (via your RHR), the fat burning zone calculation becomes much more personalized and effective.
Why RHR Matters
Your Resting Heart Rate is a strong indicator of cardiovascular health. A lower RHR typically indicates a stronger heart and better fitness. Using the Karvonen formula ensures that as you get fitter and your RHR drops, your target training zones adjust accordingly to keep you progressing.
How to Use These Numbers
Once you have your range (e.g., 125 – 145 bpm), aim to keep your heart rate within this window for the duration of your "Zone 2" cardio sessions. Effective fat-burning workouts typically last between 45 to 60 minutes. Use a smartwatch or chest strap monitor to track your pulse in real-time during exercise.