Optional: Enter 0 if unknown. Used for Karvonen Formula.
What Is the Fat Burning Zone?
The "fat burning zone" refers to a specific physiological intensity where your body prioritizes burning fat stores for fuel over carbohydrates (glycogen). This typically occurs at a lower intensity than high-speed cardio or interval training.
Physiologically, this zone is generally found between 60% and 70% of your Maximum Heart Rate (MHR). Training in this zone allows you to sustain activity for longer durations, which is crucial for maximizing total fat oxidation.
How This Calculator Works
This tool utilizes two primary methods depending on the data you provide:
Standard Method: If you only provide your age, we calculate your Maximum Heart Rate (MHR) using the formula 220 – Age and then determine the 60-70% range. This is a general estimation suitable for most beginners.
Karvonen Formula: If you provide your Resting Heart Rate (RHR), we use the Karvonen method. This is significantly more accurate because it accounts for your cardiovascular fitness level by calculating your Heart Rate Reserve (HRR) before applying the intensity percentages.
Why Lower Intensity Burns More Fat
It may seem counterintuitive that working "less hard" burns more fat, but it comes down to energy substrate utilization:
Low Intensity (Zone 2): Oxygen is plentiful. Your body has time to break down fatty acids, which is a slow process but provides massive energy.
High Intensity (Zone 4/5): Energy demand is immediate. Your body switches to glucose (sugar/carbs) because it burns fast, even though it provides less total energy per molecule than fat.
How to Measure Your Resting Heart Rate
To get the most accurate result from this calculator, measure your RHR first thing in the morning before getting out of bed. Place two fingers on your wrist or neck, count the beats for 60 seconds, or use a fitness tracker/smartwatch for an average reading.
function calculateFatBurnZone() {
var ageInput = document.getElementById('fbcAge');
var rhrInput = document.getElementById('fbcRHR');
var resultDiv = document.getElementById('fbcResult');
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
// Validation
if (isNaN(age) || age 120) {
resultDiv.style.display = 'block';
resultDiv.innerHTML = 'Please enter a valid age between 10 and 120.';
return;
}
var mhr = 220 – age;
var lowerZone, upperZone, methodUsed;
// Check if RHR is provided and valid for Karvonen
if (!isNaN(rhr) && rhr > 30 && rhr < mhr) {
// Karvonen Formula
// Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR
var hrr = mhr – rhr; // Heart Rate Reserve
lowerZone = Math.round((hrr * 0.60) + rhr);
upperZone = Math.round((hrr * 0.70) + rhr);
methodUsed = "Karvonen Formula (Personalized based on Fitness Level)";
} else {
// Standard Formula
// Target Heart Rate = Max HR * %Intensity
lowerZone = Math.round(mhr * 0.60);
upperZone = Math.round(mhr * 0.70);
methodUsed = "Standard Age-Based Method";
}
var html = '
Your Results
';
html += '
';
html += '
Fat Burning Zone
';
html += '
' + lowerZone + ' – ' + upperZone + ' BPM
';
html += '
';
html += '
';
html += '
Estimated Max Heart Rate:' + mhr + ' BPM
';
html += '
Zone Intensity:60% – 70%
';
html += '
Calculation Method:' + methodUsed + '
';
html += '
';
html += '*Note: This range represents the "Zone 2" intensity where lipid oxidation (fat burning) is most efficient. Maintain this heart rate for 30-60 minutes for best results.';
resultDiv.style.display = 'block';
resultDiv.innerHTML = html;
}