This range represents the "sweet spot" where your body primarily uses stored fat for fuel.
Understanding Your Fat Burn Zone
Finding the optimal heart rate for burning fat is a key component of an effective weight loss strategy. While high-intensity workouts burn more calories overall, training in specific lower-intensity zones encourages the body to utilize fat stores as its primary energy source rather than glycogen (carbohydrates).
How This Calculator Works
This calculator determines your target heart rate zone using two different methods depending on the data you provide:
Standard Method (Age Only): Uses the classic 220 minus age formula to estimate Maximum Heart Rate (MHR), then calculates 60-70% of that figure. This provides a general baseline suitable for most beginners.
Karvonen Formula (Age + Resting Heart Rate): This is the "Gold Standard" for accuracy. It calculates your Heart Rate Reserve (HRR) by subtracting your resting heart rate from your max. It then applies the intensity percentage to the reserve and adds the resting rate back in. This accounts for your individual fitness level.
Physiologically, your body switches fuel sources based on exercise intensity:
Fat Burn Zone (60-70% MHR): This low-to-moderate intensity zone allows the body to effectively oxidize fat because sufficient oxygen is available. It is ideal for long-duration cardio sessions like brisk walking, light jogging, or steady cycling.
Cardio Zone (70-80% MHR): Improves cardiovascular fitness and endurance. You burn more total calories per minute here, but a lower percentage comes from fat.
Peak Zone (80%+ MHR): High-intensity interval training (HIIT). Primarily burns glycogen.
Tips for Maximizing Fat Loss
To get the most out of your fat burn zone training:
Duration Matters: Since the intensity is lower, aim for longer sessions (45-60 minutes) to burn a significant amount of total energy.
Consistency: Low-impact steady-state cardio can often be performed more frequently than HIIT without risking overtraining.
Measure Correctly: For the most accurate results, measure your Resting Heart Rate (RHR) first thing in the morning before getting out of bed.
function calculateFatBurnZone() {
// 1. Get input values
var ageInput = document.getElementById('inputAge').value;
var rhrInput = document.getElementById('inputRHR').value;
// 2. Validate inputs
if (ageInput === "" || isNaN(ageInput)) {
alert("Please enter a valid age.");
return;
}
var age = parseFloat(ageInput);
// Basic validation for realistic age
if (age 120) {
alert("Please enter a realistic age between 10 and 120.");
return;
}
// 3. Calculate Max Heart Rate (MHR)
// Standard formula: 220 – Age
var mhr = 220 – age;
var lowerBound = 0;
var upperBound = 0;
var methodUsed = "";
// 4. Logic branching: Karvonen vs Standard
if (rhrInput !== "" && !isNaN(rhrInput)) {
var rhr = parseFloat(rhrInput);
// Validation for RHR
if (rhr = mhr) {
alert("Please check your Resting Heart Rate. It should be between 30 and your Max Heart Rate.");
return;
}
// Karvonen Formula
// Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR
var hrr = mhr – rhr; // Heart Rate Reserve
lowerBound = (hrr * 0.60) + rhr;
upperBound = (hrr * 0.70) + rhr;
methodUsed = "Karvonen Formula (More Accurate)";
} else {
// Standard Percentage Formula
lowerBound = mhr * 0.60;
upperBound = mhr * 0.70;
methodUsed = "Standard Age-Based Estimate";
}
// 5. Display Results
document.getElementById('resMHR').innerHTML = Math.round(mhr) + " bpm";
document.getElementById('resMethod').innerHTML = methodUsed;
// Format the zone range
var zoneString = Math.round(lowerBound) + " – " + Math.round(upperBound) + " bpm";
document.getElementById('resZone').innerHTML = zoneString;
// Show result box
document.getElementById('resultsArea').style.display = "block";
// Smooth scroll to results on mobile
document.getElementById('resultsArea').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}