Entering your resting HR uses the Karvonen formula for higher accuracy.
Target Fat Burning Heart Rate
— BPM
Based on Max HR of — BPM
Keep your heart rate within this range for 30+ minutes to maximize fat oxidation.
function calculateFatBurn() {
var ageInput = document.getElementById('fbc-age').value;
var rhrInput = document.getElementById('fbc-rhr').value;
var resultBox = document.getElementById('fbc-results');
var targetZoneDisplay = document.getElementById('fbc-target-zone');
var mhrDisplay = document.getElementById('fbc-mhr-display');
// Validation
if (!ageInput || ageInput 120) {
alert("Please enter a valid age.");
return;
}
var age = parseFloat(ageInput);
var rhr = parseFloat(rhrInput);
var maxHR = 220 – age;
var lowerBound, upperBound;
// Check if RHR is provided and valid (Karvonen Method)
if (!isNaN(rhr) && rhr > 30 && rhr < 150) {
// Karvonen Formula: Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR
var hrr = maxHR – rhr; // Heart Rate Reserve
// Fat burn zone is typically 60% to 70% intensity
lowerBound = Math.round((hrr * 0.60) + rhr);
upperBound = Math.round((hrr * 0.70) + rhr);
} else {
// Simple Method (MHR Percentage)
// Fat burn zone is typically 60% to 70% of Max HR
lowerBound = Math.round(maxHR * 0.60);
upperBound = Math.round(maxHR * 0.70);
}
// Display Results
resultBox.style.display = "block";
targetZoneDisplay.innerHTML = lowerBound + " – " + upperBound + " BPM";
mhrDisplay.innerHTML = "Estimated Max Heart Rate: " + maxHR + " BPM";
}
What Should Your Heart Rate Be to Burn Fat?
Finding the optimal heart rate to burn fat is one of the most effective ways to maximize your workout efficiency. While high-intensity cardio burns calories quickly, your body relies on different energy sources depending on your exertion level. To target fat stores specifically, you need to train in the "Fat Burning Zone."
Understanding the Fat Burning Zone
The Fat Burning Zone is a specific range of heart rate intensity where your body primarily uses stored fat for fuel rather than carbohydrates (glycogen). This zone is generally lower in intensity than standard cardio training.
Intensity Level: Moderate
Heart Rate Range: 60% to 70% of your Maximum Heart Rate (MHR).
Feeling: You should be able to hold a conversation while exercising, though your breathing will be heavier than normal.
How the Calculation Works
This calculator utilizes two primary methods to determine your optimal zone:
The Standard Formula: This calculates your Maximum Heart Rate (220 minus your age) and takes 60-70% of that number. It is a general estimate suitable for most beginners.
The Karvonen Formula: If you input your Resting Heart Rate, the calculator switches to this more advanced method. It accounts for your cardiovascular fitness level by calculating your Heart Rate Reserve (HRR), providing a highly personalized target zone.
Why Lower Intensity Burns More Fat
It may seem counterintuitive, but working out harder doesn't always mean burning more fat percentage-wise. At lower intensities (Zone 2), your body has enough oxygen available to oxidize fat for energy. As intensity increases (Zone 3, 4, and 5), your body requires faster energy and switches to burning glycogen (carbs) because it is a quicker fuel source than fat.
Tips for Success
Duration Matters: Since the intensity is lower, try to maintain this heart rate for at least 30 to 45 minutes. It takes time for the body to switch into full fat-oxidation mode.
Consistency: Aim for 3-4 sessions per week in this zone.
Monitor: Use a smartwatch or chest strap to ensure you stay within the calculated BPM range. If you go too high, you switch to cardio; too low, and you aren't stimulating change.