Best measured in the morning before getting out of bed.
Your Optimal Fat Burning Zone
Max Heart Rate: BPM
function calculateFatBurnZone() {
// Get input values
var ageInput = document.getElementById('inputAge');
var rhrInput = document.getElementById('inputRHR');
var resultBox = document.getElementById('resultBox');
var resultValue = document.getElementById('resultValue');
var resultDetails = document.getElementById('resultDetails');
var displayMaxHR = document.getElementById('displayMaxHR');
// Parse values
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
// Validation
if (isNaN(age) || age 110) {
alert("Please enter a valid age between 10 and 110.");
return;
}
if (isNaN(rhr) || rhr 200) {
alert("Please enter a valid resting heart rate (usually between 40 and 100 BPM).");
return;
}
// Calculation Logic (Karvonen Formula)
// 1. Calculate Maximum Heart Rate (MHR) = 220 – Age
var maxHeartRate = 220 – age;
// 2. Calculate Heart Rate Reserve (HRR) = MHR – Resting Heart Rate
var heartRateReserve = maxHeartRate – rhr;
// 3. Calculate Fat Burn Zone (Typically 60% to 70% intensity)
var minIntensity = 0.60;
var maxIntensity = 0.70;
// Karvonen Formula: Target HR = (HRR * %Intensity) + RHR
var minZone = Math.round((heartRateReserve * minIntensity) + rhr);
var maxZone = Math.round((heartRateReserve * maxIntensity) + rhr);
// Display Results
resultBox.style.display = 'block';
resultValue.innerHTML = minZone + " – " + maxZone + " BPM";
resultDetails.innerHTML = "Maintain this heart rate for optimal fat oxidation.";
displayMaxHR.innerHTML = maxHeartRate;
}
Understanding Your Fat Loss Heart Rate Zone
When it comes to exercise, working harder isn't always better for fat loss. To maximize the amount of fat your body burns as fuel, you need to train at a specific intensity known as the "Fat Burning Zone." This calculator uses the Karvonen formula to determine your specific target heart rate range based on your age and resting heart rate.
What is the Fat Burning Zone?
The fat burning zone is generally considered to be 60% to 70% of your heart rate reserve (HRR). At this lower intensity (often called Zone 2 training), your body relies primarily on fat stores for energy rather than carbohydrates (glycogen).
The Physiology: Oxygen is required to burn fat. When you exercise at a very high intensity (anaerobic), you cannot intake enough oxygen to metabolize fat quickly enough, so your body switches to burning carbohydrates. By keeping your heart rate in the "aerobic" zone, you ensure maximum fat oxidation.
How the Calculation Works (Karvonen Method)
While many gym machines use a generic formula (220 minus age multiplied by a percentage), this calculator uses the Karvonen Method. This is superior because it factors in your Resting Heart Rate, making it customized to your fitness level.
Step 3: Calculate the 60% and 70% thresholds based on the reserve, then add the resting heart rate back in.
How to Use This Information
Once you have calculated your range (e.g., 125 – 138 BPM), follow these steps to maximize fat loss:
Duration is Key: Because the intensity is lower, you should aim for longer sessions. Aim for 30 to 60 minutes of continuous activity.
Monitor Your HR: Use a smart watch, chest strap, or the pulse sensors on cardio machines to ensure you stay strictly within your calculated numbers.
Consistency: Perform this type of steady-state cardio 3-4 times per week.
Don't Exceed the Limit: If you go above the upper limit of your zone, you transition into cardio-endurance training. While excellent for heart health, you begin burning a higher percentage of carbs and a lower percentage of fat.
Measuring Your Resting Heart Rate
For the most accurate results, measure your resting heart rate (RHR) immediately after waking up in the morning, before you sit up in bed or drink caffeine. Do this for three days and take the average.