Calculate your optimal heart rate zone for maximum fat oxidation.
Measure your pulse in the morning before getting out of bed. If unknown, leave blank.
Estimated Max Heart Rate:— bpm
Fat Burning Zone (60%):— bpm
Fat Burning Zone (70%):— bpm
Target Range: —
Understanding the Fat Burning Zone
The "Fat Burning Zone" refers to a specific physiological state during exercise where the body primarily utilizes stored fat as its main fuel source. While carbohydrates (glycogen) and fats are both burned during exercise, the ratio shifts depending on the intensity of your workout.
How It Works
Your body burns fuel based on oxygen availability. Fat oxidation requires more oxygen than carbohydrate metabolism. Therefore, lower intensity exercises—where you can breathe comfortably—allow your body to oxidize fat more efficiently.
Low Intensity (60-70% MHR): The "Fat Zone". Approximately 85% of calories burned come from fat, though total calorie burn is lower.
Moderate Intensity (70-80% MHR): The "Cardio Zone". Roughly 50% of calories come from fat, but total calorie burn per minute is higher.
High Intensity (80%+ MHR): Anaerobic training. Primarily burns carbohydrates/glycogen.
The Karvonen Formula vs. Standard Formula
This calculator employs two methods depending on your input. The standard method (220 minus age) provides a rough estimate. However, if you input your Resting Heart Rate (RHR), we use the Karvonen Formula. This is generally considered more accurate because it takes your specific fitness level into account via your Heart Rate Reserve (HRR).
Use a Monitor: A chest strap or reliable smartwatch is the best way to track real-time data.
The Talk Test: If you can carry on a conversation but cannot sing, you are likely in the fat burning or lower aerobic zone.
Consistency: For fat loss, duration matters more than intensity in this specific zone. Aim for 45–60 minutes of steady-state activity.
When to Adjust
As you get fitter, your resting heart rate will drop, and your heart becomes more efficient. Re-calculate your zones every few months to ensure you are training at the correct intensity for your current physiology.
function calculateFatZone() {
// 1. Get Inputs
var ageInput = document.getElementById('ageInput');
var rhrInput = document.getElementById('rhrInput');
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
// 2. Validation
if (isNaN(age) || age 120) {
alert("Please enter a valid age between 10 and 120.");
return;
}
// 3. Calculation Logic
var maxHR = 220 – age;
var minZone = 0;
var maxZone = 0;
var method = "standard";
// Check if RHR is provided and valid
if (!isNaN(rhr) && rhr > 30 && rhr < 120) {
method = "karvonen";
// Karvonen Formula
// HRR = MaxHR – RHR
// Target = (HRR * %) + RHR
var hrr = maxHR – rhr;
minZone = Math.round((hrr * 0.60) + rhr);
maxZone = Math.round((hrr * 0.70) + rhr);
} else {
// Standard Formula
// Target = MaxHR * %
minZone = Math.round(maxHR * 0.60);
maxZone = Math.round(maxHR * 0.70);
}
// 4. Output Results
document.getElementById('resMaxHR').innerHTML = maxHR + " bpm";
document.getElementById('resLow').innerHTML = minZone + " bpm";
document.getElementById('resHigh').innerHTML = maxZone + " bpm";
document.getElementById('rangeDisplay').innerHTML = minZone + " – " + maxZone + " BPM";
// Show results area
document.getElementById('results-area').style.display = 'block';
}