Leave blank to use the standard age-based formula. Enter value to use the Karvonen formula.
Maximum Heart Rate (MHR):— bpm
Target Fat Burning Zone (60-70%):— bpm
function calculateFatBurnZone() {
// 1. Get input values
var ageInput = document.getElementById('userAge');
var rhrInput = document.getElementById('restingHeartRate');
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
// 2. Validate Age
if (isNaN(age) || age 120) {
alert("Please enter a valid age between 10 and 120.");
return;
}
// 3. Calculate Maximum Heart Rate (Standard Formula: 220 – Age)
var maxHeartRate = 220 – age;
var lowerBound = 0;
var upperBound = 0;
var methodText = "";
// 4. Determine Calculation Method
// If Resting Heart Rate is provided and valid, use Karvonen Formula
if (!isNaN(rhr) && rhr > 30 && rhr < 150) {
// Heart Rate Reserve (HRR) = Max HR – Resting HR
var heartRateReserve = maxHeartRate – rhr;
// Target = (HRR * Intensity%) + Resting HR
// Fat burn zone is typically 60% to 70% intensity
lowerBound = (heartRateReserve * 0.60) + rhr;
upperBound = (heartRateReserve * 0.70) + rhr;
methodText = "Calculation based on the Karvonen Formula, which accounts for your resting heart rate for higher accuracy.";
} else {
// Standard Percentage Method (MHR * Intensity%)
lowerBound = maxHeartRate * 0.60;
upperBound = maxHeartRate * 0.70;
methodText = "Calculation based on standard Percentage of Maximum Heart Rate. For a more personalized result, enter your resting heart rate.";
}
// 5. Update the UI
document.getElementById('displayMHR').innerHTML = Math.round(maxHeartRate) + " bpm";
document.getElementById('displayZone').innerHTML = Math.round(lowerBound) + " – " + Math.round(upperBound) + " bpm";
document.getElementById('methodNote').innerHTML = methodText;
// Show results
document.getElementById('resultContainer').style.display = 'block';
}
How to Calculate Your Heart Rate for Fat Burning
Finding the efficient "sweet spot" for exercise can significantly improve your ability to burn fat. While High-Intensity Interval Training (HIIT) burns calories quickly, exercising at a moderate intensity—known as the Fat Burning Zone—optimizes the body's ability to use fat stores as its primary fuel source.
What is the Fat Burning Zone?
The fat burning zone is a heart rate range where your body relies primarily on fat for energy rather than carbohydrates (glycogen). This zone is typically defined as 60% to 70% of your Maximum Heart Rate (MHR).
When you exercise within this range, you are working at a moderate intensity. You should feel like you are working, but you should still be able to carry on a conversation (often called the "talk test"). If you are gasping for air, you have likely pushed into the cardio or aerobic zone, which burns more glycogen than fat.
The Math Behind the Calculation
There are two primary ways to calculate your target heart rate zones. Our calculator above automatically selects the best method based on the data you provide.
1. The Standard Method (Age-Predicted)
This is the most common method used in gyms and on cardio machines. It estimates your Maximum Heart Rate (MHR) based solely on your age.
Formula: MHR = 220 – Age
Fat Burn Low End: MHR × 0.60
Fat Burn High End: MHR × 0.70
Example: For a 40-year-old:
MHR = 220 – 40 = 180 bpm
Fat Burn Zone = 108 to 126 bpm
2. The Karvonen Formula (Heart Rate Reserve)
This method is considered more accurate for individuals with varying fitness levels because it incorporates your Resting Heart Rate (RHR). It calculates the Heart Rate Reserve (HRR), which is the difference between your max and resting heart rates.
By factoring in your resting heart rate, this formula tailors the zone to your specific cardiovascular health. A lower resting heart rate usually indicates better fitness, and the formula adjusts the target zone upward accordingly.
How to Measure Your Resting Heart Rate
To get the most accurate result from the calculator above, you should input your Resting Heart Rate. Here is how to measure it:
Timing: The best time to measure is in the morning, right after waking up, before you get out of bed or drink coffee.
Pulse Point: Place your index and middle fingers on your wrist (radial artery) or the side of your neck (carotid artery).
Count: Count the beats for 60 seconds, or count for 30 seconds and multiply by 2.
Average: For the best data, measure this over 3 days and take the average.
Frequently Asked Questions
Will I lose weight if I only exercise in the fat burning zone?
Yes, provided you are in a calorie deficit. While the percentage of fat burned is higher in this zone, total calories burned are lower compared to high-intensity exercise. Consistency and duration are key when training in the fat burning zone.
How long should I stay in the fat burning zone?
Because the intensity is lower (moderate), you can sustain this activity for longer periods. Most fitness experts recommend sessions lasting between 45 to 60 minutes for optimal fat utilization.
What if I exceed the upper limit of the zone?
If you go above 70% of your max heart rate, you enter the aerobic or cardio zone. You are still burning calories—often more per minute—but the body shifts to burning a higher percentage of glycogen (carbs) and a lower percentage of fat.