body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 1200px;
margin: 0 auto;
padding: 20px;
background-color: #f9f9f9;
}
.calculator-wrapper {
background: #ffffff;
border-radius: 12px;
box-shadow: 0 4px 20px rgba(0,0,0,0.1);
padding: 30px;
margin-bottom: 40px;
border: 1px solid #e0e0e0;
}
.calc-title {
text-align: center;
color: #2c3e50;
margin-bottom: 25px;
font-size: 28px;
font-weight: 700;
}
.input-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
margin-bottom: 25px;
}
@media (max-width: 600px) {
.input-grid {
grid-template-columns: 1fr;
}
}
.input-group {
display: flex;
flex-direction: column;
}
.input-group label {
font-weight: 600;
margin-bottom: 8px;
color: #555;
}
.input-group input {
padding: 12px;
border: 2px solid #ddd;
border-radius: 8px;
font-size: 16px;
transition: border-color 0.3s;
}
.input-group input:focus {
border-color: #e74c3c;
outline: none;
}
.input-hint {
font-size: 12px;
color: #888;
margin-top: 4px;
}
.calc-btn {
width: 100%;
padding: 15px;
background-color: #e74c3c;
color: white;
border: none;
border-radius: 8px;
font-size: 18px;
font-weight: bold;
cursor: pointer;
transition: background-color 0.3s;
margin-bottom: 25px;
}
.calc-btn:hover {
background-color: #c0392b;
}
.results-container {
background-color: #fdf2f1;
border: 1px solid #fadbd8;
border-radius: 8px;
padding: 20px;
display: none;
}
.result-row {
display: flex;
justify-content: space-between;
align-items: center;
margin-bottom: 15px;
padding-bottom: 15px;
border-bottom: 1px solid #fadbd8;
}
.result-row:last-child {
border-bottom: none;
margin-bottom: 0;
padding-bottom: 0;
}
.result-label {
font-weight: 600;
color: #555;
}
.result-value {
font-weight: 800;
font-size: 20px;
color: #e74c3c;
}
.content-section {
background: #fff;
padding: 30px;
border-radius: 12px;
box-shadow: 0 2px 10px rgba(0,0,0,0.05);
}
h2 {
color: #2c3e50;
border-bottom: 2px solid #e74c3c;
padding-bottom: 10px;
margin-top: 30px;
}
h3 {
color: #34495e;
margin-top: 25px;
}
p, li {
color: #555;
font-size: 17px;
}
.highlight-box {
background-color: #e8f6f3;
border-left: 5px solid #1abc9c;
padding: 15px;
margin: 20px 0;
}
function calculateFatBurn() {
var ageInput = document.getElementById('calcAge');
var rhrInput = document.getElementById('calcRHR');
var resultsArea = document.getElementById('resultsArea');
var resMHR = document.getElementById('resMHR');
var resRange = document.getElementById('resRange');
var resMethod = document.getElementById('resMethod');
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
if (isNaN(age) || age 120) {
alert("Please enter a valid age between 10 and 120.");
return;
}
// Standard Max Heart Rate Formula (Fox Formula)
var mhr = 220 – age;
var lowerBound = 0;
var upperBound = 0;
var methodText = "";
// Check if Resting Heart Rate is provided to use Karvonen Formula
if (!isNaN(rhr) && rhr > 30 && rhr < 150) {
// Karvonen Formula
// Target HR = ((MHR – RHR) * %Intensity) + RHR
var hrr = mhr – rhr; // Heart Rate Reserve
lowerBound = (hrr * 0.60) + rhr;
upperBound = (hrr * 0.70) + rhr;
methodText = "Karvonen Formula (More Accurate)";
} else {
// Standard Percentage of MHR
// Target HR = MHR * %Intensity
lowerBound = mhr * 0.60;
upperBound = mhr * 0.70;
methodText = "Standard MHR Percentage";
}
// Display Results
resultsArea.style.display = "block";
resMHR.innerHTML = Math.round(mhr) + " BPM";
resRange.innerHTML = Math.round(lowerBound) + " – " + Math.round(upperBound) + " BPM";
resMethod.innerHTML = methodText;
}
How to Calculate Your Fat Burning Zone Heart Rate
Understanding your specific heart rate zones is one of the most effective ways to optimize your workout for weight loss. The "Fat Burning Zone" refers to a specific intensity level where your body prioritizes oxidizing fat stores for energy rather than burning readily available sugars (glycogen). This calculator helps you define that physiological sweet spot.
What is the Fat Burning Zone?
The fat burning zone typically occurs at a lower intensity, specifically between 60% and 70% of your Maximum Heart Rate (MHR). At this pace, the body has enough oxygen available to break down fat cells efficiently. As intensity increases (above 70-80%), the body switches to anaerobic metabolism, burning carbohydrates because they can be converted to energy faster than fat.
Key Insight: While higher intensity workouts (like HIIT) burn more calories overall per minute, the percentage of calories coming from fat is highest in the lower-intensity fat burning zone. This makes it ideal for endurance sessions and sustainable weight loss.
The Mathematics of Heart Rate Training
There are two primary ways to calculate your zones, both of which are supported by this calculator:
1. The Standard Method (Fox Formula)
This is the simplest estimation and is widely used in general fitness.
- Step 1: Calculate Maximum Heart Rate (MHR) = 220 – Age.
- Step 2: Calculate 60% of MHR (Lower Limit).
- Step 3: Calculate 70% of MHR (Upper Limit).
Example: For a 40-year-old, MHR is 180 BPM. The fat burning zone is 108 to 126 BPM.
2. The Karvonen Formula (Heart Rate Reserve)
This method is considered more accurate, especially for individuals who are fit or have a lower resting heart rate, because it accounts for your baseline cardiovascular health.
- Step 1: Determine MHR (220 – Age).
- Step 2: Measure Resting Heart Rate (RHR) upon waking.
- Step 3: Calculate Heart Rate Reserve (HRR) = MHR – RHR.
- Step 4: Apply the formula:
(HRR × Intensity %) + RHR.
Practical Example
Let's look at a realistic scenario for a 30-year-old individual with a resting heart rate of 60 BPM.
- Max Heart Rate: 220 – 30 = 190 BPM.
- Heart Rate Reserve: 190 – 60 = 130 BPM.
- Lower Limit (60%): (130 × 0.60) + 60 = 138 BPM.
- Upper Limit (70%): (130 × 0.70) + 60 = 151 BPM.
Using the Karvonen method, this individual should aim to keep their heart rate between 138 and 151 BPM during their steady-state cardio sessions to maximize fat oxidation.
How to Measure Your Heart Rate
To use these numbers effectively, you need to monitor your pulse during exercise. You can do this by:
- Wearable Tech: Smartwatches and chest straps provide real-time data.
- Manual Check: Place two fingers on your radial artery (wrist) or carotid artery (neck), count the beats for 10 seconds, and multiply by 6.
Conclusion
While the "Fat Burning Zone" is excellent for metabolizing fat, remember that total calorie deficit is the ultimate driver of weight loss. Combining zone-based cardio with strength training and proper nutrition yields the best long-term results.