function calculateHRZones() {
var ageInput = document.getElementById('ageInput');
var rhrInput = document.getElementById('rhrInput');
var resultDiv = document.getElementById('result');
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
// Validation
if (isNaN(age) || age 120) {
resultDiv.style.display = 'block';
resultDiv.innerHTML = 'Please enter a valid age between 10 and 120.';
return;
}
// Standard Max Heart Rate Formula
var maxHR = 220 – age;
// Determine method: Standard vs Karvonen
var useKarvonen = !isNaN(rhr) && rhr > 30 && rhr < 100;
var methodText = useKarvonen ? "Karvonen Formula (Using Resting HR)" : "Standard Maximum Heart Rate Formula";
// Calculation Logic
var zones = [];
var percentages = [
{ min: 0.50, max: 0.60, name: "Zone 1", desc: "Recovery", color: "zone-1" },
{ min: 0.60, max: 0.70, name: "Zone 2", desc: "Aerobic", color: "zone-2" },
{ min: 0.70, max: 0.80, name: "Zone 3", desc: "Tempo", color: "zone-3" },
{ min: 0.80, max: 0.90, name: "Zone 4", desc: "Threshold", color: "zone-4" },
{ min: 0.90, max: 1.00, name: "Zone 5", desc: "Anaerobic", color: "zone-5" }
];
for (var i = 0; i < percentages.length; i++) {
var p = percentages[i];
var minVal, maxVal;
if (useKarvonen) {
// Target HR = ((Max HR − Resting HR) × %Intensity) + Resting HR
var hrr = maxHR – rhr;
minVal = Math.round((hrr * p.min) + rhr);
maxVal = Math.round((hrr * p.max) + rhr);
} else {
// Target HR = Max HR * %Intensity
minVal = Math.round(maxHR * p.min);
maxVal = Math.round(maxHR * p.max);
}
zones.push({ name: p.name, desc: p.desc, range: minVal + " – " + maxVal + " BPM", color: p.color });
}
// Build HTML Output
var html = '
Your Heart Rate Zones
';
html += 'Estimated Max HR: ' + maxHR + ' BPM';
html += 'Method: ' + methodText + ";
html += '
';
html += '
Zone
Type
Target Range
';
html += '';
for (var j = 0; j < zones.length; j++) {
html += '
';
html += '
' + zones[j].name + '
';
html += '
' + zones[j].desc + '
';
html += '
' + zones[j].range + '
';
html += '
';
}
html += '
';
if (!useKarvonen) {
html += 'Tip: Enter your Resting Heart Rate for more accurate results matching Apple Watch\'s Heart Rate Reserve setting.';
}
resultDiv.innerHTML = html;
resultDiv.style.display = 'block';
}
Does Apple Watch Calculate Heart Rate Zones? (And How to Verify Them)
A common question among new Apple Watch users and fitness enthusiasts is: "Does the Apple Watch calculate heart rate zones automatically?"
The short answer is Yes. Starting with watchOS 9, Apple introduced native Heart Rate Zones within the Workout app. The watch automatically calculates these zones using your health data, specifically relying on the Heart Rate Reserve (HRR) method if your Resting Heart Rate and Maximum Heart Rate are available in the Health app.
However, understanding how these zones are calculated is crucial for training effectively. You can use the calculator above to verify your zones or manually configure them in your watch settings if the automatic detection feels off.
How Apple Watch Calculates Zones
By default, Apple Watch uses the Heart Rate Reserve method. This is generally considered more accurate than the basic "220 minus age" formula because it accounts for your fitness level via your resting heart rate.
The Calculation Logic:
Max Heart Rate (MHR): Usually estimated based on age, or determined automatically by your watch during high-intensity workouts.
Resting Heart Rate (RHR): The average number of times your heart beats per minute while you are at rest.
If you have not worn your watch enough to establish a resting heart rate, or if you manually switch the settings, it may default to the standard Max Heart Rate method.
Understanding the 5 Heart Rate Zones
Whether you are looking at your Apple Watch while running or using the calculator above, here is what the zones represent:
Zone 1 (50-60%): Recovery. Very light effort. Used for warm-ups, cool-downs, and active recovery. You can easily hold a conversation.
Zone 2 (60-70%): Aerobic. Light effort. This is the "fat burning" zone where you build endurance. It should feel sustainable for long periods.
Zone 3 (70-80%): Tempo. Moderate effort. Breathing becomes heavier. This zone improves blood circulation and skeletal muscle efficiency.
Zone 4 (80-90%): Threshold. Hard effort. You are nearing your lactate threshold. Muscles begin to tire, and breathing is labored. Sustainable for shorter bursts.
Zone 5 (90-100%): Anaerobic. Maximum effort. Used for sprinting or very short intervals. Cannot be sustained for long.
How to View and Edit Zones on Apple Watch
To see your zones during a workout, you must have watchOS 9 or later installed. While in a workout (like Outdoor Run), turn the Digital Crown to scroll through the workout views until you see the Heart Rate Zone screen.
To Edit Zones Manually:
Open the Watch app on your iPhone.
Tap on the My Watch tab.
Scroll down and select Workout.
Tap Heart Rate Zones.
Select Manual to enter the specific BPM ranges calculated by the tool above.
Frequently Asked Questions
1. Why are my Apple Watch zones different from the calculator?
Apple updates your Max Heart Rate automatically on the first of every month based on your activity. If the calculator above uses a generic "220 – Age" formula and your actual max HR is higher or lower, the zones will differ. Input your specific Resting Heart Rate for closer accuracy.
2. Should I use Manual or Automatic zones?
For most users, Automatic is best as it adjusts as you get fitter (and your resting heart rate drops). However, elite athletes often prefer Manual configuration based on laboratory VO2 Max testing.
3. How do I know my Resting Heart Rate?
Open the Health app on your iPhone, browse to "Heart", and select "Resting Heart Rate" to see your average over the last week.