Based on the Heart Rate Reserve (HRR) method used by watchOS
Heart Rate Reserve (Apple Standard)
0Max HR
0Resting HR
0HR Reserve
Zone
Intensity
Heart Rate Range (bpm)
Zone 5
Anaerobic / Peak
—
Zone 4
Threshold
—
Zone 3
Aerobic
—
Zone 2
Fat Burn / Endurance
—
Zone 1
Recovery
—
function calculateAppleZones() {
// Get Inputs
var ageInput = document.getElementById('calc_age').value;
var rhrInput = document.getElementById('calc_rhr').value;
var customMaxInput = document.getElementById('calc_max_hr').value;
var resultArea = document.getElementById('results_area');
// Validation
if (!ageInput && !customMaxInput) {
alert("Please enter your Age or a Custom Max Heart Rate.");
return;
}
if (!rhrInput) {
alert("Please enter your Resting Heart Rate. Apple uses the Heart Rate Reserve method which requires this.");
return;
}
var age = parseFloat(ageInput);
var rhr = parseFloat(rhrInput);
var maxHr = 0;
// Determine Max Heart Rate (MHR)
// If user provides custom, use it. Otherwise use 220 – Age (Standard fallback)
if (customMaxInput && parseFloat(customMaxInput) > 0) {
maxHr = parseFloat(customMaxInput);
} else {
maxHr = 220 – age;
}
// Safety check
if (rhr >= maxHr) {
alert("Resting Heart Rate cannot be higher than or equal to Maximum Heart Rate.");
return;
}
// Heart Rate Reserve Calculation (HRR)
// HRR = MaxHR – RestingHR
var hrr = maxHr – rhr;
// Apple Zone Logic (Based on Karvonen / HRR method typically used in watchOS 9+)
// Zone 1: 50% – 60%
// Zone 2: 60% – 70%
// Zone 3: 70% – 80%
// Zone 4: 80% – 90%
// Zone 5: 90% – 100%
var z1_low = Math.round((hrr * 0.50) + rhr);
var z1_high = Math.round((hrr * 0.60) + rhr);
var z2_low = z1_high + 1; // Start where Z1 ends
var z2_high = Math.round((hrr * 0.70) + rhr);
var z3_low = z2_high + 1;
var z3_high = Math.round((hrr * 0.80) + rhr);
var z4_low = z3_high + 1;
var z4_high = Math.round((hrr * 0.90) + rhr);
var z5_low = z4_high + 1;
var z5_high = Math.round(maxHr);
// Update DOM
document.getElementById('display_max_hr').innerText = maxHr;
document.getElementById('display_rhr').innerText = rhr;
document.getElementById('display_hrr').innerText = hrr;
document.getElementById('row_z1').innerText = z1_low + " – " + z1_high;
document.getElementById('row_z2').innerText = z2_low + " – " + z2_high;
document.getElementById('row_z3').innerText = z3_low + " – " + z3_high;
document.getElementById('row_z4').innerText = z4_low + " – " + z4_high;
document.getElementById('row_z5').innerText = z5_low + " – " + z5_high + "+";
// Show Results
resultArea.style.display = 'block';
}
How Does Apple Calculate Heart Rate Zones?
With the release of watchOS 9, Apple significantly updated how the Apple Watch tracks fitness intensity. While older versions relied heavily on standard generic formulas, the modern Apple Watch utilizes the Heart Rate Reserve (HRR) method as its default setting. This calculator replicates that logic to help you plan your training without needing to look at your wrist mid-run.
The Heart Rate Reserve (HRR) Method Explained
Unlike basic calculators that simply take a percentage of your maximum heart rate (e.g., "Zone 2 is 60-70% of Max HR"), Apple's algorithm accounts for your fitness level by including your Resting Heart Rate (RHR) in the equation.
The formula for Heart Rate Reserve is:
Max HR (MHR): Often calculated as 220 minus your age, or automatically detected by your watch during intense workouts.
Resting HR (RHR): The average number of times your heart beats per minute while you are at rest.
HRR: MHR – RHR.
By subtracting your resting rate from your max, Apple determines the total "reserve" beats you have available for exercise. Your zones are then calculated as percentages of this reserve, added back to your resting rate.
Apple's Zone Breakdowns
Once your Heart Rate Reserve is established, Apple divides your effort levels into five distinct zones:
Zone 1 (Recovery): 50% to 60% of HRR. Used for warm-ups and cooldowns.
Zone 2 (Aerobic Base): 60% to 70% of HRR. The "conversational pace" crucial for building endurance and burning fat.
Zone 3 (Aerobic): 70% to 80% of HRR. A moderate pace where training improves blood circulation and efficiency.
Zone 4 (Threshold): 80% to 90% of HRR. Hard effort where your body begins to produce lactic acid faster than it can clear it.
Zone 5 (Anaerobic): 90% to 100% of HRR. All-out effort sustainable for only very short periods.
Why Your Apple Watch Zones Change Automatically
One of the best features of the Apple Watch is that it updates your zones automatically on the first of every month. It does this by analyzing your health data:
If your Resting Heart Rate drops (indicating improved cardiovascular health), your Heart Rate Reserve increases, shifting your zones slightly.
If your recorded Max Heart Rate during a workout is higher than the age-estimated formula, the watch updates your MHR, which also recalibrates your zones.
If you prefer to use a manual Max HR or want to switch strictly to a "Percentage of Max Heart Rate" method (ignoring resting HR), you can modify these settings in the Watch app on your iPhone under Workout > Heart Rate Zones.