Zone 2 heart rate training is the "sweet spot" for building an aerobic base and improving mitochondrial efficiency. It is typically defined as 60% to 70% of your maximum heart rate. At this intensity, your body primarily uses fat as fuel and develops the metabolic flexibility needed for long-distance endurance.
How to Calculate Zone 2 Heart Rate
There are two primary methods for calculating your Zone 2 range:
The Age-Based Method (Fox Formula): A simple calculation: 220 minus your age gives your estimated Maximum Heart Rate (MHR). Zone 2 is then 60-70% of that number.
The Karvonen Formula (HRR): This is more accurate as it accounts for your fitness level by including your Resting Heart Rate (RHR). It calculates your Heart Rate Reserve (MHR – RHR) and applies the percentages to that value before adding the RHR back in.
Example Calculation (Age 40, RHR 60)
Using the Karvonen Formula:
Max HR: 220 – 40 = 180 bpm
Heart Rate Reserve (HRR): 180 – 60 = 120 bpm
60% Level: (120 x 0.60) + 60 = 132 bpm
70% Level: (120 x 0.70) + 60 = 144 bpm
Zone 2 Range: 132 – 144 bpm
Benefits of Zone 2 Training
Benefit
How it helps
Mitochondrial Health
Increases the number and efficiency of mitochondria in muscle cells.
Fat Oxidation
Trains the body to burn fat more efficiently at higher intensities.
Lactate Clearance
Improves the body's ability to clear lactate, delaying fatigue.
Faster Recovery
Builds a cardiovascular system that recovers more quickly between hard efforts.
The "Talk Test"
A simple way to verify you are in Zone 2 without a heart rate monitor is the talk test. In Zone 2, you should be able to maintain a full conversation without gasping for air, though you should sound slightly more breathless than when at rest. If you can only speak in short sentences, you have likely drifted into Zone 3.
function calculateZoneTwo() {
var age = document.getElementById("z2_age").value;
var rhr = document.getElementById("z2_rhr").value;
var resultBox = document.getElementById("z2_results");
var rangeDisplay = document.getElementById("z2_range_display");
var lowDisplay = document.getElementById("z2_low");
var highDisplay = document.getElementById("z2_high");
var methodNote = document.getElementById("z2_method_note");
if (!age || age 115) {
alert("Please enter a valid age.");
return;
}
var mhr = 220 – age;
var z2Low, z2High;
if (rhr && rhr > 30 && rhr < 130) {
// Karvonen Formula (More accurate)
var hrr = mhr – rhr;
z2Low = Math.round((hrr * 0.60) + parseInt(rhr));
z2High = Math.round((hrr * 0.70) + parseInt(rhr));
methodNote.innerText = "Calculated using the Karvonen Formula (includes Resting HR for accuracy).";
} else {
// Standard Age-based Formula
z2Low = Math.round(mhr * 0.60);
z2High = Math.round(mhr * 0.70);
methodNote.innerText = "Calculated using the Standard Age-Based Formula.";
}
rangeDisplay.innerText = z2Low + " – " + z2High + " BPM";
lowDisplay.innerText = z2Low + " BPM";
highDisplay.innerText = z2High + " BPM";
resultBox.style.display = "block";
// Smooth scroll to results
resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}