Half Marathon Heart Rate Calculator

Half Marathon Heart Rate Training Zones

Calculate your personalized training zones using the Karvonen formula based on your age and resting heart rate.

Best measured first thing in the morning.

If left blank, we will estimate using the Tanaka formula (208 – 0.7 * age).

function calculateHMHeartRates() { var ageInput = document.getElementById('hmAge').value; var rhrInput = document.getElementById('hmRHR').value; var knownMhrInput = document.getElementById('hmKnownMHR').value; var resultDiv = document.getElementById('hmHRResult'); var age = parseFloat(ageInput); var rhr = parseFloat(rhrInput); var knownMhr = parseFloat(knownMhrInput); if (isNaN(rhr) || rhr 120) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid Resting Heart Rate (typically between 30-120 bpm).'; return; } var mhr; var mhrSource; if (!isNaN(knownMhr) && knownMhr > rhr && knownMhr 10 && age < 100) { // Tanaka Formula for estimating MHR mhr = 208 – (0.7 * age); mhrSource = "Estimated (Tanaka Formula)"; } else { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid Age or a known Maximum Heart Rate.'; return; } mhr = Math.round(mhr); // Heart Rate Reserve (Karvonen method base) var hrr = mhr – rhr; // Calculate Zones (Karvonen Formula: (HRR * %) + RHR) var z1_low = Math.round((hrr * 0.50) + rhr); var z1_high = Math.round((hrr * 0.60) + rhr); var z2_low = Math.round((hrr * 0.60) + rhr) + 1; var z2_high = Math.round((hrr * 0.70) + rhr); var z3_low = Math.round((hrr * 0.70) + rhr) + 1; var z3_high = Math.round((hrr * 0.80) + rhr); var z4_low = Math.round((hrr * 0.80) + rhr) + 1; var z4_high = Math.round((hrr * 0.90) + rhr); var z5_low = Math.round((hrr * 0.90) + rhr) + 1; var resultHTML = '

Your Training Profile

'; resultHTML += '

'; resultHTML += '
• Resting HR: ' + rhr + ' bpm
'; resultHTML += '
• Maximum HR: ' + mhr + ' bpm (' + mhrSource + ')
'; resultHTML += '
• Heart Rate Reserve: ' + hrr + ' bpm
'; resultHTML += '

'; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += ''; resultHTML += '

Zone	Intensity (%)	Range (BPM)	Typical Training Use
Z1 Recovery	50-60%	' + z1_low + ' – ' + z1_high + '	Warmup, cool-down, active recovery runs.
Z2 Aerobic Base	60-70%	' + z2_low + ' – ' + z2_high + '	Long runs, easy mileage. Builds endurance.
Z3 Tempo / Sweet Spot	70-80%	' + z3_low + ' – ' + z3_high + '	Primary Half Marathon Pace zone for many runners. Comfortably hard.
Z4 Threshold	80-90%	' + z4_low + ' – ' + z4_high + '	Threshold runs. Often race pace for experienced half marathoners.
Z5 VO2 Max	90-100%	' + z5_low + ' – ' + mhr + '	Short intervals, final race kick. Very hard effort.

'; resultDiv.innerHTML = resultHTML; resultDiv.style.display = 'block'; }

Why Train by Heart Rate for a Half Marathon?

Training for a half marathon (13.1 miles or 21.1 km) requires a balance of building endurance and increasing lactate threshold speed. While many runners rely solely on pace per mile, training by heart rate offers a more physiological approach that accounts for daily variations in fatigue, weather, and terrain.

By monitoring your heart rate, you ensure that easy days remain truly easy, allowing for recovery, and hard days reach the necessary intensity to spur adaptation. This calculator uses the Karvonen method, considered more accurate for athletes than standard age-based formulas because it incorporates your Resting Heart Rate (RHR) to determine your Heart Rate Reserve (HRR).

Understanding Your Key Metrics

To get the most out of the Half Marathon Heart Rate Calculator above, it helps to understand the inputs:

• Resting Heart Rate (RHR): Your heart's beats per minute when fully relaxed. The best time to measure this is immediately upon waking up, before getting out of bed. A typical adult RHR is between 60-100 bpm, though trained runners often see values between 40-60 bpm.
• Maximum Heart Rate (MHR): The highest number of beats per minute your heart can achieve during maximal effort. If you do not know this from a lab test or a recent maximal field test, the calculator estimates it using the Tanaka formula (208 – 0.7 × Age), which is generally more accurate for active adults than the old "220 minus age" rule.

Applying These Zones to Half Marathon Training

Once you have calculated your zones, you can structure your training plan effectively. Successful half marathon training isn't about running hard every day.

For most runners, the bulk of weekly mileage should be run in Zone 2 (Aerobic Base). This builds the capillary networks and mitochondrial density necessary for running 13.1 miles efficiently.

Your specific half marathon race pace will likely fall into Zone 3 (Tempo) or lower Zone 4 (Threshold), depending on your experience level. Beginners may race mostly in Zone 3, while competitive runners often sustain Zone 4 efforts for the duration of the race. The calculator highlights these zones to help you identify your target race intensity.