Target Heart Rate Calculator

Understanding Heart Rate Zones

This calculator uses the Karvonen formula, which accounts for your heart rate reserve (HRR = max HR − resting HR). Zone 1 (50–60% HRR) is active recovery. Zone 2 (60–70% HRR) is the aerobic base — conversational, sustainable effort. Zone 3 (70–80% HRR) feels comfortably hard. Zone 4 (80–90% HRR) is threshold — you can sustain it for 20–60 minutes. Zone 5 (90–100% HRR) is maximum effort, sustainable only in short bursts. Most training plans target 80% of volume in zones 1–2 and 20% in zones 4–5 — the "80/20" polarized approach backed by extensive endurance research.

Zone 2 Training: The Aerobic Base

Zone 2 is where the most important long-term adaptations happen: mitochondrial density increases, fat oxidation efficiency improves, and cardiac stroke volume grows. These adaptations make every higher-intensity workout more productive. Elite endurance athletes spend 75–80% of training in zone 2. The problem is that most recreational athletes train too hard — they push into zone 3, which feels like work but doesn't produce the same adaptive stimulus. A useful check: if you can't hold a full conversation, you're likely above zone 2. Drop the pace until you can speak in complete sentences without effort.

How to Measure Your Resting Heart Rate

Resting heart rate (RHR) should be measured before getting out of bed, after at least 5 minutes lying still. Check for 60 seconds with two fingers on your neck (carotid) or wrist (radial). A heart rate monitor worn overnight is the most accurate consumer method. Average adult RHR is 60–100 bpm; trained athletes often sit at 40–60 bpm due to larger stroke volume. RHR trending upward by 5+ beats over a few days can indicate illness, overtraining, or poor sleep — it's one of the earliest signs of physiological stress before other symptoms appear.

The MAF Method

Phil Maffetone's Maximum Aerobic Function (MAF) method sets a training cap at 180 minus your age in bpm — for a 35-year-old, that's 145 bpm. The rationale is that staying below this threshold keeps you fully aerobic and prevents chronic cortisol elevation that accompanies hard training. MAF practitioners run exclusively below this threshold for 3–6 months, often seeing pace improve dramatically at the same heart rate as their aerobic base builds. It's slower and humbling initially — but the method has genuine support among ultra-endurance coaches and athletes who prioritize longevity and sustainable performance over short-term results.