"Stay in Zone 2" has become gym-floor and running-app gospel, but most people using heart rate zones have no idea where their zones actually start and end, let alone why the number matters. Zones aren't arbitrary — they map directly to which fuel source and which physiological system your body is leaning on at a given intensity. Get them right and every workout has a clear purpose.
Step 1: Find Your Maximum Heart Rate
The most common estimate is the age-predicted formula:
For a 40-year-old, that's 180 bpm. It's not perfect — genetics, fitness level, and even the specific study population used to derive the formula create real variance — but it's a solid, widely-used starting point for structuring training zones.
The 5 Heart Rate Zones Explained
Once you have max HR, each zone is a percentage band of that number. Here's what a 40-year-old (Max HR = 180) looks like across all five zones:
| Zone | % of Max HR | BPM (age 40) | Feels Like |
|---|---|---|---|
| Zone 1 — Warm-up | 50-60% | 90-108 | Easy, conversational |
| Zone 2 — Fat burn | 60-70% | 108-126 | Comfortable, can talk in full sentences |
| Zone 3 — Cardio/Aerobic | 70-80% | 126-144 | Moderately hard, short sentences only |
| Zone 4 — Threshold | 80-90% | 144-162 | Hard, few words possible |
| Zone 5 — Peak/VO2 max | 90-100% | 162-180 | Maximum effort, unsustainable |
A More Personalized Option: The Karvonen Method
The 220-minus-age method ignores one thing: your resting heart rate, which reflects your actual cardiovascular fitness. Two 40-year-olds with the same max HR can have very different resting rates — a sedentary person might rest at 75 bpm while a trained runner rests at 50 bpm. The Karvonen method (also called heart rate reserve) accounts for this:
Example: a 40-year-old with Max HR 180 and Resting HR 60 wanting 70% intensity (Zone 2/3 border):
Target HR = ((180 − 60) × 0.70) + 60 = 144 bpm — noticeably different from the flat 70% of 180 (126 bpm) used in the simple method.
Fat Burn Zone Myth: Percentage vs. Total Calories
Zone 2 burns a higher percentage of calories from fat (roughly 60-70% fat-sourced) compared to Zone 4-5 (which leans more on carbohydrates). But Zone 4-5 burns significantly more total calories per minute. A 30-minute Zone 2 walk might burn 150 total calories with 100 from fat. A 30-minute Zone 4 run might burn 350 total calories with only 140 from fat — still more total fat calories burned despite the lower percentage. For fat loss specifically, weekly total calorie burn matters more than chasing a specific zone percentage.
How to Actually Measure It
Chest strap monitors (like Polar H10 or Garmin HRM) use electrical signals and are the gold standard for accuracy, especially during intervals or strength training. Wrist-based optical sensors on smartwatches are convenient and generally accurate within 5-10 bpm during steady cardio, but can lose accuracy during motion-heavy or high-intensity work due to how they detect blood flow through skin.