Heart rate zone training is one of the simplest concepts in fitness. It is also one of the most misunderstood.
The idea is straightforward: different heart rate ranges produce different physiological adaptations. Train at a low heart rate and you build your aerobic base. Train at a high heart rate and you push your anaerobic threshold. Each zone has a purpose. Each zone trains something different.
The problem is that most people default to the same intensity every session — a moderate effort that feels productive but is actually too hard for aerobic development and too easy for meaningful intensity work. They live in the "grey zone," getting some benefit from everything but maximising nothing.
Understanding heart rate zones fixes this. It gives you a framework for training with purpose. Every session has a target. Every workout serves a specific goal.
Here is how heart rate zones work, what each one does, and how to use them to train smarter.
How to Calculate Your Max Heart Rate
Heart rate zones are expressed as percentages of your maximum heart rate (MHR). To use zones, you need to know — or at least estimate — your max.
The Simple Formula: 220 Minus Age
The most commonly cited formula: MHR = 220 - your age. A 30-year-old gets an estimated MHR of 190 BPM. A 50-year-old gets 170 BPM.
This formula is easy to remember and roughly right for large populations. But it has a standard deviation of about 10-12 BPM, which means your actual max could be 10-12 BPM higher or lower than the formula predicts. Knowing your resting heart rate is also important, as it forms the baseline from which your zones are calculated. For a 30-year-old, that is a range of 178 to 202 BPM — wide enough to significantly affect zone calculations.
Better Formulas
Several alternatives have been developed:
- Tanaka formula: MHR = 208 - (0.7 x age). Slightly more accurate across age groups.
- Gulati formula (for women): MHR = 206 - (0.88 x age). Developed from a female-only study population.
These are improvements but still estimates.
The Best Method: Find It Empirically
The most accurate approach is to discover your max heart rate during a structured test. This typically involves a graduated exercise protocol — progressive increases in intensity until you reach your absolute maximum.
A simple field test: after a thorough warm-up (10-15 minutes), perform 3-4 uphill intervals of 2-3 minutes at maximum effort with brief recovery between them. Your peak heart rate during the final intervals should approximate your true MHR.
Important: Maximal heart rate tests are intense. If you have cardiovascular concerns or are new to exercise, consult a doctor before attempting one.
A Note on Resting Heart Rate
Some training methodologies use heart rate reserve (HRR) rather than simple percentage of max. HRR is calculated as: zone percentage x (MHR - resting heart rate) + resting heart rate. This Karvonen method accounts for fitness level and is considered more personalised. However, the percentage-based zones described below are the most commonly used and easier to apply.
The Five Heart Rate Zones
Zone 1: Recovery (50-60% of Max HR)
What it feels like: Very easy. You can hold a full conversation without any breathlessness. Walking, very light jogging, gentle cycling.
What it trains: Active recovery. This zone promotes blood flow to muscles without adding meaningful stress. It helps clear metabolic waste products from previous training sessions.
When to use it: Rest days where you want to move rather than be completely sedentary. Cool-downs after intense sessions. Recovery walks between hard training days.
Common mistake: Dismissing Zone 1 as "not real exercise." Active recovery in Zone 1 genuinely enhances recovery between hard sessions. It is not wasted time.
Zone 2: Aerobic Base (60-70% of Max HR)
What it feels like: Comfortable. You can talk in full sentences, though it requires a bit more effort than Zone 1. You might be slightly warm and breathing a bit deeper than at rest.
What it trains: This is the zone getting the most attention right now, and for good reason.
Zone 2 is where your body primarily uses fat as fuel. It develops your aerobic engine — the mitochondrial density in your muscles, the capillary networks that deliver oxygen, and the efficiency of your fat oxidation pathways.
The benefits of consistent Zone 2 training include:
- Increased mitochondrial density and function
- Improved fat oxidation (your body gets better at burning fat for fuel)
- Enhanced cardiovascular efficiency (lower resting heart rate over time)
- Better endurance at all intensity levels
- Improved metabolic health and insulin sensitivity
- Foundation for higher-intensity work
When to use it: The majority of your training volume. Endurance athletes often spend 70-80% of their training time in Zone 2. Even for non-endurance athletes, 2-3 Zone 2 sessions per week of 30-60 minutes builds a cardiovascular base that supports all other training.
Common mistake: Going too hard. True Zone 2 feels almost too easy. If you cannot hold a conversation, you are above Zone 2. Many people think they are training in Zone 2 but are actually in Zone 3 because they do not want to "waste time" going slow.
Zone 3: Tempo (70-80% of Max HR)
What it feels like: Moderately hard. You can speak in short sentences but not hold a sustained conversation. Breathing is noticeably elevated. Comfortable but purposeful.
What it trains: Improved aerobic capacity and lactate clearance. Zone 3 sits at and around your aerobic threshold — the point where lactate begins to accumulate faster than your body can clear it.
When to use it: Tempo runs, sustained efforts, moderate-intensity group workouts. Zone 3 has its place, but it is often overused by recreational exercisers.
Common mistake: Living in Zone 3. This is the "grey zone" — hard enough to feel productive, but not hard enough for maximum adaptation. Spending too much time here means you are too tired for proper Zone 2 recovery work but not pushing hard enough for Zone 4/5 adaptations. The 80/20 principle (80% easy, 20% hard) deliberately avoids excessive Zone 3 work.
Zone 4: Threshold (80-90% of Max HR)
What it feels like: Hard. You can speak only in single words or short phrases between breaths. You are aware that this effort is not sustainable for much longer than 20-40 minutes.
What it trains: Lactate threshold and VO2max. Zone 4 is where you improve your body's ability to sustain high-intensity effort. It pushes the point at which lactate accumulates faster than it is cleared, effectively raising your ceiling.
When to use it: Interval training, tempo intervals, hill repeats. Structured sessions with work periods of 3-8 minutes followed by recovery periods.
Common mistake: Doing Zone 4 work every session. High-intensity training creates significant physiological stress. Without adequate recovery, it leads to overtraining and diminishing returns. Two to three Zone 4 sessions per week is typically the maximum for most people.
Zone 5: Maximum Effort (90-100% of Max HR)
What it feels like: All-out effort. You cannot speak. You are breathing as hard as you can. This effort is sustainable for only 30 seconds to 2-3 minutes at most.
What it trains: Maximum cardiovascular output, anaerobic capacity, neuromuscular power. Zone 5 pushes your heart and lungs to their absolute limits.
When to use it: Sprint intervals, high-intensity interval training (HIIT), competitive efforts. Very short work periods (15-60 seconds) with substantial rest periods (2-4 minutes).
Common mistake: Trying to spend too much time in Zone 5. This zone creates enormous physiological stress. One to two Zone 5 sessions per week is sufficient for most people. More than that increases injury and overtraining risk without proportional benefit.
Why Zone 2 Is Having a Moment
Zone 2 training has exploded in popularity, largely driven by the research and public communication of exercise physiologists like Dr Iñigo San Millán and the broader longevity medicine movement.
The argument is compelling: Zone 2 training targets mitochondrial function — the ability of your cells to produce energy efficiently. Mitochondrial dysfunction is implicated in nearly every chronic disease: type 2 diabetes, cardiovascular disease, cancer, and neurodegenerative conditions.
By building your mitochondrial capacity through Zone 2 training, you are not just improving athletic performance. You are building metabolic health that protects against disease and supports healthy ageing.
The practical appeal is also significant. Zone 2 training does not require gym membership, fancy equipment, or athletic talent. Walking briskly, easy cycling, light jogging, or swimming at a conversational pace all qualify. It is accessible to virtually everyone.
For athletes, Zone 2 builds the aerobic base that makes higher-intensity training more effective. A bigger aerobic engine means you can recover faster between hard efforts, sustain moderate-intensity work for longer, and build endurance without accumulating excessive fatigue.
How Heart Rate Zones Connect to Strain Scores
If you use a wearable that tracks strain, understanding zones helps you interpret the numbers.
Strain scores measure cardiovascular load — essentially how hard your heart worked during the day. The calculation is based on time spent in each heart rate zone, with higher zones weighted more heavily.
Penng tracks continuous heart rate during workouts and throughout the day, producing a strain score on a 0-100 scale. Time spent in higher zones (4 and 5) contributes more to your strain score than the same duration in lower zones. This makes sense — 30 minutes of Zone 5 sprints is far more stressful than 30 minutes of Zone 2 walking.
Understanding this connection helps you predict and plan your strain:
- A Zone 2 session will add modest strain — appropriate for high-volume training days
- A Zone 4/5 interval session will add significant strain — appropriate when your recovery score supports it
- Matching your training zone to your recovery status is one of the most effective ways to balance training stress with recovery
How Penng Tracks Heart Rate During Workouts
Penng uses a PPG optical heart rate sensor on the wrist to measure heart rate continuously. During a workout, you manually start and end the activity in the app. The band records continuous heart rate data throughout the session.
In the app, you see:
- Real-time heart rate (if you check during the workout)
- Average and peak heart rate for the session
- Time spent in different heart rate zones
- Strain score for the workout and for the day overall
Penng supports 109+ workout types across 15 categories, including running, cycling, strength training, swimming, yoga, and more. Most workout types support GPS tracking.
A limitation to be aware of: wrist-based optical heart rate sensors can be less accurate during activities involving significant wrist movement (like rowing or certain strength exercises) or tight gripping. For activities where wrist-based HR tracking is potentially compromised, some users pair a chest strap with their phone for higher accuracy during the workout while using Penng for daily and sleep tracking.
Also note that Penng requires manual workout detection — you need to start and stop workouts yourself in the app. It does not auto-detect activities.
How to Use Zones for Different Goals
Fat Loss
A common misconception is that you should stay in Zone 2 for fat loss because it burns the highest percentage of fat. While Zone 2 does burn proportionally more fat, higher-intensity zones burn more total calories per minute, including more total fat calories.
The best approach for fat loss is a combination:
- Zone 2 for volume (3-4 sessions per week of 30-60 minutes) — builds fat oxidation capacity
- Zone 4/5 for intensity (1-2 sessions per week) — increases total calorie burn and EPOC (excess post-exercise oxygen consumption)
- Strength training (2-3 sessions per week) — builds muscle, which increases resting metabolic rate
Diet remains the primary driver of fat loss. Heart rate zone training optimises the exercise component, but you cannot out-exercise a poor diet.
Endurance
Follow the 80/20 principle. Roughly 80% of training volume in Zones 1-2. Roughly 20% in Zones 4-5. Minimise time in Zone 3 (the grey zone). This is how most elite endurance athletes train, and the evidence supporting it is strong.
General Fitness
Aim for variety. Two to three Zone 2 sessions, one to two higher-intensity sessions, and one to two strength sessions per week provides a well-rounded programme. Use your HRV data and recovery score to guide intensity — green recovery days are for hard efforts, red recovery days are for Zone 1-2 work.
Strength Training
Heart rate zones are less directly applicable to strength training because heart rate during lifting is influenced by the Valsalva manoeuvre, rest periods, and acute cardiovascular responses that do not map neatly to aerobic zones. However, monitoring heart rate during strength sessions still provides useful data about overall cardiovascular load and contributes to your daily strain.
Frequently Asked Questions
How do I know what heart rate zone I am in during a workout?
A continuous heart rate monitor is the most reliable method. Wearables like Penng track heart rate in real-time during workouts using a PPG optical sensor. Without a monitor, the talk test is a reasonable proxy: Zone 2 means full conversation, Zone 3 means short sentences, Zone 4 means single words, Zone 5 means no speaking possible.
Is Zone 2 training really that important?
The evidence strongly supports Zone 2 as the foundation of cardiovascular fitness. It builds mitochondrial density, improves fat oxidation, and enhances aerobic efficiency — all of which support performance at every intensity level. Most people underdo Zone 2 and overdo moderate-intensity work. Adding 2-3 dedicated Zone 2 sessions per week is one of the highest-value changes most people can make.
How long should I train in each zone?
Zone 2 sessions: 30-90 minutes. Zone 4 intervals: 20-40 minutes total (including rest periods). Zone 5 intervals: 10-20 minutes total (including rest periods). Total training duration depends on your fitness level and goals. Most recreational exercisers benefit from 3-5 hours per week distributed across zones.
Can I use heart rate zones for strength training?
Heart rate zones are designed for cardiovascular exercise and do not map perfectly to strength training. Heart rate during lifting is affected by breath-holding, rest periods, and the nature of the exercise. However, tracking heart rate during strength sessions provides useful data about cardiovascular load and contributes to daily strain calculations. Some people use heart rate to ensure rest periods are adequate (waiting for HR to drop below a threshold before the next set).
Why does my heart rate seem too high or too low for the effort?
Several factors affect heart rate beyond exercise intensity: caffeine, dehydration, heat, altitude, fatigue, stress, and illness can all elevate heart rate. Sleep deprivation and overtraining can also cause elevated heart rate at a given intensity. If your heart rate is consistently higher than expected for the same effort, check your recovery status and consider whether you need additional rest. Conversely, very fit individuals may have difficulty reaching Zone 5 heart rates during certain activities.
Curious how your heart rate data compares? Take the free quiz at penng.ai/quiz and discover your training profile in 2 minutes.