Heart rate zones are ranges of heartbeats per minute that correspond to different exercise intensities. Training in specific zones targets different adaptations—from fat burning to maximum power output. Understanding your zones helps you train smarter, not just harder.
What Are Heart Rate Zones?
Heart rate zones divide your exercise intensity into 5-6 distinct ranges, each defined as a percentage of your maximum heart rate (or heart rate reserve). Each zone trains your body differently:
| Zone | Name | Intensity | What It Trains | |------|------|-----------|----------------| | Z0 | Recovery | Below 50% | Rest and recovery between efforts | | Z1 | Warm Up | 50-60% | Light activity, active recovery, warm-up | | Z2 | Fat Burn | 60-70% | Aerobic base, efficient fat oxidation | | Z3 | Cardio | 70-80% | Cardiovascular fitness, endurance | | Z4 | Performance | 80-90% | Lactate threshold, race pace | | Z5 | Peak | 90-100% | Maximum power, VO2 max, sprint capacity |
How Zolt Calculates Your Zones
Zolt uses the Heart Rate Reserve (HRR) method, also known as the Karvonen formula. This approach is more accurate than simple percentage-of-max calculations because it accounts for your resting heart rate.
The Formula
Target HR = Resting HR + (Intensity % × Heart Rate Reserve)
Where Heart Rate Reserve = Max HR - Resting HR
Example Calculation
For someone with:
- Max HR: 185 bpm
- Resting HR: 60 bpm
- Heart Rate Reserve: 125 bpm (185 - 60)
Zone 3 (70-80% intensity) would be:
- Lower bound: 60 + (0.70 × 125) = 148 bpm
- Upper bound: 60 + (0.80 × 125) = 160 bpm
Why HRR Is Better
The simple percentage method (just multiplying max HR by intensity) ignores fitness level. Two people with the same max HR of 185 would get identical zones—but if one has a resting HR of 50 and another has 70, their actual fitness levels differ significantly.
The HRR method gives the fitter person (lower resting HR) higher zone thresholds, which more accurately reflects their cardiovascular capacity.
Understanding Each Zone
Zone 0-1: Recovery and Warm Up (50-60%)
What it feels like: Easy conversation pace. You could talk in full sentences without getting winded.
Benefits:
- Active recovery between hard workouts
- Warm-up before intense sessions
- Promotes blood flow without adding training stress
- Good for beginners building an aerobic base
When to use it: Recovery days, warm-ups, cool-downs, and the majority of easy runs.
Zone 2: Fat Burn / Aerobic Base (60-70%)
What it feels like: Comfortable but purposeful. You can talk, but prefer shorter sentences.
Benefits:
- Optimal fat oxidation (your body burns the highest percentage of calories from fat)
- Builds aerobic efficiency
- Improves mitochondrial density
- Sustainable for long durations
When to use it: Long runs, endurance rides, and base-building phases. Many elite athletes spend 80% of their training time here.
Zone 3: Cardio / Tempo (70-80%)
What it feels like: Moderate effort. You can speak in short phrases but would rather not.
Benefits:
- Improves cardiovascular capacity
- Increases lactate clearance
- Builds sustainable race pace
- Good balance of intensity and volume
When to use it: Tempo runs, steady-state cardio, and longer intervals.
Zone 4: Performance / Threshold (80-90%)
What it feels like: Hard. Speaking is difficult—maybe one or two words at a time.
Benefits:
- Raises lactate threshold
- Improves time-to-exhaustion at high intensities
- Builds race-specific fitness
- Increases power at threshold
When to use it: Threshold workouts, race-pace efforts, and interval training.
Zone 5: Peak / VO2 Max (90-100%)
What it feels like: Maximum effort. You cannot speak. This is unsustainable beyond a few minutes.
Benefits:
- Maximizes VO2 max
- Improves anaerobic capacity
- Builds sprint power
- Increases stroke volume
When to use it: Short intervals, hill sprints, and finishing kicks. Use sparingly—this zone creates significant training stress.
Finding Your Maximum Heart Rate
The classic formula 220 - age gives a rough estimate, but individual variation is huge—actual max HR can differ by ±10-15 bpm from this prediction.
Better Methods
-
Field test: After a thorough warm-up, do 3-4 intervals of increasing intensity (e.g., 3 minutes each), with the final interval at all-out effort. Your peak heart rate during the last interval approximates your max.
-
Race data: Your highest heart rate during a competitive all-out effort (like a 5K race) is often close to your max.
-
Lab test: The gold standard, but requires access to exercise physiology equipment.
In Zolt
Zolt can auto-calculate your max HR based on your highest recorded heart rates during workouts. You can also manually set it if you know your true max from testing.
Resting Heart Rate Matters
Your resting heart rate (RHR) is equally important for accurate zones. A lower RHR generally indicates better cardiovascular fitness.
How to measure RHR:
- Measure first thing in the morning, before getting out of bed
- Average several days for accuracy
- Use a wearable that tracks overnight heart rate (Zolt uses your sleep heart rate data)
What affects RHR:
- Fitness level (fitter = lower)
- Stress and recovery status
- Sleep quality
- Illness
- Caffeine and alcohol
- Overtraining
Zolt tracks your 30-day average RHR and can auto-update your zones as your fitness changes.
Customizing Your Zones in Zolt
Zolt allows full customization of your heart rate zones:
- Set Max HR: Manually enter your known max or let Zolt calculate from your data
- Set Resting HR: Enter manually or use Zolt's 30-day average from your sleep data
- Auto-calculate: One tap recalculates all zones based on your current max and resting HR
- Fine-tune boundaries: Adjust individual zone thresholds if the defaults don't match your perceived effort
When to Adjust Zones
- After a max HR test: If you've done a proper test and know your true max
- Significant fitness changes: If your RHR has dropped substantially (or increased)
- Perceived effort mismatch: If Zone 2 feels too easy or too hard, your zones may need adjustment
Training Distribution
How much time should you spend in each zone? Research suggests an 80/20 split works well for most athletes:
- 80% in Zones 1-2: Easy aerobic work builds your base without accumulating fatigue
- 20% in Zones 4-5: High-intensity work drives performance adaptations
This distribution applies across a training week or block. Many recreational athletes make the mistake of going too hard on easy days and too easy on hard days—ending up stuck in Zone 3 for most workouts.
How Zolt Uses Your Zones
Beyond just displaying your zones during workouts, Zolt uses them for:
- Training load calculation: Higher zones contribute more to daily training stress
- TRIMP scoring: Time in each zone is weighted to calculate training impulse
- Activity score: Your daily activity score factors in time spent at different intensities
- AHA goal tracking: American Heart Association guidelines are tracked using Zone 2+ minutes
- Workout analysis: Each workout shows zone distribution and time in each zone
Common Mistakes
Training Too Hard Too Often
Spending too much time in Zones 3-4 (the "gray zone") provides moderate stimulus but high fatigue. You're not easy enough to recover, but not hard enough to drive adaptation.
Ignoring Zone 2
Zone 2 feels too easy for many people, so they skip it. But this zone builds the aerobic base that supports all other training. Elite endurance athletes spend the majority of their time here.
Using Generic Zones
Default zones based on age formulas may not match your physiology. If a zone feels completely wrong (Zone 2 feels like Zone 4), your settings need adjustment.
Not Updating Zones
As fitness improves, RHR typically drops and zones shift. Review your zones every few months or after significant training blocks.
The Bottom Line
Heart rate zones are a practical tool for training at the right intensity. The key insights:
- Know your numbers: Accurate max HR and resting HR make zones useful
- Personalize: Generic formulas are starting points, not gospel
- Respect easy days: Most training should be in Zones 1-2
- Use hard days wisely: Zones 4-5 drive adaptation but create fatigue
- Trust the data: If your zone distribution looks wrong (all Zone 3), adjust your training
Zolt calculates and applies your zones automatically across all your workouts, giving you insight into whether you're training at the right intensities to reach your goals.