Step 1: estimate maximum heart rate
estimated HRmax = 208 − (0.7 × age)Tanaka and colleagues derived this regression from pooled laboratory data and a large validation sample. For a 40-year-old, the estimate is 180 beats per minute. It describes a population relationship; it does not prove that a particular 40-year-old's measured maximum is 180.
Step 2: choose a zone method
| Method | Calculation | What it requires |
|---|---|---|
| Percent of estimated maximum | HRmax × intensity % | Age only |
| Karvonen / heart-rate reserve | resting HR + (HRmax − resting HR) × intensity % | Age and a representative resting heart rate |
Example
Estimated maximum: 208 − 28 = 180 bpm
Resting heart rate: 60 bpm
Heart-rate reserve: 180 − 60 = 120 bpm
Karvonen 70% point: 60 + (120 × 0.70) = 144 bpm.
Percent-max 70% point: 180 × 0.70 = 126 bpm.
The methods answer slightly different questions, so their boundaries should not be mixed. ToolBullet allows zone percentages to be configured rather than presenting one five-zone scheme as universal.
Why age equations have wide individual error
Validation work on common age-predicted equations has found wide limits of agreement even when average bias is modest. Two people of the same age can have meaningfully different measured maximum heart rates. The uncertainty of the maximum flows directly into every calculated zone.
Factors that change heart-rate response
- Medication, particularly drugs that alter heart-rate response
- Heat, dehydration, altitude, illness and fatigue
- Caffeine and other stimulants
- Exercise mode: cycling, running and swimming can produce different responses
- Sensor error, poor contact or optical-watch lag
How to use zones responsibly
Use zones as broad feedback alongside perceived effort, breathing, pace or power. A measured maximum from an appropriately supervised test can improve individualisation, but a number alone still does not determine whether exercise is safe. Stop and seek appropriate help for concerning symptoms.
Compare percent-max and Karvonen zones in the Heart Rate Zone Calculator.
Resting heart rate and measured maximums
Karvonen calculations depend on resting heart rate, so use a calm, repeatable measurement rather than a value taken immediately after movement, caffeine or emotional stress. Wearable readings can also be affected by fit, skin contact and motion artefact. A properly measured individual maximum may be more informative than an age equation, but maximal testing can be inappropriate without suitable screening and supervision. ToolBullet therefore keeps age prediction clearly labelled and never presents a calculated maximum as a clinical measurement.
Why zone labels differ
Five-zone systems do not share one universal set of boundaries or physiological names. Percentage-of-maximum and heart-rate-reserve methods can place the same pulse in different bands, and ventilatory or lactate thresholds are separate measured concepts. Use the configurable boundaries to reproduce a documented coaching system, then keep the method unchanged when comparing sessions. Do not interpret a one-beat boundary as a biological switch.
How it works and sources
Method and sources reviewed 7 July 2026. ToolBullet uses these sources to explain the method and its limits; it does not replace coaching, medical advice or official governing-body records.