How Fitbit Calculates Your Max Heart Rate (HRmax)
Fitbit estimates HRmax with 220 − age, then personalises your zones using heart rate reserve. That second step is why its zones look wrong to runners.
Fitbit estimates your maximum heart rate with 220 − age, the oldest formula there is. Then it does something most brands do not: it personalises your zones using heart rate reserve, the gap between that maximum and your own resting heart rate. Google's documentation states this directly, which makes Fitbit one of the few brands to say out loud what its zones are actually percentages of.
Two different numbers
This trips people up constantly, so it is worth separating.
The maximum comes from an age formula. 220 − 40 = 180 for a 40-year-old. It carries an individual error of roughly ±10 to 12 bpm and is biased high for the young and low for the old.
The zones are not percentages of that maximum. They are placed inside your heart rate reserve, which is maximum minus resting. A 40-year-old with a maximum of 180 and a resting heart rate of 55 has a reserve of 125 beats, and the zone boundaries are drawn inside that range and then added back onto 55.
The practical effect is that Fitbit's zone boundaries sit higher in bpm than a plain percentage-of-maximum system would put them. That is why a Fitbit "fat burn" zone often feels harder than a Garmin zone 2, and why people comparing the two assume one of them is broken.
Reserva de frecuencia cardíaca
135bpm
185 − 50
Frecuencia cardíaca máxima
185bpm
Frecuencia cardíaca en reposo
50bpm
| Zona | Rango | Qué entrena |
|---|---|---|
| Z1 | Recuperación 118–131 bpm | Recuperación activa y calentamiento; favorece el flujo sanguíneo sin añadir fatiga. Muy fácil, RPE 1–2. Conversación completa, casi sin agitarte. |
| Z2 | Resistencia 131–145 bpm | Base aeróbica, oxidación de grasas y densidad capilar; la zona fundamental del día a día. Cómodo, RPE 3–4. Puedes mantener frases completas. |
| Z3 | Tempo 145–158 bpm | Capacidad aeróbica y resistencia muscular; trabajo continuo 'moderadamente duro'. Exigente, RPE 5–6. Solo frases cortas. |
| Z4 | Umbral 158–172 bpm | Umbral de lactato, tu techo sostenible de ritmo de competición. Duro, RPE 7–8. Unas pocas palabras seguidas. |
| Z5 | VO₂max 172–185 bpm | Potencia aeróbica máxima y VO₂max mediante intervalos cortos e intensos. Muy duro a máximo, RPE 9–10. Sin hablar. |
Z1 · Recuperación. Recuperación activa y calentamiento; favorece el flujo sanguíneo sin añadir fatiga. Muy fácil, RPE 1–2. Conversación completa, casi sin agitarte.
Z2 · Resistencia. Base aeróbica, oxidación de grasas y densidad capilar; la zona fundamental del día a día. Cómodo, RPE 3–4. Puedes mantener frases completas.
Z3 · Tempo. Capacidad aeróbica y resistencia muscular; trabajo continuo 'moderadamente duro'. Exigente, RPE 5–6. Solo frases cortas.
Z4 · Umbral. Umbral de lactato, tu techo sostenible de ritmo de competición. Duro, RPE 7–8. Unas pocas palabras seguidas.
Z5 · VO₂max. Potencia aeróbica máxima y VO₂max mediante intervalos cortos e intensos. Muy duro a máximo, RPE 9–10. Sin hablar.
- Karvonen ancla las zonas a tu reserva de frecuencia cardíaca (HRmax − FC en reposo), por lo que personaliza las zonas según tu condición física mejor que el simple %HRmax: un atleta en forma con una FC en reposo baja obtiene objetivos proporcionalmente más bajos.
- Mide la frecuencia cardíaca en reposo a primera hora de la mañana, tumbado, antes de la cafeína. Un error de 5 bpm en la FC en reposo desplaza todas las zonas.
Un PDF con tus resultados personalizados, más un código QR para volver a abrirlos cuando quieras.
Enter your maximum and your resting heart rate to see the boundaries the reserve method produces, and compare them against plain %HRmax zones.
Why the reserve method is the better half of Fitbit's system
Using resting heart rate is a genuine improvement. Two people with identical maximums and very different fitness get different zones, because the fitter one has a lower resting heart rate and therefore a larger reserve. A pure percentage-of-maximum model cannot make that distinction at all.
The weak link is the other input. Fitbit personalises the bottom of the range with your measured resting heart rate and leaves the top of the range as an age guess. So half the calculation is yours and half is a population average.
How to fix the half that is wrong
Set a custom maximum. In the Fitbit app, open your heart rate zone settings and choose a custom maximum rather than the default. From there you can also define a custom zone range.
Test the number first. Build to hard over 10 to 15 minutes, finish with an all-out 60 to 90 second effort, take the highest reading off a chest strap rather than the wrist, repeat within two weeks and keep the higher number. Not for anyone with cardiac risk or new to hard training. The protocol is in finding your maximum heart rate.
Check your resting heart rate is real. The reserve method only works if the bottom number is right. A device worn loosely overnight, or not worn at all, gives Fitbit a poor resting estimate and shifts every zone with it.
FAQ
What formula does Fitbit use for max heart rate? 220 − age. You can replace it with a custom maximum in the app.
Does Fitbit use heart rate reserve? Yes. Google's documentation says Fitbit personalises your zones using heart rate reserve, the difference between your maximum and resting heart rates.
Why are Fitbit's zones higher than my other watch's zones? Because reserve-based boundaries sit higher in bpm than plain percentage-of-maximum boundaries. The same effort can be "fat burn" on one device and zone 3 on another.
Can I set custom heart rate zones on Fitbit? Yes. Set a custom maximum, or define a custom zone range directly in the heart rate settings.
Is Fitbit's max heart rate accurate? The default is an age estimate with ±10 to 12 bpm of individual error. The zone method around it is better than most. Fix the maximum and the whole system improves.
Other brands
- Garmin uses Tanaka and raises HRmax from recorded peaks.
- COROS recommends Gellish and offers a reserve model too.
- Apple Watch publishes nothing, though the community thinks it also uses reserve.
- Polar and Suunto both default to 220 − age.
Sources
- Google Health Help on tracking heart rate with Fitbit
- Google Health Help on Active Zone Minutes
- Karvonen, Kentala & Mustala (1957), the heart rate reserve method
Cover photo by PamD, CC BY-SA 4.0, via Wikimedia Commons.
