A 25-year-old training in Zone 2 should target roughly 115–134 bpm, calculated as 60–70% of an estimated HRmax of 191. This is the aerobic-base zone where mitochondrial density, fat oxidation and stroke volume adapt most strongly — and the intensity you can hold in full conversation for 60–90 minutes.
60% of 191 = 115 bpm. 70% of 191 = 134 bpm. Stay inside this band for easy aerobic development.
| Zone | Name | % HRmax | BPM (age 25) |
|---|---|---|---|
| 1 | Recovery | 50–60% | 96–115 |
| 2 | Endurance | 60–70% | 115–134 |
| 3 | Aerobic | 70–80% | 134–153 |
| 4 | Threshold | 80–90% | 153–172 |
| 5 | Maximum | 90–100% | 172–191 |
If you know your resting heart rate, Karvonen gives a more accurate Zone 2. Example for a 25-year-old with RHR 60: Zone 2 = 60 + (191 − 60) × (0.60 to 0.70) = 139–152 bpm.
Most recreational athletes get the biggest return from 3–5 Zone 2 sessions of 45–75 minutes per week, plus one Zone 4 session for lactate-threshold stimulus. Sleep quality and recovery capacity become the limiter — not intensity.
Typical resting heart rate: 56–72 bpm.
Typical VO2max reference (good/very good): Men 43–50 ml/kg/min (good); women 36–42 ml/kg/min (good). Source: ACSM norms and HUNT Fitness Study.
Why HRmax sits where it does at 25: HRmax is only 1–2 bpm below the age-20 value. Individual variation (±10 bpm) is much larger than the age effect at this stage.
Two easy Zone 2 sessions of 45 minutes on weekdays, one long Zone 2 ride/run of 75–90 minutes on the weekend, and one Zone 4 threshold session. Add strength twice weekly.
If you know your resting HR, Karvonen (heart rate reserve) is more accurate — especially if RHR is below 60. For a rough starting point, %HRmax is fine.
A 25-year-old should aim for approximately 115–134 bpm in Zone 2, calculated as 60–70% of an estimated HRmax of 191 bpm (Tanaka).
Using the Karvonen method with a resting HR of 60, a 25-year-old's Zone 2 becomes approximately 139–152 bpm. Karvonen is generally more accurate than %HRmax for trained athletes.
Most research supports 3–5 Zone 2 sessions of 45–75 minutes per week, plus one Zone 4 threshold session. Recovery capacity becomes the main limiter as you age, not intensity.
Use the talk test: in Zone 2 you should be able to speak in full sentences comfortably. If you can only manage short phrases, you have drifted into Zone 3 or 4.
If you know your resting HR, Karvonen (heart rate reserve) is more accurate — especially if RHR is below 60. For a rough starting point, %HRmax is fine.
Disclaimer: This calculator is for informational purposes only and does not provide medical advice. Always consult with a healthcare professional before starting any new exercise program.
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