Heart Rate Zones Calculator

How Heart Rate Training Zones Work

Heart rate zones are percentage ranges of your maximum heart rate (MHR). Different intensities drive different adaptations: fat oxidation and capillary density at the bottom, lactate clearance and VO2max at the top.

Maximum heart rate estimation:

The better-validated formula (Tanaka, 2001), and the default here:

MHR = 208 − (0.7 × Age)

The older Fox formula (220 − Age) came from a 1971 literature review rather than an experiment, and it drifts in opposite directions at the two ends of the age range.

The gap between them works out as (0.3 × Age) − 12, which means the two agree exactly at age 40 and diverge from there. At 20, Fox reads 6 bpm high. At 65 it reads about 8 bpm low, and at 80 it is 12 bpm low. Enough to shift a zone boundary either way, so the calculator lets you pick and shows what the other formula would have given.

If you have a measured max heart rate from a field or lab test, use that instead of either formula. Individual variation is roughly ±10 to 15 bpm around any age-based estimate, which is larger than the gap between the two formulas.

Worked example: 38-year-old runner:

MHR = 208 − (0.7 × 38) = 208 − 26.6 = 181 bpm

The 5 training zones:

Key training principles:

• Zone 2 is where most training volume should occur (60–80% of weekly mileage), it builds aerobic capacity with minimal injury risk
• Zone 4 is where 10K and half-marathon race pace typically falls
• Zone 5 intervals (30 sec to 4 min) are used sparingly to increase maximum oxygen uptake

Heart rate reserve method (Karvonen: more precise):

Target HR = ((MHR − Resting HR) × Zone%) + Resting HR

For a person with resting HR of 55 bpm targeting Zone 3 at 70%:

Target HR = ((181 − 55) × 0.70) + 55 = 88.2 + 55 = 143 bpm

Fill in the optional resting heart rate above and the calculator switches to this method. Notice the direction of the change: plain percentage-of-max gives 127 bpm for that same 70%, so Karvonen lands 16 bpm higher. Percentage-of-max effectively assumes a resting rate of zero, which nobody has, and the fitter you are the more it under-prescribes. That is why a well-trained athlete following percentage-of-max zones often finds Zone 2 feels like barely moving.

Use a chest strap monitor for accuracy. Wrist optical sensors can lag by 10–15 seconds during interval training.

Finding your true max heart rate

The Tanaka formula is an estimate; individual MHR can vary ±10–15 bpm from the predicted value. To find your actual max:

• Field test (running): After a 15-minute warm-up, run 3 × 800 m all-out with 90 seconds of recovery between each. Record the highest HR seen. The third interval usually produces it.
• Field test (cycling): 20-minute warm-up, then 5 minutes building to all-out effort on a steady climb. Hold the maximum sustainable HR for the last minute.
• Lab test: A VO2max test in an exercise physiology lab gives the most accurate MHR plus your ventilatory thresholds.

Re-test once a year. Max HR drops about 0.7 bpm per year of age, in line with the Tanaka formula.

Long-course endurance pacing

For triathlon and marathon distance racing, most volume sits in Zones 1–2 (the aerobic base). Race-day pacing per discipline:

• Ironman bike: 70–80% of max HR, which is Zone 3 on the table above
• Ironman run: 75–80% of max HR, the upper half of Zone 3
• Marathon race pace: about 80–85% of max HR, the bottom of Zone 4
• Half-marathon race pace: 85–90% of max HR, the top of Zone 4

Going above Zone 3 on the Ironman bike almost always buys a painful run. The legs are gone by mile 10 even when the cardiovascular system still feels fine, and no amount of discipline on the run gets that back.