This calculator will help you to define your heart rate zones using the Karvonen formula. A full description of the formula can be found here.
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The Karvonen formula is one of the bests that can be used to define your heart rate zones and anaerobic threshold. It is based on the heart rate reserve, so it takes into account both your resting heart rate and maximum heart rate. Therefore it gives a much better result than the formulas that use only the maximum heart rate.