This section groups two related but different questions: how hard you are working right now, and how much energy a session actually cost. The heart rate calculators — maximum, reserve and target — deal with intensity: what your heart is capable of, and what zone a given effort falls into. The calorie calculators — running, cycling, swimming, hiking, weightlifting — deal with energy expenditure, each built from a MET value specific to that activity, applied against your body weight and the time spent doing it.
Calculators in this section
The two sets of numbers are not interchangeable. A hard, short effort and a long, easy one can burn a similar number of calories while training completely different systems, and heart rate alone will not tell you which one you just did. Body weight drives the calorie figures more than most people expect — moving at a given pace or through a given pattern of effort simply costs more energy in a heavier body — while the heart rate figures are driven mostly by age and genetics rather than fitness, which is why training barely moves your maximum even as it changes how hard a given heart rate feels to sustain.
None of these are lab measurements. They are published formulas — Tanaka's age-based maximum, the Karvonen reserve method, standard MET tables — applied to figures you supply, most useful for comparing one session against another rather than for treating any single output as exact.