Heat Acclimatization Calculator

Heat acclimatization is a genuine physiological adaptation, not just "getting used to" the heat — with repeated heat exposure the body expands blood plasma volume, starts sweating earlier and more heavily, and lowers both core temperature and heart rate at a given workload. This calculator estimates how much of that adaptation you can realistically achieve given the days you have before your event. A full adaptation baseline is set at 14 days, adjusted up or down by expected humidity (higher humidity extends the timeline since evaporative cooling is less effective). Your progress toward that baseline follows a diminishing-returns curve — an exponential approach where the bulk of the benefit arrives in the first week or so and the remainder trails off, rather than a straight linear ramp.

From your calculated adaptation percentage, the calculator scales four physiological benchmarks to their typical maximums at full acclimatization: up to a 12% plasma volume increase, up to 25% more sweat output, up to 0.5°C lower core temperature, and up to 20 bpm lower heart rate at matched effort — each reduced proportionally if you haven't reached full adaptation. It also outputs a starting training intensity (50%, building to full by around day 10) plus session-level fluid and sodium targets — fluid intake scales with your daily exercise duration and the expected humidity, while the extra sodium target scales with both the temperature gap between your training and race environments and the expected humidity. Because these are physiological averages, individual response to heat training varies — some people acclimate faster or slower than the 14-day baseline, so treat the day-count outputs as a planning guide, not a guarantee.