Sweat rate is not set by how hard exercise feels. It is set by how much heat your body has to get rid of through evaporation, which researchers call the evaporative requirement for heat balance. This calculator works that out in four steps.

1. Heat you produce. Your activity has a MET value from the Compendium of Physical Activities, the standard reference for the energy cost of activities. Multiplied by your body mass, that gives metabolic rate in watts. Part of that becomes mechanical work, the rest becomes heat. Cycling converts more into useful work than running does, so the same MET value produces different heat.

2. Heat you lose without sweating. Some heat leaves by convection and radiation, depending on the gap between your skin and the air. That is why the same run is far sweatier at 95°F than at 50°F. Above roughly 95°F the gradient reverses and the environment starts adding heat to you. Sun adds radiant load on top.

3. What evaporation has to handle. Whatever heat is left has to leave as evaporated sweat. Evaporating a gram of sweat removes about 2,426 joules.

4. Sweat that drips instead of evaporating. In humid air, sweat rolls off without cooling you, so your body produces more to compensate. That is why humidity raises sweat losses even when the temperature is unchanged.

Age. Sweating capacity declines measurably from around age 40 and significantly between 45 and 70, so the estimate is reduced accordingly. Sweating less is not protective. It means you shed heat less effectively and run hotter for the same work.

How much you personally sweat. This is the largest single factor and the reason the tool asks. Two people of the same size doing identical work in identical conditions can differ twofold, and no equation predicts which one you are. Your answer also quietly accounts for fitness and heat acclimatisation, since trained and heat-adapted people sweat earlier and more, which is a protective adaptation rather than a problem.

Electrolytes. Sweat sodium is calculated at about 850 mg per litre, derived from published sodium loss rates across sports. Individuals range from roughly 200 to 2,000 mg per litre, so a salty sweater loses well over this and a light one well under. Potassium runs near 200 mg per litre and magnesium near 10, both far more consistent between people.

Reference sweat rates from athlete testing, for comparison with your result:

Every estimate above is a model. Your actual sweat rate can be double or half of it. Measuring takes one session: weigh yourself in minimal clothing before, towel off and weigh again after, and track what you drank.