Outdoor Heat Index & Sweat Loss Calculator

How it works

The heat index uses the Rothfusz formula, which combines temperature and humidity into a "feels like" temperature. In Fahrenheit it is: −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², where T is temperature in Fahrenheit and R is relative humidity in percent, with the standard low-humidity and high-humidity adjustments. Temperatures entered in Celsius are converted to Fahrenheit first, and the result is shown in both units. Below 80°F (about 27°C) the heat index is close to the air temperature itself, so the tool simply reports the air temperature there.

Sweat loss is an estimate, stated here in full. A base sweat rate per hour is set by intensity for a 70 kg person: light 0.4 litres per hour, moderate 0.8 litres per hour, vigorous 1.2 litres per hour. That rate is scaled by body weight (your weight divided by 70), and then by heat: for every degree Celsius the heat index sits above 27°C, the rate rises by 3%, so a heat index of 37°C multiplies the rate by 1.30. Estimated sweat loss is that hourly rate multiplied by duration in hours.

Fluid replacement guidance follows a common rule of thumb: plan to drink about 1.2 to 1.5 litres for every litre of sweat lost, spread before, during, and after activity rather than all at once at the end. For sessions over an hour, or where sweat loss exceeds about 1 litre, include some electrolytes — from food or a sports drink — rather than plain water alone, because sweat carries salt with it.

How to use your result

Use the heat index to judge the session, not just the thermometer: a 32°C day at high humidity can feel like 40°C or more, and that is when pacing, shade breaks, and shorter sessions matter most. Drink on a schedule during the activity instead of waiting for thirst, and weigh the guidance against how you actually feel — if you feel dizzy, nauseous, confused, or unusually fatigued, stop, move to shade, cool down, and seek help if symptoms do not pass quickly. This is an estimate: individual sweat rates vary widely, and acclimatisation, clothing, wind, and direct sun all shift the real number.

Related guides

• The Heat Index, Explained: Why Humidity Makes It Worse
• Sweat Loss and Rehydration After Hot Workouts
• Heat Exhaustion: Warning Signs and First Response