Loading page
About this tool
Estimate humid-weather hydration needs from body size, heat exposure, workout duration and sweat-risk factors.
The Hydration Calculator Humid Climate estimates a day's fluid target for humid heat by adding sweat losses to a body-weight baseline of 35 mL per kg per day (30 mL/kg from age 65), then scaling those sweat losses by the US National Weather Service heat index computed from your air temperature and relative humidity. It exists because humidity is the part ordinary hydration maths ignores: sweat only cools you when it evaporates, and as the air approaches saturation you sweat more for less cooling. It is for anyone training or working in muggy conditions who wants a planning number, plus a sodium estimate and a safe drinking ceiling — informational only, not medical advice.
Open Hydration Calculator Humid Climate on AltFTool — it loads instantly in your browser.
Enter Sex, Age, Body weight (kg) and Waking hours, then Air temperature (°C) and Relative humidity (%) so the NWS heat index can be computed.
Set Training intensity today, Training hours today, Hours outdoors or without AC, and How salty does your sweat run? — 500, 900 or 1,500 mg of sodium per litre.
Read Drink today in litres and 250 ml glasses, with the sodium estimate and a warning if the pace would pass the 1.0 L per hour the kidneys can clear, then press Copy plan.
The full NWS Rothfusz heat index regression, including both published corrections, drives the sweat adjustment — so 80% humidity changes your number, not just the advice text.
Sweat sodium is estimated at 500, 900 or 1,500 mg per litre depending on whether you run light, typical or salty, which is what decides if water alone will do.
Because healthy kidneys clear only about 0.8–1.0 L of water per hour, the plan warns when your pace would exceed that, the scenario behind exercise-associated hyponatraemia.
Start from about 35 mL per kilogram of body weight per day — roughly 2.4 L for a 70 kg adult — and add your sweat losses on top. This calculator estimates those losses from training intensity and hours, hours spent outdoors, and a heat-index-based multiplier, so a hot humid training day lands well above the baseline while a mild indoor day stays near it.
Yes, and it is the more dangerous error during long sweaty sessions. Healthy kidneys excrete a maximum of roughly 0.8 to 1.0 litres of water per hour; drinking faster than that, especially plain water, dilutes blood sodium and risks exercise-associated hyponatraemia. The plan flags any pace above 1.0 L/hour and any daily total above 10 litres.
Once estimated sweat losses reach about 1.5 litres in a day. Sweat carries roughly 460 to 1,840 mg of sodium per litre depending on the person, and for prolonged sweating the ACSM suggests a drink containing about 460 to 690 mg of sodium per litre. If you leave white salt marks on your kit, assume you are at the higher end.
Because sweat cools you only when it evaporates. In humid air evaporation slows, sweat drips off without carrying heat away, and the body compensates by sweating harder for less cooling — so fluid loss outruns what the thermometer suggests. The heat index captures exactly that combination, which is why it, rather than air temperature, drives the sweat multiplier here.
Add the Hydration Calculator Humid Climate widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/hydration-calculator-humid-climate"
title="Hydration Calculator Humid Climate — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/hydration-calculator-humid-climate?utm_source=embed&utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>