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About this tool
Daily fluid and sodium replacement plan for hot, very dry desert conditions, from body weight, temperature, humidity and exposure hours.
The Desert Climate Hydration Calculator estimates how much fluid and sodium you need to replace during exposure to hot, very dry air, by adding heat-scaled sweat loss and dry-air respiratory loss on top of a 35 mL/kg daily maintenance baseline. It reports a per-hour drinking rate, a sip size every 15 minutes, and the sodium carried out in that sweat using ACSM's 200-2000 mg per litre sweat sodium range. It is aimed at desert hikers, pilgrims, outdoor workers and anyone travelling somewhere the humidity sits below 20 percent.
Open Desert Climate Hydration Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Adds up to 0.12 L per hour of respiratory and skin loss below 10 percent humidity, the loss desert travellers never see as sweat.
Reports sweat sodium separately so you know when plain water alone would put you at risk of hyponatraemia.
Warns when the plan crosses the 1.5 L per hour absorption ceiling or the 12 L per day heat-illness guideline.
For moderate activity at around 40 C, roughly 0.8 to 1.2 litres per hour is typical, and hard work can push it towards 2 litres per hour. The absolute ceiling is about 1.5 litres per hour — the gut cannot absorb more than that, and drinking faster raises the risk of dilutional hyponatraemia rather than fixing dehydration.
In air below about 20 percent relative humidity, sweat evaporates the instant it reaches the skin, so you never feel wet and never get the visual cue that you are losing fluid. Thirst also lags actual loss by roughly 1 to 2 percent of body mass, which is why desert guidance is to drink on a fixed schedule rather than on demand.
Water is enough for short exposures, but once you lose more than about 3,500 mg of sodium — roughly three litres of sweat for an unacclimatised person — you need salt as well. Sweat sodium averages about 1,150 mg per litre unacclimatised and around 575 mg per litre once heat acclimatised, so an electrolyte drink or salted food matters more the longer you are out.
Weigh yourself before and after the exposure: a loss of more than 2 percent of body mass, which is 1.4 kg for a 70 kg person, is the threshold ACSM treats as performance-impairing dehydration. Dark urine and a dry mouth are supporting signs, but confusion, fainting or sweating that stops in the heat are emergencies — get medical help immediately rather than relying on any calculator.