Loading page
About this tool
Estimate daily fluid needs in cold, dry air from respiratory water loss, sweat under layers and cold-induced diuresis.
The Cold Weather Hydration Calculator estimates how much fluid you lose on a cold day by adding four separate losses: your baseline need of about 35 ml per kg of body mass, the extra water carried off by breathing dry sub-zero air, sweat trapped under insulating layers, and cold-induced diuresis. The breathing loss is calculated from psychrometrics rather than a rule of thumb — exhaled air leaves the airway saturated at 37 °C while air at -10 °C holds under 2 g of water per cubic metre, and that gap is water you never get back. It is aimed at hikers, skiers, construction and site crews and anyone who stops feeling thirsty once the temperature drops.
Open Cold Weather 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.
Respiratory loss comes from the Magnus saturation-vapour equation and your actual minute ventilation, with the indoor resting loss subtracted so nothing is double-counted.
Separates what to drink during the outdoor session from what to drink afterwards, and caps the hourly rate at a safe 800 ml.
Estimates sweat sodium loss so you know when plain water alone is the wrong replacement.
Yes — cold air is extremely dry, so you lose more water through breathing, and cold exposure triggers cold-induced diuresis that raises urine output. The bigger problem is that cold blunts thirst by roughly 40% in laboratory studies, so people under-drink even when their losses are unchanged or higher.
At rest indoors, breathing costs around 250-350 ml of water a day. Working hard outdoors at -10 °C, minute ventilation can rise five-fold and each cubic metre of air you exhale carries about 42 g more water than it brought in, which typically adds 150-500 ml over a few hours of activity.
Insulating layers trap heat, so once you start moving your body still has to dump the heat your muscles make. Sweat rates of 0.5-1.0 L per hour during winter hiking or shovelling are normal, and because the sweat is absorbed by clothing rather than dripping visibly, most people badly underestimate it.
Plain water is fine below roughly 1.5 L of sweat loss. Above that, sweat sodium at around 40 mmol per litre adds up to more than a gram of sodium, so an electrolyte drink or salty food alongside water reduces the risk of dilutional hyponatraemia. This tool is informational — if you have a kidney, heart or blood-pressure condition or take diuretics, ask your doctor about your own targets.