Hydration Calculator
How much water should you carry? Enter the temperature, your effort level, pack weight and hike length — get an hourly and daily water target plus a sodium guide.
Estimate your water needs
Works for day hikes, ridge runs and multi-day approaches. All numbers are editable — try the example first.
Results
How is this calculated?
Base hourly intake starts at 400 ml/h, adds 30 ml for every °C above 15 °C, and 10 ml for every kg of pack above 5 kg. This is multiplied by an effort factor (easy 1.0, moderate 1.25, hard 1.5) and clamped to a sensible range of 250–1500 ml/h. Total = hourly rate × hours + a 500 ml reserve. Estimated sweat equals 80% of planned intake, and sweat sodium is assumed at 0.6 g per litre.
ml/h = clamp((400 + max(0,T−15)×30 + max(0,pack−5)×10) × effort, 250, 1500)
total = ml/h × hours + 500 ml
Sources: Wilderness Medical Society practice guidelines on hydration; trail-hiking sweat-rate literature (0.5–1.2 L/h typical). Estimates only — thirst and urine colour are your real guide.