How much water to carry, and when to refill
Baseline drinking rates, how heat and effort change them, the weight tradeoff of carrying water, and how to plan refills safely.
Start from a per-hour rate
The most reliable way to plan trail water is per hour of movement, not per mile, because terrain changes your pace far more than your sweat rate. A relaxed walk in mild weather runs close to half a liter an hour for many hikers, and this calculator uses that as its baseline. From there you scale up rather than down: hot sun, thin air at altitude, a heavy pack and steep climbing all pull the number higher. Track your own bottles across a few known hikes and you will quickly learn a personal baseline that beats any generic chart.
How heat and effort stack up
Temperature usually matters more than difficulty. Moving through direct sun on an exposed ridge can double your losses compared with the same effort in cool shade, which is why the Hot setting scales higher than Strenuous in this tool. Effort still counts, since carrying a loaded pack uphill raises your core temperature and breathing losses. The two combine, so a hot and strenuous day can push you well past three quarters of a liter an hour. When both apply, use the custom rate field and enter a figure that reflects the harder of the two plus a margin.
The weight tradeoff
Water is heavy, about one kilogram or 2.2 pounds per liter, so carrying a full day of it has a real cost in fatigue and joint strain. That weight is also why refilling matters: topping up at a reliable stream lets you carry less between sources instead of hauling the whole day from the trailhead. The art is balancing risk against load. Carry the minimum you need to reach the next dependable water with a safety margin, and lean toward more, not less, in remote or unfamiliar terrain where a source might be dry.
Planning and treating refills
Before a hike, map known water sources against your per-hour estimate so you know the longest dry stretch you must cover. Seasonal springs and small creeks can vanish in late summer, so check recent trip reports rather than trusting an old map. Never drink untreated wild water: filter it, use chemical treatment, or boil it to kill the bacteria, protozoa and viruses that cause backcountry illness. A lightweight filter plus a backup treatment method is a common setup that keeps you covered if one fails.