Estimate water supply for a multi-day trail.
On a multi-day trail the number that matters is not total consumption but carrying capacity, because you refill along the way. Capacity is set by the longest gap between reliable sources, converted to time using your daily pace: a source spacing of 18 km at 22 km a day is roughly 20 hours of walking, so you need most of a day's water in hand. Getting that conversion right is what distinguishes a comfortable trip from a rationed one. Total trip consumption is shown for planning purification capacity and filter life, not for deciding what to carry — water at a kilogram a litre is the last thing you want to over-provision.
Time between sources
Hours = source spacing km / daily distance km x 24
Carrying capacity
Litres per person = daily consumption x hours between sources / 24 + 0.5 L reserve
General planning guidance, not medical or safety advice. Always treat water from natural sources by filtering, boiling or chemical purification. Hydration needs vary with individual physiology, heat, altitude and exertion, and both dehydration and hyponatraemia are dangerous. Verify current water availability with recent trail reports before committing to a route.
Because the longest gap is the one that leaves you dry. Averages are irrelevant for a constraint that only has to be violated once to become a serious problem. Read the water report or trail notes for your route and size to the worst carry, then top up at every reliable source.
Highly variable. Seasonal streams and springs shown on maps routinely run dry by late summer, and small tanks can be fouled or empty. Use current trail reports and recent hiker updates rather than the map alone, and treat any single unconfirmed source as optional rather than planned.