Source spacing, not thirst, sets the carry
Two people with identical bodies and identical gear can walk the same trail in the same weather and carry weights that differ by seven or eight pounds, because one of them is walking a section with a creek every three miles and the other is walking the September version of that section with nothing running until the road. The daily rate is a personal constant. The carry is a route property.
That is why the calculation here starts from the gap rather than from the day. Your rate per hour comes from your own daily figure divided by the hours you walk, the gap divided by your pace gives the hours, and the two multiply. Add a dry camp if you sleep in the middle of the gap, because an evening and a morning without a source is often more water than the walking between them.
The weight, in numbers you can hold in your head
| Carried | Weight | Roughly the same as |
|---|---|---|
| 0.5 L | 1.1 lb | A headlamp and a phone |
| 1 L | 2.2 lb | A three-season sleeping pad |
| 2 L | 4.4 lb | A one-person shelter, poles and stakes |
| 4 L | 8.8 lb | Shelter plus quilt plus pad, together |
| 6 L | 13.2 lb | An entire lightweight base weight |
The right-hand column is the point. Somebody who spends a winter and several hundred dollars taking three pounds off a shelter can undo the whole exercise by filling up an extra litre and a half at a source they did not need to. The reverse is also true, which is the useful direction: knowing that the next spring is running lets you leave with two litres instead of five and drop seven pounds for free.
The reserve is for the source being wrong, not for drinking
The percentage on top is not extra thirst. It is what covers a spring that has stopped, a side trail that turned out to be longer than the description, a hotter afternoon than forecast, or an hour spent looking for a camp. Twenty percent on a nine-mile carry is under a litre and costs two pounds. On a twenty-mile carry it is more than two litres and starts to be a real decision, which is the point at which people carry less reserve and pay closer attention to whether the next source is confirmed.
What no reserve percentage covers is a single source with no alternative. If the whole plan rests on one spring and the spring is dry, arithmetic is not the tool — a second option is, whether that is a longer walk to a known creek, a cache placed beforehand, or a different route.
Filtering time is real time
A group of four leaving a source with three litres each is twelve litres through one filter. At a clean rate of a litre and a half a minute that is eight minutes, which is nothing. At the half-litre a minute a clogged cartridge often delivers it is twenty-four minutes, in the cold, at the point in the day when everybody wants to be walking. Gravity systems trade weight for not having to stand there. Chemical treatment trades weight for waiting. Both are legitimate and the choice tends to be made at the source rather than at home.
Questions people ask
How much does a litre of water weigh?
About 2.2 pounds, or one kilogram, which is close enough to exact that nothing in trip planning turns on the difference. A US gallon is 3.785 litres and weighs about 8.34 pounds. These are the only weights in a pack that cannot be reduced by buying something better, which is why water planning gives a bigger return than gear shopping on most trips.
How much water should I carry per mile?
That figure is derived, not given: it is your own hourly rate divided by your pace. If you use three and a half litres across eight hiking hours, that is 0.44 litres an hour, and at 2.2 miles an hour it works out around 0.2 litres a mile. Change any of the three inputs and the per-mile number changes with it. The reason this page asks for a daily figure rather than a per-mile one is that people know their daily figure from experience and nobody has ever measured their own per-mile rate.
Where does the daily water figure come from?
From you. What a specific person needs to drink depends on their body, the heat, the effort and things a web page has no access to, and it is a question for a doctor or dietitian rather than a calculator. The field on this page takes whatever number your own experience on similar days has produced and multiplies it by the hours the gap will take. It does not suggest a value and it will not tell you whether the answer is enough.
How much extra water for a dry camp?
Enough for dinner, drinking through the evening, whatever you use overnight, breakfast, and the first stretch of walking before you reach water. That is a separate figure from the walking rate because most of it is not walking, and for most people it lands somewhere in the region of one to three litres. Cooking is the part people forget: a freeze-dried dinner and a hot drink can be most of a litre before anybody has drunk anything.
Is it worth carrying a spare filter for a group?
It depends less on failure risk than on how long you are willing to stand at sources. One filter serving four people at three litres each means twelve litres per stop, and a partly clogged cartridge turns that into a twenty-minute wait several times a day. A second filter, or a gravity setup that works while you do other things, buys back that time. As a redundancy question it is a different argument, and chemical treatment carried as a backup weighs almost nothing.