Carry It or Filter It? What Water Weighs on a Day Hike
Leave-no-trace thinking and light-pack thinking arrive at the same question about water: do you carry everything you need from the trailhead, or carry less and refill from streams and springs on the way? Carrying it all is simple and certain. Refilling saves weight but depends on a source being there and on treating the water properly. This guide puts numbers on the trade-off with three of the site's calculators, using a hypothetical warm-weather day.
Step 1: how much water does the day need?
Suppose a six-hour day on the trail, including stops, at 26°C (about 79°F) on a rolling route. The Water Needs Calculator gives:
- Temperature factor: 1.3 (the 20 to 28°C band).
- Rate: 0.65 litres per hour at moderate effort.
- Total: 3.9 litres.
That is a planning figure for an adult, not a prescription. Heat, sun exposure, humidity, altitude and how much you personally sweat can all push the real figure higher. But it is a sound starting point, and it is already a lot of weight: a litre of water weighs about a kilogram, so 3.9 litres is about 3.9 kg.
Step 2: what does it do to the pack?
The Backpack Weight Calculator splits a load into base weight (gear that stays the same all day) and consumables (food, water, fuel). Take a 70 kg hiker with a 6 kg base weight, a fairly full day pack with layers, first aid and a few comforts, and 0.8 kg of food.
| Plan | Water carried at the start | Total pack | Share of body weight |
|---|---|---|---|
| Carry it all | 3.9 kg | 10.7 kg | 15.29% |
| Carry 2 litres, refill once | 2 kg | 8.8 kg | 12.57% |
Both plans sit within the calculator's 20% body-weight guideline, which is a rough guide rather than a medical limit. The refill plan takes almost 2 kg off the heaviest part of the day, the first climb with a full load. On a hotter day, where the water figure might be 6 litres or more, the difference grows to the point where carrying it all becomes genuinely hard work.
Remember too that water weight falls as you drink. If you carry everything, the pack is heaviest at the start and lightest at the end. If you refill, it is heavy twice, but never as heavy as the full-carry start.
Step 3: what does the weight cost in energy?
The Hiking Calorie Burn Calculator uses the Pandolf equation, which counts pack weight directly. For the same 70 kg hiker walking the moving part of the day, 16 km at 3.5 km per hour on level, rocky trail (terrain factor 1.2):
- With the full 10.7 kg pack: about 968 kcal.
- With the 8.8 kg refill pack: about 950 kcal.
The energy saving is small, under 20 kcal, and that is an honest result worth stating: on level ground, a couple of kilograms of water barely changes the energy cost of walking. The equation's load term grows with the square of the pack's share of body weight, so the cost of extra weight rises steeply only for heavy loads. The real benefits of carrying less are elsewhere: less strain on knees and shoulders, a more comfortable climb, better balance on rough ground, and a pack that is easier to manage over a long day. On steep climbs, where every kilogram is lifted, the difference is also larger than this flat-ground figure suggests.
When refilling makes sense
Refilling is a good plan when all of these are true:
- The source is reliable. You know from recent, trustworthy information that it is flowing. Seasonal streams and springs dry up, often in exactly the hot, dry spells when you need them most. Check the land manager's current information, and treat a map symbol as a possibility, not a promise.
- It comes early enough. You can reach it with the water you carry from the start, plus a margin, even if you are slower than planned.
- You can treat the water. Never drink untreated water from natural sources, however clear and cold it looks. Filters, chemical tablets or drops, UV devices and boiling all work when used as the maker intends; carry a backup method in case a filter clogs or breaks.
- There is a plan B. If the source is dry, you have enough water to turn around and get back.
If any of these is uncertain, carry it all. Two kilograms of extra water is a nuisance; arriving at a dry stream with an empty bottle in the heat is dangerous.
A practical refill rule
For the refill plan above: carry 2 litres from the start. The calculator's rate is 0.65 litres per hour, so 2 litres covers about three hours. If the source is, say, two hours in, you arrive with a litre of margin. Refill to 2 litres, and the remaining time is covered with margin to spare. If the source is dry when you get there, you have enough to walk back to the trailhead on the same route. That is the test: never let reaching an uncertain source be the only way to finish the hike safely.
A second rule worth carrying with you: if you have drunk half your water and are not yet halfway to the next sure supply, turn back or change the plan.
Drinking right, not just carrying right
Having water in the pack is only useful if you drink it. Drink regularly at stops, and do not save water "for later" until you are already dehydrated. On long, hot, hard days, sweat carries away salt as well as water: eat salty snacks, and consider electrolytes on days the water calculator flags them.
It is also possible to overdo it. Drinking large amounts of plain water without replacing salt can lead to hyponatremia, low blood sodium, whose early signs (nausea, headache, confusion, tiredness) look like dehydration. Drinking to thirst, with food, is a sensible guide for most healthy adults.
Watch for heat exhaustion: heavy sweating, weakness, dizziness, headache, nausea, cramps. Stop in shade, cool down, sip water, eat something salty and rest; then take the easiest way out. Confusion, collapse or very hot skin may be heatstroke, an emergency: call the local emergency number and cool the person aggressively while you wait. Our guide to water on a hot hike covers the signs in more detail.
Leave no trace at water sources
Refilling well is also about the source itself. Collect water without trampling banks, wash and use soap well away from streams and lakes, and keep toilet stops a good distance from water. Many places have specific rules about water sources and campsites; follow the land manager's guidance where you are.
Summary
Work out how much water the day needs with the calculator. Decide whether a reliable source lets you carry less. If it does, carry enough to reach it with a margin and to get back if it is dry, treat everything you collect, and enjoy a lighter pack. If it does not, carry the full amount and accept the weight; on level ground it costs less energy than you might think, and it buys certainty. Either way, carry the basics, tell someone your route and return time, and know the local emergency number.