How much water should you carry?

Why there is no built-in answer

Most hiking pages hand you a number such as a liter per hour in the heat. The medical guidelines do not. The Wilderness Medical Society's 2024 update on heat illness recommends drinking to thirst during activity, and its guidelines on exercise-associated hyponatremia state that no specific volume of fluid has been shown to prevent that condition, so the best advice is to drink in response to thirst. The American College of Sports Medicine's 2007 position stand explains why a single figure fails: sweat rates and sweat sodium vary a great deal between individuals, so fluid plans should be personal. This calculator therefore asks for your rate and does the part that is pure arithmetic: how many hours, how many people, how much in camp, how much margin, and how heavy the result is.

The method

Water to carry = rate × hours × people + camp use × people + a reserve, where the reserve is a percentage of the first two terms. The weight uses 1 kilogram per liter, the density of water to within a fraction of a percent at ordinary temperatures. In the other mode, the hours are the water you carry divided by your rate times the number of people.

Worked example

You drink half a quart an hour and the next reliable stream is 4 hours away. That is 2 quarts on the trail, plus 1 quart for camp and a 20 percent reserve on the 3 quarts: 0.6 quarts. The total is 3.6 quarts, 7.5 pounds. Three people on the same stretch carry 10.8 quarts together, 3.6 each. Reverse the question: with 3 quarts and 0.75 quarts an hour, the water lasts 4:00 for one person.

Finding your rate

The most direct way is to weigh yourself before and after a hike of the kind you plan, count what you drank, and use the sweat rate calculator. The American College of Sports Medicine's stand aims to keep water loss below about 2 percent of body weight; the National Park Service suggests eating often, not forcing fluids and taking at least one 10-minute break each hour on Grand Canyon hikes. Rates rise with heat, effort and a heavy pack. Acclimatization changes the picture too: the 2024 guidelines note that most of it is achieved in about a week of moderate heat exposure, an hour or two a day.

Limits of this method

• Hours, not miles. The calculator works in time because that is what drinking follows. Turn a distance into hours with the hiking time calculator, and add the time for rests.
• Water sources are not guaranteed. A spring on a map can be dry in late summer, and a reservoir can be unsafe to drink. Check recent reports from rangers or other hikers and carry extra when in doubt.
• Treatment takes time and fuel. The CDC says to boil clear water for one minute, or three minutes above 6,500 feet. Chemical treatment and filters have their own wait times and limits; follow the label.
• Food and salt. Water alone is not the whole story on a long, sweaty day. The Park Service recommends salty snacks together with water on any hike over 30 minutes.

Common questions

How much water should I drink per hour when hiking?

There is no figure in the guidelines to quote. The Wilderness Medical Society's 2024 heat illness guidelines recommend a drink-to-thirst approach, and its hyponatremia guidelines say that no fixed amount of fluid has been shown to prevent illness. Sweat rates differ a lot between people, which is why the American College of Sports Medicine favors a personal plan. Measure your own loss with the sweat rate calculator, or note how much you drink on a similar hike, and type that in. The 0.5 quarts an hour in the calculator is only a placeholder.

Can you drink too much water on a hike?

Yes. Drinking far more than you sweat can dilute the sodium in the blood, a condition called exercise-associated hyponatremia. The Wilderness Medical Society advises limiting drinking to thirst. The National Park Service tells Grand Canyon hikers not to force fluids, to eat often, and to have salty snacks and water or a sports drink on any hike lasting longer than 30 minutes.

How much does the water weigh?

A liter of water weighs a kilogram, which is 2.2 pounds, and a quart weighs 2.1 pounds. The example on this page, a 4-hour stretch for one person with camp use and a 20 percent reserve, comes to 3.6 quarts, about 7.5 pounds. Three people on the same stretch carry 10.8 quarts, 22.5 pounds.

How do I know how long my water will last?

Switch the calculator to the second mode. Carrying 3 quarts and drinking 0.75 quarts an hour lasts 4:00 of hiking for one person. For a group, the time is the water divided by the combined rate. Aim to reach the next source with water left, since a source that turns out to be dry is a risk worth planning for.

Do I need to treat water from a stream?

Yes, unless you have confirmed it is safe. The Centers for Disease Control and Prevention says to boil clear water for one minute, and for three minutes above 6,500 feet (2,000 m), where water boils at a lower temperature. Boiling does not remove chemicals. The altitude and boiling point calculator shows why the extra time matters at elevation.

How is this different from the sweat rate calculator?

The sweat rate calculator measures how much fluid you lost in a workout from your weight before and after. This one starts from a rate you have chosen and turns it into a volume and a weight for a stretch of trail. Use the first to find your rate and the second to pack for it.

This is a planning aid, not a safety assessment or medical advice. If someone is confused, vomiting or stops sweating in the heat, treat it as an emergency and call for help.

Sources: Wilderness Medical Society, Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness, 2024 update (Wilderness & Environmental Medicine 35(1_suppl)), as summarized in American Family Physician 2025;112(6):700–702; Wilderness Medical Society hyponatremia guidelines, 2019 update, as summarized in American Family Physician, February 15, 2021, p. 252 (the guidelines themselves not opened); American College of Sports Medicine, "Exercise and Fluid Replacement," Medicine & Science in Sports & Exercise 39(2):377–390 (2007), abstract level; National Park Service, Grand Canyon, "Hiking Tips"; Centers for Disease Control and Prevention, boil water guidance; U.S. Geological Survey, Water Science School. Related: Sweat Rate Calculator, Water Intake Calculator, Hiking Time Calculator, Backpack Weight Calculator, Altitude Pressure and Boiling Point Calculator.