How much water should you carry per hour hiking?

Carry between 0.5 and 1.4 litres for every hour of hiking, chosen by how hot it is and how hard you are working. That range is the US Army's heat doctrine, TB MED 507, converted from quarts: half a quart an hour for light work in the mildest heat category, up to 1.5 quarts an hour for loaded marching in the hottest, and never more than that. Multiply your hourly rate by the hours to the next reliable source.

How much water per hour does the Army table say?#

The Army Public Health Center prints the two fluid tables from TB MED 507, the Army's heat stress manual, on a one-page poster. Table 1 covers training up to 4 continuous hours and builds rest into every hour (30 minutes on, 30 off, for example). Table 2 covers continuous work beyond 4 hours, with no rest cycle prescribed. A hiker walking most of each hour is closer to Table 2, and its quarts convert to litres like this:

The poster defines the work rates by example. Easy is weapons maintenance, marksmanship training, drill and ceremony: work done standing. Moderate is "patrolling with a 30-pound load" (13.6 kg). Heavy is patrolling with a 45-pound load (20.4 kg), a four-person litter carry, or jogging at 4 mph. Walking a trail with a day pack sits between easy and moderate; a backpacking load is moderate or heavy. Neither table has a row below 78 degrees F WBGT, so the doctrine says nothing about cool days.

Why harder work does not mean more water#

Across any row, the water stops rising once you leave easy work. In Category 3, moderate, heavy and very heavy work all get 0.95 litres an hour. What changes is the time limit. In Table 2, moderate work is unlimited until Category 4 (180 minutes) and Category 5 (70 minutes); heavy work is capped at 100 minutes in Category 1, falling to 45 in Category 5; very heavy work at 45 minutes, falling to 20.

The reason is the ceiling. The TRADOC version of the same table, reprinted by the Military Health System in May 2026, carries two cautions: "Hourly fluid intake should not exceed 1½ qts" and "Daily fluid intake should not exceed 12 qts". Montain, Latzka and Sawka described the 1999 revision of the Army table. The Army had paid "little consideration" to overhydration, and there was "an increasing incidence of water intoxication during military training". The table was redesigned to sustain hydration "while protecting the soldier from becoming sick from overdrinking".

For a hiker, a steep climb in the heat is managed with pace and shade breaks, not with more than 1.42 litres an hour.

What changes the number for you?#

The TRADOC notes give two corrections, each a quarter of a quart (0.24 L) an hour:

• Individual differences. A heavy sweater adds 0.24 L; a light one subtracts it.
• Sun or shade. Full sun adds 0.24 L; full shade subtracts it.

Stack both and one cell spans a full quart. Category 3, moderate work, is 0.95 L at the centre: 0.47 L for a light sweater in shade, 1.42 L for a heavy sweater in full sun. Category 4 for the same heavy sweater in sun would come to 1.66 L, past the ceiling, so the ceiling wins and the answer becomes slowing down or waiting for shade.

The table was built for an "average-sized and heat-acclimatized service member" in uniform. A child, a small adult or someone on their first hot day of the year is outside that, and the doctrine gives no correction for them; weighing yourself (below) does.

WBGT is not the temperature on your phone#

The categories are in WBGT, wet bulb globe temperature. OSHA describes the instrument as three thermometers: dry bulb for air temperature, natural wet bulb for evaporative cooling, black globe for radiant heat. It accounts for "temperature, humidity, radiant heat, and wind"; an ordinary thermometer measures one.

• The same air temperature can sit in different categories. Humidity and sun push WBGT up; dry air, wind and cloud pull it down. A rule written in air temperature cannot capture that.
• Sun matters more than the forecast suggests. Grand Canyon's park service says sun temperatures run 15 to 20 degrees F (9 to 11 degrees C) above posted shade temperatures.

You do not need an instrument: the National Weather Service publishes an experimental WBGT forecast for direct sunlight, by region, up to 7 days ahead. Find the category and read the rate.

Half a litre or a litre an hour: which rule is right?#

Each is right somewhere. Advnture's rule is "one liter of water for every 2 hours of hiking", 0.5 L an hour, raised by 150 to 200 ml an hour for every 10 degrees F above a comfortable temperature. Australian Hiker's author allows 1 litre an hour in "very hot weather (typically 30 degrees celsius +)" and reports averaging about 600 ml an hour on a 13-hour, 58 km day.

Set them against the doctrine:

• 0.5 L an hour is the easy-work rate for Categories 1 and 2 (0.47 L). For anyone carrying a pack in WBGT above 78 degrees F, it is too low: moderate work starts at 0.71 L.
• 1 L an hour is the moderate-work rate for Categories 2 and 3 (0.95 L). It is the right planning figure for a loaded hike on a hot day, but too low for Categories 4 and 5, where the doctrine goes to 1.18 and 1.42 L.

The half-litre rule survives because most hiking happens in cool conditions the tables never cover. It fails on exactly the days when running short matters.

How much does the water weigh between sources?#

Water is 1 kg per litre (2.2 lb), so the carry weight is the rate times the hours. These figures exclude the bottles.

Six hours at the ceiling is 8.5 kg, heavier than many complete base weights. That is the case for planning a hot route around water rather than carrying for the whole day, and the base weight calculator shows how much of your total load the water becomes.

A refill rule you can work on a map#

Our rule: carry the hourly rate times the hours to the next reliable source, plus one hour's ration in reserve. The reserve is our choice, not doctrine. It covers a slow pace, a dry source or an early hot afternoon.

Worked example. A Category 3 forecast, a pack, so 0.95 L an hour. The next reliable source is 15 km away; at an assumed 4 km an hour that is 3.75 hours. Add the reserve hour: 4.75 hours times 0.95 is 4.5 litres, 4.5 kg. The half-litre rule would give 2.4 litres, 2.1 kg lighter and two and a half hours short.

Count hours, not kilometres, at your own pace on similar ground: a climb takes longer and needs more water. Use the category for the hottest part of the stretch. Cooking water is extra, worked through in how much stove fuel to carry, and every litre from a stream needs treating by a filter that still works: see how long a backpacking water filter lasts.

What happens when sweat outruns the ceiling?#

Grand Canyon's park service says "Hikers can lose one or two quarts (liters) of water per hour." Two quarts is 1.89 litres against a 1.42 ceiling, so a hiker drinking at the limit still falls 0.47 litres an hour behind.

The American College of Sports Medicine sets the goal of drinking during exercise as preventing more than 2 percent loss of body weight from water deficit. For a 70 kg hiker, our example, that is 1.4 kg. At a 0.47 L an hour shortfall, 2 percent arrives in about 3 hours. Carrying more water does not stop it. The park service's answer is not to hike between 10 am and 4 pm, and it says most of the people who need emergency help for heat illness in the canyon were hiking in those hours.

The daily cap sets a second limit. 12 quarts is 11.4 litres, which is 8 hours at the hourly ceiling. A full day in Category 5 hits the daily cap before the day ends.

Can you drink too much water while hiking?#

Yes, and that is what the ceiling is for. Grand Canyon's page describes hyponatremia as low blood sodium "caused by drinking too much water and losing too much salt through sweating", with symptoms that mimic early heat exhaustion: fatigue, nausea, vomiting, confusion, frequent urination. Among Grand Canyon hikers studied by Backer and colleagues, 7 of 44 patients whose blood was analysed had symptomatic hyponatremia below 130 mmol/L, 3 of them with grand mal seizures, and the authors report indirect evidence that they were overhydrated. Seizures and altered mental state distinguished them from heat exhaustion.

The park service's page carries both sides of the familiar argument. One heading reads "Don't Force Fluids, Drink When You Are Thirsty", and the text below it says to "sip water regularly throughout your hike, even if you don't feel thirsty." The tables settle it in practice: drink steadily to the rate for the category, eat salty food with it as the park service advises, and do not force water past 1.42 litres an hour to catch up.

Measure your own sweat rate#

The ACSM position stand recommends customised fluid plans because sweat rates differ so much between people, and gives the method: weigh before and after exercise.

1. Weigh yourself in minimal clothing before a hike of at least an hour, on a day as hot as the trips you plan.
2. Record every drink from a measured bottle.
3. Weigh again at the end in the same clothing, towelled dry, before eating or drinking.
4. Add the water you drank to the weight you lost. Each kilogram is about a litre of sweat (less any urine).
5. Divide by the hours walked for your sweat rate at that heat and load.
6. Compare it with the table. Well above the cell for that day's category, plan on the cell plus 0.24 L and expect the ceiling to bind earlier. More than 2 percent of your starting weight lost means you drank too little.

Sweat also costs heat as it evaporates, which is the reason the moving layer has to pass vapour.

Common mistakes#

Using air temperature as the heat category. The tables are in WBGT, which counts humidity and sun. Look up the WBGT forecast.

Planning a loaded hot hike on 0.5 litres an hour. That is the easy-work rate. With a pack, Category 1 already needs 0.71 L and Category 3 needs 0.95 L.

Drinking more to make up for a hard climb. The doctrine answers harder work with shorter work periods and rest. Above 1.42 L an hour, more water is a hyponatremia risk.

Drinking plain water all day in heat without eating. Grand Canyon's guidance pairs water with electrolytes and salty snacks.

Frequently asked questions#

How much water should I drink per hour while hiking?#

In heat, 0.71 to 1.42 litres an hour with a pack, by the US Army's TB MED 507 table, depending on the WBGT heat category. For easy walking it is 0.47 to 0.95 litres. Add or subtract 0.24 litres for a heavy or light sweater, and again for full sun or shade, but never drink more than 1.42 litres an hour or 11.4 litres a day.

How much water should I bring on a 4 hour hike?#

Four times your hourly rate, plus a reserve if there is no source on the way. In mild conditions at 0.5 litres an hour that is 2 litres; on a hot day with a pack, Category 3, it is 3.8 litres for the walking hours and 4.7 with a reserve hour. Each litre weighs 1 kg.

Is a litre an hour too much water?#

Not on a hot day with a pack. A litre an hour is close to the Army's 0.95 litre rate for moderate work in heat Categories 2 and 3 (WBGT 82 to 87.9 degrees F). For easy walking in mild heat the doctrine gives 0.47 litres. The limit to respect is 1.42 litres an hour.

What is the maximum water you should drink in an hour?#

US Army guidance caps drinking at 1.5 quarts an hour, about 1.42 litres, and 12 quarts a day, about 11.4 litres. The tables were revised in the late 1990s because water intoxication was increasing in military training. Drinking past that does not keep up with heavy sweating; it adds a risk of low blood sodium.

How far apart can water sources be?#

As far apart as the water you can carry lasts. Divide what you are willing to carry by your hourly rate to get hours, take off one hour for a reserve, and turn hours into distance with your own pace. At 0.95 litres an hour and a 4 litre carry, that is about 3.2 hours of walking between reliable sources.

Should I drink before I feel thirsty when hiking?#

Drink steadily to a planned rate rather than waiting for thirst or forcing water. Grand Canyon's guidance says to sip regularly even if not thirsty, and also not to force fluids. The Army table gives the rate: 0.47 to 1.42 litres an hour by heat and work. Steady drinking at that rate, with salty food, avoids both dehydration and overdrinking.