How much water to bring hiking

The short answer: carry roughly 500 ml (half a liter) per hour of hiking, adjust for conditions, and replace electrolytes on any outing over two hours. Everything else is dialing that formula to your specific day.

The baseline and why sipping matters

A moderate hike in mild weather burns through roughly 500 ml of water per hour. Strenuous hiking in high heat can push that to 1 liter or more per hour. Those numbers come from REI's hydration guidance and align with the practical experience of most trail users.

The delivery schedule matters as much as the total volume. Drinking 6–12 oz every 15–20 minutes keeps pace with loss. Waiting until you are thirsty and then gulping a liter does not: your gut absorbs water at a limited rate, and thirst is already a lagging indicator by the time it registers.

That 2% figure deserves a moment. For a 180-pound hiker, it is only 3.6 pounds of fluid loss, enough to reduce strength and power by 2–5.5%. You can lose that much on a warm morning hike without feeling particularly thirsty.

Adjusting for heat, humidity, and altitude

Three conditions require you to revise your estimate upward before you leave the trailhead.

Heat and humidity increase sweat rate substantially. On exposed trails in hot conditions, sweat losses can exceed 2 liters per hour. If the forecast is warm and the route has little shade, plan on at least 1 liter per hour and carry capacity to match.

Altitude creates a compounding problem. At elevation, hikers lose approximately 20% more fluid through increased respiration in dry mountain air. At the same time, the thirst mechanism is suppressed, so you feel less thirsty even as you dehydrate faster. The fix is to drink on a schedule, not on demand.

Steep, sustained climbing pushes breathing and sweat rate regardless of temperature. On a long climb even in moderate weather, lean toward the higher end of the range.

When to carry vs. filter on trail

The two strategies are not competing. They answer different questions: how much capacity do you need, and how do you refill efficiently?

For most half-day hikes with mapped sources, a 2-liter hydration bladder like the HydraPak Contour or two 1-liter bottles covers the baseline comfortably. If you are planning longer days or moving through dry terrain, 3 liters of capacity plus a filter is a reasonable floor; hikers who train under a weighted pack run similar setups, and Ruck Authority's hydration bladder picks for rucking cover that variant. For gear options, see our guide to the best hydration bladders.

Electrolytes: why plain water is not enough on longer days

Water carries sweat out of your body. Sweat also carries sodium, potassium, and other electrolytes. On hikes over one to two hours, especially in heat, you need to replace those minerals alongside the fluid.

Hyponatremia (dangerously low blood sodium) is caused by drinking too much plain water without replacing sodium. It shares symptoms with dehydration but has the opposite fix: restrict fluid and eat salty food, do not drink more water.

The practical threshold is straightforward. Under two hours in mild conditions, plain water is fine. Once you cross two hours, or if conditions are hot or strenuous, add electrolytes. A sports drink, an electrolyte tablet dissolved in your water, or even a handful of salty crackers at a rest stop all work. The goal is to keep blood sodium from being diluted as you drink.

Knowing the difference: dehydration vs. hyponatremia

Both conditions share overlapping symptoms: headache, nausea, fatigue, confusion. The cause and treatment differ completely.

Dehydration means you have not drunk enough. The fix is water, ideally with electrolytes.

Hyponatremia means you have drunk large amounts of plain water and diluted your blood sodium too far. The fix is to stop drinking plain water and consume salty food. Drinking more water in this state makes the situation worse.

If you are on a long hot day and have been drinking steadily but feel off, consider what you have been drinking before deciding what to do next.

FAQ

For more on selecting a hydration system, see our hike gear hub. Details on how we research and rate every guide are on our how we research and rate page.