The short answer: 1 to 1.5 liters (34–50 fl oz) covers a 5-mile hike for most people in mild weather. The honest answer: that number can double before you’ve changed anything but the forecast. Here’s how to get your number instead of the internet’s average.

Start with time, not distance

Your body doesn’t drink by the mile — it drinks by the hour. Wilderness medicine references and the US National Park Service converge on about 0.5 to 1 liter per hour of moderate hiking.

So the first step is honest time math. A flat 5-miler at a 3 mph stroll is under two hours — the bottom of the range. The same 5 miles with 2,000 ft of climbing is closer to a 3.5-hour outing, and your water plan should know that before you do it the hard way. (Our hiking time calculator does this conversion with Naismith’s rule.)

Then multiply for conditions

Three factors move the 0.5 L/h baseline more than anything else:

  • Heat. Sweat loss rises steeply past about 25 °C (77 °F). Plan +50% in genuinely hot weather and +80% in desert heat — and shift your start earlier instead of carrying a swimming pool.
  • Altitude. Above ~2,500 m (8,200 ft) you lose extra water with every breath and your thirst signal weakens. Add roughly 10% per additional 1,000 m, and drink on schedule rather than on thirst.
  • Load. A heavy overnight pack raises your work rate — and your sweat rate — by 10–20% compared to a daypack on the same trail.

A worked example

Take a real 5-miler: 2,000 ft of gain, 28 °C, a light daypack, mixed sun.

  • Time: about 3 h 20 min for an average hiker (Naismith with terrain factor)
  • Rate: 0.5 L/h × 1.5 (heat) × 1.1 (partial sun) ≈ 0.83 L/h
  • Total: ≈ 2.7 liters — nearly double the generic advice

That difference — 1.5 L versus 2.7 L — is the gap between “finished thirsty” and cutting a hike short with a headache. The water intake calculator runs this exact math from your own inputs in seconds.

Two mistakes to avoid

Carrying nothing but water on a hot day. Long, sweaty efforts flush sodium as well as fluid. Past roughly 3 liters of intake, add electrolyte tabs or salty snacks — hyponatremia (over-diluted blood sodium) is rarer than dehydration but more dangerous.

Counting on water sources you haven’t verified. A stream on the map is not a stream in August. If your plan depends on refilling, check a recent trip report — and still start with a full liter of margin.

Pack for the hike you’re actually doing — the temperature, the climb, the altitude — and 5 miles stops being a guessing game.