Water calculator

How much water you need

Sweat loss on a hike is set mostly by body weight, hours on the move and temperature. The common planning formula is:

Water (mL) = body weight (kg) × 5 × hours × temperature factor

60 kg for 6 hours is 1.8 L. In midsummer (30°C and above) multiply by 1.5 for 2.7 L. The goal is not to finish every drop but to drink little and often so you never get behind.

Drink before you are thirsty

Thirst arrives late, after you have already lost 1–2% of your body weight in fluid. Aim for 150–200 mL every 30 minutes. A hydration bladder with a hose makes this effortless; if you prefer bottles, keep one in a side pocket you can reach without stopping.

Water on Japanese trails

Marked springs (水場, mizuba) on Japanese maps can run dry in summer, and mountain huts sell water for ¥100–200 per litre or more at high altitude, where everything is carried in by helicopter. Reduce what you carry only when you have current information that a source is flowing. Drinking straight from streams is risky below huts or toilets; a small filter widens your options.

Salt matters too

Sweat takes sodium with it. Drinking large volumes of plain water can dilute your blood sodium and make you feel worse, not better. Mix salty snacks into your trail food (Japanese convenience stores sell salt tablets and umeboshi pickled plums) or carry a diluted sports drink.