Hiking water calculator: body weight × hours × 5
Most hiking advice says "bring plenty of water" without a number. Japan's Hokkaido Government, warning summer hikers about mountain accidents, gives one: water needed (ml) = body weight (kg) × hours of activity × 5. Enter your weight and planned hours (including breaks). Everything is calculated in your browser.
Worked examples
| Body weight | Hours | Water |
|---|---|---|
| 110 lb (50 kg) | 3 hours | About 0.8 L (25 fl oz) |
| 110 lb (50 kg) | 6 hours | About 1.5 L (51 fl oz) |
| 130 lb (59 kg) | 3 hours | About 0.9 L (30 fl oz) |
| 130 lb (59 kg) | 6 hours | About 1.8 L (60 fl oz) |
| 150 lb (68 kg) | 3 hours | About 1.0 L (34 fl oz) |
| 150 lb (68 kg) | 6 hours | About 2.0 L (69 fl oz) |
| 180 lb (82 kg) | 3 hours | About 1.2 L (42 fl oz) |
| 180 lb (82 kg) | 6 hours | About 2.5 L (83 fl oz) |
| 210 lb (95 kg) | 3 hours | About 1.4 L (48 fl oz) |
| 210 lb (95 kg) | 6 hours | About 2.9 L (96 fl oz) |
Assumptions
- The formula is Hokkaido Government's, published with its summer hiking safety advice. We use it as is.
- It covers drinking water while moving, not cooking water at camp.
- Hokkaido also advises starting early because summer mountain weather tends to turn in the afternoon. Carry a margin in case your trip runs long (our suggestion).
- NPS lists water and water treatment supplies among the Ten Essentials; see The Ten Essentials and Treating water in the backcountry.
Everyday
Before each hike, use this to decide how many bottles to pack.
When it counts
On a long walk to a shelter or home after a disaster, you have a rough idea of the water you need.
Sources and when we checked them
- Hokkaido Government (Japan), beware of summer mountain accidents (2025-11-20, in Japanese) (checked 2026-10-31)
- National Park Service, Ten Essentials (checked 2026-10-31)
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