How Much Water to Bring Hiking: Calculate It With Body Weight × Hours × 5
When you pack for a day hike, the hardest call is often how many litres of water to carry. Too little can lead to dehydration and leg cramps; too much makes your pack heavy and wears you out. This guide shows how to estimate the water you lose through sweat and breathing with body weight × hours walking × 5, gives a quick-reference table by weight and time, explains why you only need to drink 70 to 80% of what you lose, covers how much spare water to carry, and shows with real numbers how your pace and the season change the amount.
Estimate fluid loss as body weight (kg) × hours walking × 5 ml
Working out how much water you need for a hike starts with estimating how much water leaves your body while you walk (your fluid loss). A widely used guideline from Masayoshi Yamamoto of the National Institute of Fitness and Sports in Kanoya, who researches the exercise physiology of mountaineering, is: fluid loss (ml) = body weight (kg) × activity time (h) × 5. It covers not just sweat but also the moisture you breathe out.
For example, a 60 kg person walking for 5 hours loses 60 × 5 × 5 = 1,500 ml. The heavier you are and the longer you walk, the more you lose, in direct proportion. "Hours walking" here means time actually on the move, not breaks. If you can keep to the standard course time on the map, use that; if you walk slowly, allow a little longer.
Quick-reference table by body weight and hours walking
The table shows the fluid loss, how much to drink while walking (70 to 80% of the loss), and the water to carry: the top of the drinking range plus 500 ml of spare water. The water to carry is rounded to the nearest 50 ml.
| Body weight | Hours walking | Fluid loss | Drink (70 to 80%) | Water to carry (incl. 500 ml spare) |
|---|---|---|---|---|
| 50 kg | 3 hours | 750 ml | 525 to 600 ml | 1,100 ml (three 500 ml bottles) |
| 50 kg | 5 hours | 1,250 ml | 875 to 1,000 ml | 1,500 ml (3 bottles) |
| 50 kg | 8 hours | 2,000 ml | 1,400 to 1,600 ml | 2,100 ml (5 bottles) |
| 60 kg | 3 hours | 900 ml | 630 to 720 ml | 1,200 ml (3 bottles) |
| 60 kg | 5 hours | 1,500 ml | 1,050 to 1,200 ml | 1,700 ml (4 bottles) |
| 60 kg | 8 hours | 2,400 ml | 1,680 to 1,920 ml | 2,400 ml (5 bottles) |
| 70 kg | 3 hours | 1,050 ml | 735 to 840 ml | 1,350 ml (3 bottles) |
| 70 kg | 5 hours | 1,750 ml | 1,225 to 1,400 ml | 1,900 ml (4 bottles) |
| 70 kg | 8 hours | 2,800 ml | 1,960 to 2,240 ml | 2,750 ml (6 bottles) |
| 80 kg | 5 hours | 2,000 ml | 1,400 to 1,600 ml | 2,100 ml (5 bottles) |
As a rough guide, a 3-hour low-level walk needs a little over 1 L, a 5-hour day hike 1.5 to 2 L, and a long 8-hour day 2 to 3 L. You often hear "1 litre is enough for a day hike", but a 60 kg person walking for 5 hours needs to drink more than 1 L on the trail alone.
Why you only need to drink 70 to 80% of your fluid loss
You do not have to replace every drop as you go. Your body has some reserve, and drinking 70 to 80% of what you sweat out while walking is generally considered enough to prevent dehydration serious enough to affect how your body works. The rest you can make up with food and drinks after the hike.
- Drinking 200 to 300 ml at each break is absorbed better than drinking a lot at once
- By the time you feel thirsty, you are already starting to run short. Drink before you get thirsty
- Sweat also costs you salt, so on long routes or hot days combine water with sports drinks or salt sweets
- Drinking well under 70% makes leg cramps, headaches and sudden exhaustion more likely
To work out how much to drink at each break, divide the drinking amount by the number of breaks. For a 60 kg person walking 5 hours, the upper drinking amount is 1,200 ml: five breaks at 60-minute intervals means 240 ml each time, and ten breaks every 30 minutes means 120 ml each time. Planning breaks together with food is covered in the trail food article.
Spare water, and how much more to carry for pace and season
The drinking amount you calculate assumes everything goes to plan. Add spare water for getting lost, waiting for an injured companion or a hotter day than expected. For a day hike, around 500 ml is a sensible amount, plus extra if you plan to make instant noodles or coffee at the summit.
| Walking pace (60 kg, 5-hour course time) | Hours walking | Fluid loss | Water to carry |
|---|---|---|---|
| Fast (0.8 × course time) | 4 hours | 1,200 ml | 1,450 ml |
| On course time | 5 hours | 1,500 ml | 1,700 ml |
| Slow (1.2 ×) | 6 hours | 1,800 ml | 1,950 ml |
| Very slow (1.5 ×) | 7 hours 30 minutes | 2,250 ml | 2,300 ml |
Walking slowly feels as if it should mean less sweat, but because you are out longer, your total fluid loss for the day goes up. If you are a beginner or hiking with children and expect to take longer than the course time, recalculate using your own pace. How to work out your own pace factor is explained in the course time article.
How heavy water is, and how to split it between bottles
One litre of water weighs almost exactly 1 kg. If you carry 1.7 L of water, that alone adds 1.7 kg to your pack. Rather than cutting water to save weight, it is safer to rethink the rest of your gear.
- 1
Split it into 500 ml bottles
Finishing one bottle at a time shows at a glance how much is left, and counting the bottles remaining helps you pace yourself.
- 2
Keep one bottle within easy reach
A side pocket or a hydration bladder (a water reservoir with a drinking tube) lets you sip regularly, not just at breaks.
- 3
Put the spare bottle at the bottom of your pack
Treat the spare as if it were not there. If everything goes to plan, you come down without touching it.
- 4
Note how much was left after the hike
Recording how much you had left over or ran short each time helps you find the right amount for you on future hikes.
Enter the course time, distance and elevation gain along with your body weight and pack weight into the hiking water and trail food calculator, and it works out your fluid loss, how much to drink and how much water to carry, right down to the number of 500 ml bottles. You can change the spare amount too, so adjust it for the season and route. To get an idea of how hot or cold it will be, see the summit temperature article.
FAQ
- Is 1 litre of water enough for a day hike?
- It is often enough for a low-level walk of about 3 hours, but if you walk for 5 hours or more it probably will not be. A 60 kg person walking for 5 hours loses about 1,500 ml and should drink around 1,050 to 1,200 ml. Including spare water, carrying 1.5 to 2 L is safer.
- How much water should I drink at a time while hiking?
- Divide your drinking amount (70 to 80% of your fluid loss) by the number of breaks. For a 60 kg person walking 5 hours with a break every 60 minutes, that is about 240 ml each time. Drink a little at every break, before you get thirsty.
- How do I work out water for children?
- Use the same formula with the child’s body weight. Children are not as good at regulating their temperature as adults and may not say they are thirsty, so remind them to drink regularly and carry extra spare water.
- Is water or a sports drink better?
- On short routes water alone is fine, but in summer or on long routes where you sweat a lot, carrying some sports drink to replace salt and sugar helps prevent cramps and exhaustion. Many hikers carry one bottle of each.
- If I have water left over, did I bring too much?
- Having your spare left over is normal. If a lot of your drinking water (not counting the spare) was left, you can carry a little less next time. If you finished even your spare, carry more next time or rethink how you drink at breaks.
Tools in this article
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