Water is the single heaviest item in most backpacks. At 2.2 pounds per liter, carrying too much is a brutal tax on your back and knees. Carrying too little is dangerous. Most backpackers default to “fill up everything” — which is fine on a short trip, but wasteful on a thru-hike where you’re making that choice 20+ times.
Here’s how to actually figure out what you need.
The Baseline Formula
Start with this as a floor, then adjust for conditions:
0.5 liters per hour of active hiking = a solid baseline for moderate climate, moderate effort.
For an 8-hour hiking day, that’s 4 liters consumed during movement. Add 1–2 liters for camp use (cooking, drinking, hygiene) = 5–6 liters per day total.
Now here’s the trick: you almost never need to carry 6 liters at once. You carry between water sources, then refill. Your target isn’t “a day’s water” — it’s “enough to reach the next source plus a buffer.”
The Real Calculation: Source-to-Source
Plan your water this way:
- Identify water sources on your map (streams, springs, cached water, lakes). Use Gaia GPS, Caltopo, or FarOut (formerly Guthook) for crowdsourced reliability.
- Calculate the longest dry stretch between sources on your route.
- Multiply by 0.5 L/hour for the hours it’ll take you to cover that stretch.
- Add a 1-liter safety buffer — always.
- Fill enough to reach the next source + buffer, not more.
Example: You’re hiking a 12-mile day with water at mile 0, 5, and 12. You plan 3 mph including breaks.
- Mile 0 → 5: ~1.5 hours = 0.75 L consumed → carry 1.5 L leaving the source
- Mile 5 → 12: ~2.5 hours = 1.25 L consumed → carry 2.25 L leaving mile 5
You never carry more than 2.25 L on this day, versus the 6 L you’d carry if you packed “a full day” from the start.
Adjustments by Climate
Desert (Southwest, Great Basin, southern CDT)
Double your baseline: 1.0 L/hour. Some hikers carry 1.5 L/hour in peak sun.
Desert hiking evaporates water through sweat faster than you register thirst. Your urine is the telltale — if it’s darker than pale yellow, you’re behind. In the desert, plan for the worst-case reliability of each water source. “Seasonal spring” in July often means dry.
Practical approach: carry enough for the longest stretch + a full 2-liter buffer. When in doubt, fill up. A heavy pack leaving a reliable source is cheaper than a dehydration emergency 8 miles out.
Alpine / High Mountain (Sierra, Rockies, North Cascades)
0.4–0.5 L/hour at altitude in cool temps. Thin air and dry conditions can still dehydrate you despite the cold.
Water is usually plentiful above 9,000 feet — streams from snowmelt, tarns, high-altitude springs. Carry less between sources but drink consistently. Altitude sickness symptoms (headache, nausea) overlap with dehydration — treat the dehydration first.
Humid Forest (Appalachian, Pacific Northwest, Ozarks)
0.4 L/hour in cool humid conditions. Less in cold rain.
Paradox of humid climates: you sweat constantly because it can’t evaporate off your skin. You may not feel thirsty but you’re still losing water. Force yourself to drink on schedule (every 30–60 min), not just on thirst.
Water is usually abundant in these zones, so you can carry minimal buffer. But treat everything — beaver fever (giardia) is rampant in forest streams.
Hot & Humid (Southeast in summer, jungle regions)
0.8–1.0 L/hour. Heat + humidity is the worst-case for hydration. Your body can’t cool efficiently, so it loses water aggressively through sweat.
Add electrolytes — plain water isn’t enough. Nuun, LMNT, or Liquid IV work. In serious heat, one full electrolyte dose per liter isn’t excessive.
Adjustments by Terrain & Effort
Beyond climate, hard efforts cost water. Add to baseline:
| Effort | Additional water |
|---|---|
| Steep climbing (>2,000 ft/day) | +0.2 L/hour during climbs |
| Heavy pack (>35 lb) | +0.1 L/hour |
| Fast pace (>3 mph sustained) | +0.2 L/hour |
| Dry wind / low humidity | +0.2 L/hour |
| Above 10,000 ft | +0.1 L/hour |
A 2,500 ft climb in dry wind at 9,000 ft with a 40 lb pack? Easy to hit 1.0 L/hour on that stretch.
Adjustments by Body Size
The 0.5 L/hour baseline assumes a ~160-pound hiker at moderate effort. Adjust:
- Under 140 lbs: −15% (~0.4 L/hr baseline)
- 140–180 lbs: baseline
- 180–220 lbs: +15% (~0.6 L/hr)
- Over 220 lbs: +25% (~0.65 L/hr)
Women generally need slightly less per pound than men due to lower sweat rate — but not by much. Don’t underdrink based on gender assumptions.
Signs You’re Underdrinking
- Urine darker than pale straw
- Headache, especially at altitude
- Muscle cramps (can also be electrolytes)
- Fatigue disproportionate to effort
- Decreased urine volume or frequency
- Dry mouth (late signal — if this appears, you’re already 2% dehydrated)
Pee on schedule: every 2–3 hours, pale yellow. If you haven’t peed in 4+ hours while hiking hard, drink immediately and take a longer break.
Signs You’re Overdrinking
Real, and more common than people realize:
- Nausea despite not being exerted
- Mental fog, confusion (hyponatremia — low sodium)
- Headache (confusing, because dehydration also causes this)
- Clear, copious urine every 30 min
Ultra-endurance runners die from over-drinking about as often as from under-drinking. If you’re sipping pure water every 10 minutes for 8+ hours without electrolytes, you can dilute your blood sodium dangerously. Include salt.
The Electrolyte Layer
Beyond water volume, sodium matters more than most backpackers realize. You lose 500–1,000 mg of sodium per liter of sweat. A hot day can cost 3–5 grams of sodium — serious.
Simplest approach:
- Tablets: Nuun (1 tab = 360 mg sodium), dirt cheap and light
- Powder: LMNT (1 packet = 1,000 mg sodium) — best for hot days
- DIY: 1/8 tsp salt + sugar + lemon in each bottle
Add one dose per 2 liters in moderate conditions, one per liter in heat. If you’re cramping, electrolytes before more water.
Real-World Examples
John Muir Trail (Sierra, August)
5-day segment, 70 miles, cool mornings and warm afternoons at 8,000–11,000 ft. Water sources every 2–5 miles.
A common carry strategy: 1 L Smartwater bottle + Sawyer Squeeze + 2 L Platypus soft flask. Usually 1–1.5 L between sources. A full 3 L is typical only for the Forester Pass–Diamond Mesa stretch (~8 miles dry).
Ozark Highlands Trail (Arkansas, September)
Humid, 75–85°F, moderate elevation. Water abundant in springs and creeks.
Typical strategy: 1 L max. Refill every 3–4 miles. The 0.5 L/hr baseline holds steady. Rarely a shortage on this trail.
Desert section, CDT New Mexico (April)
Cold nights, hot days (60–85°F), wind, 6,000 ft elevation. Cached water boxes + occasional windmills.
Typical strategy: 5–6 L max capacity (1 L Smartwater, 2 L CNOC, 3 L Cnoc bladder). Usual carry is 3–4 L. Daily consumption ~6 L including cooking. Urine output stays low despite high intake — the wind and sun are brutal.
Bottom Line Formula
For most trips, use this quick mental math:
Hours to next source × 0.5 L/hr
+ 1 L buffer
+ climate adjustment (−0.1 for cool, +0.3 for hot, +0.5 for desert)
+ effort adjustment (+0.2 if climbing steep)
= liters to carry leaving this source
That gives you a realistic number. Always slightly over-carry in the desert, always slightly under-carry in the Cascades. Adjust as you learn your own sweat rate.
Pro tip: Weigh your water. On a 5-day trip, the difference between “I pack 3 L between every source” and “I pack 1.5 L between sources and refill more often” is 3 pounds less on your back, all day, every day. That’s the compound return of thoughtful water planning.
Go Light. Go Far. Stay Hydrated.