Ask three overnight backpackers how much water they carry and you’ll get three different answers: 2 liters, 4 liters, “depends on the creek.” All three might be right — and all three are answering a different question than the one that actually matters once the sun goes down.

Day-hike water math stops at the last step of the trail. Overnight math doesn’t. You still have to cook dinner, make coffee, rinse a pot, and start tomorrow with something in the bottle. Skip that part of the accounting and you’ll hit camp with exactly enough water to be thirsty by 6 a.m.

Two totals, not one

An overnight trip runs up two separate water bills.

Moving water follows the same formula as any day hike: a baseline of about 0.5 liters per hour, scaled up for heat, altitude, sun, and pack weight. That’s the math our water intake calculator runs, built on the US National Park Service’s 0.5–1 L/h field guidance.

Camp water is the part day-hike advice skips entirely. Camp water isn’t optional. Rehydrating a freeze-dried dinner takes roughly 0.5–0.75 L. Coffee and breakfast the next morning takes another 0.4–0.5 L. Add rinsing a pot and topping off for the first hour back on trail, and camp alone runs 1–1.5 L — before you’ve covered a single mile of the actual hike.

A worked night

Take a real overnight: 8 miles in, climbing 1,500 ft to a camp sitting around 6,500 ft — short of the 8,200 ft (2,500 m) altitude threshold, so no altitude bump yet. A heavy pack (backpacking load, north of 20% of body weight), 30 °C (86 °F) afternoon sun, partial shade.

Moving time: call it 4.5 hours on rough trail carrying that grade and load.

Rate: 0.5 L/h base × 1.5 (heat factor, 25–32 °C) × 1.0 (altitude, below threshold) × 1.2 (heavy pack) × 1.1 (partial sun) ≈ 1.0 L/h.

Moving total: 1.0 × 4.5 ≈ 4.5 liters.

Add camp water — dinner, coffee, breakfast, a small margin for a slow morning — call it 1.5 L.

Full overnight need: roughly 6 liters. That’s 13.2 lb of water alone, on top of food and shelter. Carry that from the trailhead and you’ve spent your weight budget on liquid before the pack even closes.

Carry less, filter more

That 13-pound figure is the actual argument for a filter, not a nice-to-have. Don’t trust the map’s blue line — check a recent trip report before you assume a stream will be running. If the route has a reliable source near camp, carry enough to reach it plus a safety margin, then filter for cooking and the morning refill. A Sawyer- or Katadyn-style filter weighs a few ounces; six liters of water weighs thirteen pounds. That trade isn’t close.

If the route genuinely doesn’t have water near camp — a true dry camp — you carry the full 6 liters, full stop. No filter fixes a dry camp.

Our calculator’s total is the moving-water floor. It doesn’t know your route’s springs and streams, so add your own camp estimate on top of its number, the way we did above.

Don’t forget the pack budget

Six liters of water is real weight, and it competes with everything else on your back. Run the same trip through the gear weight budget calculator before committing to carrying it all — water that pushes you past the 20% body-weight line is water you should be filtering, not hauling. If tomorrow’s leg is longer than tonight’s, check it with the hiking time calculator before deciding how much to fill up before bed.

Electrolytes matter here too: our formula flags electrolyte replacement past 3 liters of total intake, or above 25 °C regardless of volume — this trip clears both. Salty snacks or a tab in the last liter fixes that cheaply.

The honest limit

No formula knows if the stream on the map is running in August. That’s not a math problem. It’s a scouting problem, and it’s yours to solve before the trailhead — not at a dry creek bed at mile six. Carry the number the math gives you. Then carry a little more anyway.

Plan the water before you feel the thirst.