How much water to carry backpacking
Start from 2.5 L per day, then add for two things: how far you'll go between water sources, and how hot it is. A flat "1 gallon a day" rule ignores both — a short, cool day and a long, hot dry stretch aren't the same problem.
The formula
base = 2.5 L
+ 0.5 L per 10 km between water sources ("dry stretch")
+ 0.5 L per 10°F the forecast high is above 75°F
The dry-stretch distance is the longest gap between on-route water sources within that day's own start and end points — not the day's total distance, since a day that crosses water twice needs less carried at once than one long dry push. The 75°F threshold and the two 0.5 L/10-unit rates are widely used backpacking rules of thumb, not medical guidance — stated here exactly as they're stated in the app.
Worked examples
| Dry stretch | Forecast high | Carry |
|---|---|---|
| 5 km | 68°F (below threshold) | 2.8 L |
| 8 km | 78°F | 3.1 L |
| 15 km | 85°F | 3.8 L |
Limitations
- This is a planning starting point, not a medical or physiological calculation — individual sweat rate, exertion, and altitude all shift real need.
- It assumes on-route water sources are real and reliable; a forecast source that's dry on the ground changes everything. Verify locally before you go.
- It doesn't account for cooking, or for a group sharing a filter versus each person carrying independently.
Formula current as of 2026-07-19. Source: Far's own water-carry planner, applied identically in the app.