Hydration Bladder Size Calculator
Choose the right hydration reservoir size for your outing from duration, effort and heat.
How it works
need = hours × rate(effort) × temp multiplier · pick smallest standard bladder ≥ need (1 / 1.5 / 2 / 3 L)
Hydration reservoirs come in a few standard sizes, and the right one balances carrying enough against carrying dead weight. This tool estimates your water need from time, effort and heat, then recommends the smallest common bladder that covers it. A bladder you only fill halfway is fine for short outings, but on hot, hard days it is better to carry a size up so you are not rationing water between refills.
Worked example
Four hours of moderate effort (0.5 L/hr) on a mild day needs 4 × 0.5 = 2.0 L, so a 2 L bladder is the right pick, filled nearly to the top.
Frequently asked questions
What size hydration bladder should I get?
For most day hikes a 2 L reservoir is the versatile default. Choose 1–1.5 L for short runs and trail workouts, and 3 L for long, hot days or when refills are scarce.
Should I fill it all the way?
Only carry what you expect to drink plus a margin. Water is heavy — about 2.2 lb per liter — so topping off a big bladder for a short outing just adds load.
Bladder or bottles?
Bladders make frequent sipping easy and pack flat, while bottles are simpler to refill, measure and clean. Many hikers carry a bladder plus one bottle for mixing drinks.
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This calculator is for educational purposes. Double-check important results before acting on them.