For a 15-mile walk, you’ll usually be out for 5–7 hours, so hydration matters—but carrying too much can be just as uncomfortable as too little.

A good rule of thumb:

~500 ml per hour of walking in mild weather
That works out to roughly 2.5 to 3.5 litres total for most people

That said, it’s not one-size-fits-all. Adjust based on:

Weather

Cool/cloudy: closer to 2–2.5 L
Warm/sunny: 3–4+ L (you’ll sweat more than you think)
Very hot: you may need even more, plus electrolytes

Your body and pace

If you sweat heavily or walk briskly, lean toward the higher end.

Access to refills

If your route passes cafés, churches, or taps, you don’t need to carry everything at once.
You might bring 1–2 litres and top up along the way.

Electrolytes

(often overlooked)

For anything over ~3 hours, plain water isn’t always enough.
Adding a pinch of salt, electrolyte tablets, or a sports drink can help prevent cramps and fatigue.

Practical setup

Carry 1–2 bottles or a hydration bladder

Sip regularly—don’t wait until you’re thirsty
Aim for small, frequent drinks every 15–20 minutes

Quick reality check:
Most people underestimate how heavy water is.
3 litres = 3 kg.

If that feels like too much, plan refill points instead of overloading your pack.