What size water bottle is best for hiking?

For summer hiking, 32 oz (1 liter) is the minimum practical size. It covers about 1 2 hours of hydration in hot weather. For half day hikes, bring 40 64 oz. The trade off is weight: a full 64 oz insulated bottle weighs about 3 4 pounds. Many hikers prefer two 32 oz bottles (one on each side of the pack) over one large bottle for better weight distribution.