Water is the single most time-sensitive supply in any emergency kit — the human body can only go a few days without it. Yet it's also one of the most commonly under-stocked items in home kits.
A common general guideline is one gallon of water per person, per day, covering both drinking and basic sanitation. For a minimum viable kit, aim for a 3-day supply (3 gallons per person); for a more resilient home stockpile, work toward two weeks (14 gallons per person).
Use food-grade, BPA-free containers designed for water storage. Avoid reusing containers that previously held non-food liquids. Store containers away from direct sunlight and chemicals, in a cool location.
Commercially bottled water typically has a long shelf life, but home-filled containers should generally be rotated every six months to keep taste and quality fresh, even though properly stored water rarely becomes unsafe.
In addition to stored water, consider water purification tablets or a portable filter as a backup if your stored supply runs low and you need to treat water from another source.
Factor in extra water for pets — a reasonable starting point is about half a gallon to a gallon per day depending on the animal's size.