Use a public-health baseline.
Ready.gov advises at least one gallon of water per person per day for several days. CDC says to store at least one gallon per person per day for three days, and to try for a two-week supply if possible. Both note that needs can be higher in hot weather and for people who are sick; plan for pets and other household needs as well.
Ready.gov: water ↗ · CDC: creating and storing an emergency water supply ↗
Choose a safe, manageable storage method.
CDC identifies unopened commercially bottled water as the safest and most reliable source in an emergency. If filling your own containers, CDC recommends clean, food-grade water-storage containers. Follow the container maker’s directions and CDC’s current cleaning and filling guidance.
Label it and keep it in mind.
For water you fill yourself, CDC advises labeling it as drinking water with the storage date, keeping it cool and away from direct sunlight and toxic substances, and replacing it every six months. For store-bought water, observe the product’s expiration date.
Use a clean method when dispensing water so you don’t contaminate what remains in the container.
Follow local advisories.
Stored water is not a substitute for current public-health instructions. During an outage, contamination event, or boil-water advisory, follow your local water utility and health department’s directions.