When disasters strike, clean water is the first thing to disappear. These free calculators help you size your emergency supply correctly — whether you're prepping a 72-hour kit, planning for an off-grid homestead, or fortifying your home before hurricane season.
Emergency Water Planning: What These Calculators Cover
Access to safe drinking water is the most critical factor in any emergency — yet it's also the most commonly underestimated. Most people have no idea how much water their household actually needs until the taps run dry. The six calculators in this section are built to fix that. Whether you're preparing for a natural disaster like a hurricane or flood, setting up an off-grid homestead, assembling a 72-hour emergency kit, or building a long-term water stockpile, each tool gives you a precise, personalised storage target based on your specific situation. The tools start from published guidance — Ready.gov (FEMA) and the CDC for household storage, the Sphere humanitarian standards for survival and basic needs — and then let you adjust for what generic advice misses: heat, physical activity, pets, infants and medical needs. Each page shows its formula, a worked example and its sources, and was reviewed by John Mac, PE.
Choosing the Right Tool
How to Choose the Right Emergency Calculator
Start with the Emergency Water Storage Calculator if you want a general-purpose answer to "how much water do I need to store?" It's the most comprehensive tool in this category, covering households of any size and any emergency timeline from 3 days to 6 months.
Use the 72-Hour Water Supply Calculator if your goal is the 3-day kit the CDC sets as the minimum. It's quick, and gives a clear shopping list in litres, gallons and containers.
Reach for the Water Stockpile Duration Calculator when you already have water stored and want to know how long it will last. This is useful for checking an existing supply against the 3-day minimum and the 2-week target. During an emergency, drink what you need — Ready.gov advises not to ration drinking water unless authorities tell you to.
The Bug-Out Bag Water Calculator is purpose-built for people who may need to evacuate on foot. It factors in heat and physical exertion and shows the weight each adult would carry, which home storage calculators don't.
Scenario
Best Calculator
Key Output
General disaster prep at home
Emergency Water Storage Calculator
Total litres / gallons to store
Building a 72-hour go-kit
72-Hour Water Supply Calculator
Minimum & recommended volumes
Auditing an existing stockpile
Water Stockpile Duration Calculator
Days of supply remaining
Evacuation & bug-out planning
Bug-Out Bag Water Calculator
Carry weight vs. duration balance
Off-grid homestead or cabin
Off-Grid Water Storage Calculator
Tank size & refill schedule
Hurricane / flood preparation
Hurricane & Disaster Water Prep Calculator
Storm-specific storage target
Pro tip: Run the Emergency Water Storage Calculator first for a baseline, then use the scenario-specific tool for your situation. The two figures together give you a floor and a ceiling for your planning.
Sources & Review
How These Calculators Are Built and Reviewed
Every calculator in this category starts from published guidance and shows its working, so you can check the numbers yourself. Where we add a planning step of our own — such as heat factors between "normal" and "can double" — the page says so.
Ready.gov (FEMA) — Water and Build a Kit: at least 1 gallon per person per day; needs can double in very hot temperatures; extra water for pets.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
Last updated
FAQ
Frequently Asked Questions
How much water should I store for an emergency?
Ready.gov (FEMA) and the CDC recommend at least 1 gallon (3.8 litres) of water per person per day for drinking and sanitation, for at least 3 days, and the CDC suggests a 2-week supply if possible. For 2 weeks, a family of four needs at least 56 gallons (about 212 litres). Children, nursing mothers and sick people may need more, and in very hot temperatures needs can double. Use the Emergency Water Storage Calculator to factor in your specific conditions and get an accurate personal target.
What is the 72-hour water rule for emergencies?
The CDC's minimum is a 3-day (72-hour) supply — enough to cover the first days after a disaster, when utilities and emergency services are most disrupted. At 1 gallon per person per day, that is 3 gallons (about 11 litres) per person, or 12 gallons (about 45 litres) for a family of four. It is a minimum: store more if you can, and add water for pets. Our 72-Hour Water Supply Calculator walks you through the exact numbers for your household.
How long does stored water last before it needs to be replaced?
For store-bought water, the CDC says to follow the expiration date on the bottle. Water you bottle yourself from a safe tap should be replaced every 6 months. Store containers somewhere cool — the CDC suggests 50–70 °F (10–21 °C) — and dark, away from sunlight, fuel and pesticides. Treated tap water needs no additives; label each container with the date it was filled.
How much water do I need in a bug-out bag?
An adult walking in temperate weather needs about 4 litres of drinking water a day, and much more in heat — up to the US Army's 12-quart (11.4 L) daily limit for hard work in heat. Because a few days' water is too heavy to carry on foot, most people carry a day or two and bring a filter and purification tablets to treat water they find. Our Bug-Out Bag Water Calculator helps you strike the right balance between carry weight and duration, factoring in your travel intensity and group size.
How do I calculate water needs for a hurricane or major disaster?
Hurricane water planning must account for the number of people, pets, expected outage duration, and local climate. Start from 1 gallon per person per day and raise it for heat and cleanup work — Ready.gov notes needs can double in very hot temperatures. Multiply by the days you are planning for; the CDC suggests a 2-week supply at home if possible, which suits hurricane zones. The Hurricane & Disaster Water Prep Calculator handles all these variables automatically.
What containers are best for long-term emergency water storage?
Food-grade polyethylene containers (HDPE, marked with recycling symbol #2) are the most practical choice. Dedicated water storage barrels (30–55 gallon) offer excellent volume-to-cost efficiency for home stockpiling. Never use containers that previously held non-food products, as residual chemicals can leach into water over time. Opaque or dark containers reduce algae growth. Keep all containers tightly sealed and stored away from heat sources, gasoline, pesticides, and cleaning chemicals.
How much water storage does an off-grid home need?
Multiply people × daily use × the days between refills, then add a buffer. The UN and WHO say 50–100 litres per person per day meet most basic needs, and many water-careful off-grid homes plan on around 80 litres. At that rate, a family of four bridging a 30-day dry spell needs 9,600 litres, or about 11,500 litres with a 20% buffer. The Off-Grid Water Storage Calculator works this out and counts the tanks.
