📖 How To Use
How to Use This Calculator
Planning your 72-hour emergency water supply takes under a minute:
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Enter your household size
Count adults and children separately — the calculator uses a lower daily allowance for children (under 12) since they typically require 1–1.5 L/day for drinking versus 2–3 L for adults.
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Set the number of days
The standard emergency period is 72 hours (3 days), as recommended by FEMA and most civil defense agencies. You can extend this to 7, 14, or 30 days for longer preparedness scenarios.
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Choose your activity level
Sedentary (sheltering in place, minimal movement) needs less water than active (evacuating on foot, outdoor labor in heat). Hot climate or strenuous work can triple hydration needs.
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Add special needs (optional)
Include pets, nursing or pregnant household members, or anyone ill with fever — all require meaningfully more water than the base rate.
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Read your results
The calculator returns your total in litres and gallons, broken down into drinking vs. sanitation use, and tells you exactly how many 1 L, 2 L bottles, or 20 L jerry cans to stock.
Tip: Store water in food-grade, BPA-free containers — commercial bottled water (sealed) is ready to use. Replace stored tap water every 6–12 months. Keep containers out of direct sunlight and away from chemicals or fuel.
📐 The Formula
72 Hour Water Supply Formula
The calculation is based on per-person per-day water minimums set by WHO and FEMA:
Total Water = (People × L/person/day + Pets × L/pet/day) × Days
Drinking share ≈ 60% of total (direct consumption + food prep)
Sanitation share ≈ 40% of total (hygiene, wound care)
US Gallons = Total Litres ÷ 3.785
UK Gallons = Total Litres ÷ 4.546
Daily Water Rates by Category
| Person / Animal | Sedentary | Normal | Active | Hot/Strenuous |
|---|
| Adult | 2.0 L/day | 3.0 L/day | 4.5 L/day | 6.0 L/day |
| Child (under 12) | 1.5 L/day | 2.0 L/day | 3.0 L/day | 4.0 L/day |
| Nursing/Pregnant (+extra) | +1.0 L/day | +1.5 L/day | +2.0 L/day | +2.5 L/day |
| Ill / Fever (+extra) | +1.0 L/day | +1.5 L/day | +1.5 L/day | +2.0 L/day |
| Small pet (cat/sm. dog) | 0.3 L/day | 0.5 L/day | 0.7 L/day | 1.0 L/day |
| Large dog | 0.8 L/day | 1.2 L/day | 1.8 L/day | 2.5 L/day |
These figures represent minimum survival quantities. For comfort and hygiene beyond bare essentials, multiply by 1.5–2×.
Critical: The FEMA standard of 1 US gallon (3.78 L) per person per day is a survival minimum. It covers basic drinking and minimal sanitation only. In hot climates, during physical exertion, or for nursing mothers, requirements can be 2–3× higher.
❓ FAQ
Frequently Asked Questions
How much water does FEMA recommend for a 72-hour emergency?
FEMA recommends a minimum of 1 US gallon (approximately 3.8 litres) per person per day for drinking and basic sanitation. For a household of 4 over 72 hours, that equals 12 gallons (roughly 45 litres). This is a survival minimum — for comfort and hygiene, plan for 2× that amount.
How much emergency water does a family of 4 need?
At the FEMA minimum (3.8 L/person/day) for 3 days: approximately 46 litres or 12 US gallons. For full comfort (7 L/person/day including sanitation): around 84 litres or 22 gallons. This calculator lets you fine-tune for activity level, children, and pets.
How long can I store tap water for emergencies?
Properly treated tap water stored in clean, food-grade, sealed containers in a cool, dark location is safe for 6–12 months. Add 8 drops of unscented household bleach (5–9% sodium hypochlorite) per gallon before sealing if treating tap water yourself. Commercially bottled water remains safe until the printed expiry date.
How much water do pets need in an emergency?
Cats and small dogs typically need 30–50 mL per kg of body weight per day — roughly 0.3–0.5 L/day for a small pet under normal conditions. Large dogs (30+ kg) may require 1.2–2.5 L/day, more in heat or if active. This calculator uses conservative averages; adjust upward if your pet is larger or more active than typical.
What containers are best for emergency water storage?
Use food-grade, BPA-free HDPE (marked with recycle code #2) containers with tight-sealing lids. Commercial bottled water in original sealed packaging is the most convenient option. For bulk storage, 20-litre (5-gallon) food-grade jerry cans are practical for households. Never use containers that held chemicals, cleaning products, or non-food items, as residue is nearly impossible to remove fully.
Is 72 hours enough, or should I store more water?
72 hours covers most short-term emergencies (power outages, brief evacuations, storm events). However, major infrastructure disasters — earthquakes, hurricanes, floods — can disrupt water supplies for 1–4 weeks. Many preparedness experts and agencies like the American Red Cross now recommend 2-week supplies for households in disaster-prone areas. Use this calculator with "14 days" to estimate your extended storage goal.
Do I need to include cooking and sanitation water separately?
Yes. The total water figure from this calculator already includes an allocation for both drinking/cooking (~60%) and basic sanitation (~40%). If you plan to cook meals requiring significant water (boiling pasta, making soups), add an extra 0.5–1 litre per person per day on top of the calculated result. In a survival scenario, switch to shelf-stable meals that require minimal water preparation.
How do I calculate emergency water storage for a hot climate?
In ambient temperatures above 30°C (86°F) or when performing physical labor, fluid requirements can rise to 5–8 litres per adult per day. Select the "Hot climate / strenuous" activity level in this calculator, which uses 6 L/adult/day as the base. Children and the elderly are especially vulnerable to dehydration — factor in an extra 20–30% buffer for households in tropical or desert climates.