📖 How To Use
How to Use This Calculator
Find out exactly how long your emergency water stockpile will last in under 60 seconds:
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Enter your stored water volume
Count up all your water containers — jerry cans, tanks, bottles, barrels. Enter the total. Switch the unit dropdown to match your measurement (litres, US gallons, UK gallons, or cubic metres).
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Enter number of people
Include everyone who will draw from this supply — family members, dependents, and any guests you're planning for.
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Set daily usage per person
Use a preset (FEMA minimum, WHO basic, WHO full, or comfortable household) or type in your own figure. Emergency conditions almost always mean lower usage — drinking and cooking only, no showers or laundry.
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Hit Calculate
The result shows days of supply, readiness status, weekly breakdown, and total group consumption rate. Copy or print for your emergency plan.
Tip: FEMA recommends a minimum of 1 US gallon (3.8 L) per person per day for drinking and basic sanitation. In hot climates or for sick/pregnant individuals, double that figure to at least 7.6 L per person per day.
📐 The Formula
Water Stockpile Duration Formula
The calculation is straightforward:
Days of Supply = Total Water (L) ÷ (People × Daily Usage per Person in L)
Weeks = Days ÷ 7
Total Group Daily Consumption = People × Daily Usage per Person
This calculator converts all volume inputs to litres internally. Mixed units are handled automatically — just pick the right unit in each dropdown.
WHO & FEMA Daily Usage Benchmarks
| Scenario | L/person/day | US gal/person/day | Notes |
|---|
| FEMA Minimum | 3.8 | 1.0 | Drinking + minimal sanitation only |
| WHO Survival | 7.5 | 2.0 | Absolute minimum for survival |
| WHO Basic Needs | 15 | 4.0 | Drinking, cooking, basic hygiene |
| WHO Comfort | 50 | 13.2 | Full hygiene, food prep, some laundry |
| Developed-World Average | 100–200 | 26–53 | Normal household use |
Sources: WHO Technical Notes on Drinking Water, Sanitation and Hygiene in Emergencies; FEMA Emergency Water Storage guidance.
Duration Reference — 4 People at FEMA Minimum (3.8 L/day each)
| Stored Volume | Days (FEMA Min) | Days (WHO Basic) | Days (WHO Full) |
|---|
| 50 L | 3.3 | 0.8 | 0.25 |
| 100 L | 6.6 | 1.7 | 0.5 |
| 200 L | 13.2 | 3.3 | 1.0 |
| 500 L | 32.9 | 8.3 | 2.5 |
| 1,000 L | 65.8 | 16.7 | 5.0 |
| 2,000 L | 131.6 | 33.3 | 10.0 |
🔍 Use Cases
When to Use This Calculator
Hurricane & Flood Preparedness
Authorities in flood-prone regions recommend storing at least 7 days of water before storm season. Use this calculator to verify your current stores meet that target — then size up if they fall short before an event hits.
Off-Grid Living
If you're running on rainwater collection or a borehole with uncertain yield, knowing exactly how many days your tank covers gives you a concrete trigger point for when to ration or seek a top-up supply.
Bug-Out & Go-Bag Planning
Water weighs 1 kg per litre. Every litre you carry in a bug-out bag is weight you're hauling on foot. This calculator lets you trade off days of supply against carry weight so you can make a realistic plan, not a wishful one.
Apartment Stockpiling Without a Tank
Many urban dwellers store water in bathtubs, large bottles, or WaterBOB inserts. Knowing the litres in those containers gives a real duration figure — which is almost always much shorter than people assume.
Remote Worksite Supply Planning
Construction crews, mining camps, and research stations often operate where water delivery is intermittent. Use this calculator to plan resupply intervals based on headcount and actual usage rates.
❓ FAQ
Frequently Asked Questions
How do I calculate how long my water stockpile will last?
Divide your total stored volume (in litres) by the total daily consumption of your group (people × litres per person per day). The result is your days of supply. This calculator does that math instantly and converts all common units automatically.
How much water should I stockpile per person for emergencies?
FEMA recommends a minimum of 1 US gallon (3.8 litres) per person per day, targeting a 3-day minimum and ideally a 2-week supply. WHO sets the basic needs threshold at 15 litres per person per day for drinking, cooking, and basic hygiene. For a family of 4 aiming for 14 days at WHO basic: 15 × 4 × 14 = 840 litres.
How long can stored water last before it goes bad?
Commercially sealed bottled water typically has a 1–2 year shelf life but can last much longer if stored correctly. Tap water stored in clean, sealed containers away from light and heat is generally safe for 6–12 months. Treat with chlorine (2 drops of unscented household bleach per litre) to extend safe storage. Use our Chlorine Dosage Calculator for precise treatment amounts.
What is a good water stockpile duration for emergencies?
FEMA's baseline is 3 days. Most emergency management authorities now recommend 7–14 days as the realistic minimum for natural disaster scenarios (hurricanes, earthquakes, extended outages). For serious preparedness or off-grid living, 30+ days is the target. Anything under 3 days should be treated as critically insufficient.
Does this calculator account for water for pets?
Not automatically. To include pets, convert your pet's daily water consumption to litres and either add it to the per-person daily figure, or treat each large dog as roughly 0.25–0.5 additional people in the headcount field. A medium dog needs approximately 0.5 litres per day; a cat about 0.2 litres.
How many litres of water is in a standard 55-gallon drum?
A 55 US gallon drum holds approximately 208 litres. At FEMA's minimum rate of 3.8 L/person/day, a single drum provides one person with roughly 55 days of supply, or a family of 4 with about 13.7 days. You can enter this directly into the calculator using the US gallons option.
Should I plan for more water in hot climates?
Yes — significantly more. In hot climates or during physical exertion, individual water needs can double or triple. FEMA specifically notes that children, nursing mothers, and the sick may also need more than the standard minimum. For tropical or desert environments, plan on at least 6–8 litres per person per day for drinking alone, before any cooking or hygiene.
How do I calculate water stockpile duration in gallons?
Select "US gal" in the stored volume dropdown and enter your total gallons. Then select "gal/day" in the daily usage dropdown. The calculator converts everything to litres internally and displays the correct days of supply. You don't need to do any manual unit conversion.