Enter your household details to calculate your minimum and recommended storage
Household & usage details
People
ppl
Daily Usage / Person
Quick Usage Presets
Supply & safety settings
Supply Days
days
Safety Buffer
%
Tank Size (opt.)
L
⚠ Please enter valid values for number of people, daily usage, and supply days (all must be greater than zero).
Your Storage Requirements
Recommended Total Storage
—
Litres (with safety buffer)
Minimum Storage
— L
US Gallons
— gal
UK Gallons
— gal
Total Daily Demand
— L/day
Tanks Needed
— tanks
Water Weight
— kg
Storage Breakdown
📖 How To Use
How to Use the Off Grid Water Storage Calculator
Planning your off-grid water system takes just four quick inputs:
Enter the number of people
Count everyone in your household — adults, children, and any regular guests. Children usually use less than adults, but it's safer to count them as full adults when sizing a long-term system — they grow, and guests come.
Set your daily usage per person
Use the quick presets or enter your own figure. The Sphere humanitarian standards set 15 litres per person per day as a minimum for drinking, cooking and hygiene in a crisis; the UN and WHO say 50–100 litres are needed to meet most basic needs. A water-careful off-grid home often plans on roughly 60–100 litres per person, and full comfort with daily showers and laundry can reach 150–200 litres. Your own meter readings are the best guide.
Choose your supply days
This is how many days your stored water must last between refills — whether from rainwater harvesting, a borehole, or a delivery truck. Common choices: 30 days (one month), 90 days (seasonal), or 365 days (full year independence).
Set a safety buffer (recommended: 20%)
A buffer covers guests, a longer dry spell, leaks, and the water left below each tank's outlet that you can't draw off. 20% is a common planning choice; use more if a dry season is unpredictable or the supply is critical. Optionally, enter your intended tank size to see how many tanks you'll need.
Pro tip: For rainwater-dependent off-grid systems, pair this calculator with our Rainwater Harvesting Calculator to verify your roof can actually collect enough water to refill your tanks in your local rainfall conditions.
📐 The Formula
Off Grid Water Storage Formula Explained
The calculation has three straightforward steps:
Step 1 — Minimum Storage:
Min (L) = People × Daily Usage (L/person) × Supply Days
Step 3 — Tank Count (if tank size entered):
Tanks = CEILING(Recommended ÷ Tank Size)
The "recommended" figure is the one you should actually use when purchasing tanks — the minimum alone leaves no room for unexpected consumption spikes, leaks, or supply disruptions.
EPA: an average family uses 300+ gallons a day at home
Supply Days Guide
Scenario
Supply Days
Rationale
Emergency backup
3–14
CDC: at least 3 days, a 2-week supply if possible
Short dry season
30
One month between rainfall events
Seasonal gap
90
Three-month dry season
Semi-annual
180
Six months in arid climates
Full independence
365
Complete year-round self-sufficiency
🏡 Use Cases
When to Use This Calculator
🌿
Homestead & Smallholding
Size your tank system for a property that is fully or partly off the mains. Work out animals and irrigation separately with the farm and livestock calculators, then add them.
🏕️
Remote Cabin or Holiday Home
Calculate how much storage you need for a property that is only visited occasionally and relies on tanker deliveries or infrequent well pump runs.
⚡
Emergency Preparedness
Determine how large a water reserve to maintain for grid-outage scenarios, natural disasters, or supply disruptions.
🚐
Van, Boat & Mobile Living
Plan your on-board water tank capacity for extended off-grid travel where refill opportunities are limited.
🌍
Developing-World Projects
Humanitarian and community water projects where central supply is absent — size communal tanks to serve a defined population for a set number of days.
🔋
Borehole / Pump Systems
Where your pump only runs periodically (e.g., solar-powered), calculate how much storage bridges the gap between pump cycles.
🧮 Worked Example
Worked example: a family of four through a 30-day dry spell
Daily demand: 4 people × 80 L = 320 L a day.
Minimum storage: 320 × 30 days = 9,600 L.
With a 20% buffer: 9,600 × 1.2 = 11,520 L — about 3,043 US gallons, weighing about 11.5 tonnes when full.
Tanks: 11,520 ÷ 5,000 L = 2.3, so 3 × 5,000 L tanks. Even if only 90% of each tank is usable, 3 tanks still cover it.
For a 90-day dry season the same household needs about 34,560 L — seven 5,000 L tanks. At that scale, check that your roof and rainfall can refill the tanks with the rainwater harvesting calculator, and cut daily use where you can: dropping to 60 L per person saves 8,640 L over 90 days.
❓ FAQ
Frequently Asked Questions
How much water storage do I need for off-grid living?
It depends on your household size, daily consumption, and how many days must pass between refills. A family of 4 using 80 litres per person per day needs 9,600 litres for a 30-day supply — plus a 20% safety buffer, that's about 11,500 litres. Use this calculator to get a figure specific to your situation.
What is a realistic daily water usage for an off-grid household?
Many off-grid households plan on roughly 60–100 litres per person per day, which covers drinking, cooking, short showers and hand-washing — in line with the UN/WHO view that 50–100 litres meet most basic needs. That is well below a typical US home, where the EPA says an average family uses more than 300 gallons (about 1,100 litres) a day. Low-flow fixtures and careful habits make the difference; your own meter readings are the best guide.
Why do I need a safety buffer in my off-grid water storage calculation?
Real use rarely matches the plan. Guests, a dry spell that runs long, leaks, and the water below each tank's outlet that you can't draw off all eat into storage. A 20% buffer is a common planning choice for homes; use more if your dry season is unpredictable or the water supply is critical.
How many water tanks do I need for off-grid living?
This depends on the capacity of your chosen tank. Divide your total recommended storage by the usable volume of one tank and round up. For example, 11,520 litres ÷ 5,000 litres per tank = 2.3, so you need 3 tanks. Enter your tank size in the calculator to get this number automatically.
Is underground or above-ground storage better for off-grid water?
Underground tanks maintain more stable temperature (reducing algae growth), are protected from UV degradation, and cannot be knocked over. However, they cost more to install and require a pump to retrieve water. Above-ground tanks are cheaper, easier to inspect, and can gravity-feed if elevated, but are susceptible to temperature extremes and UV damage. Many off-grid systems use both: a large underground cistern as the main store and a smaller elevated header tank for gravity pressure.
How long can water be safely stored in an off-grid tank?
An off-grid tank in daily use is constantly being drawn down and refilled, so it is less about shelf life and more about keeping the water clean: use a food-grade tank, keep it sealed and dark, screen the inlet, overflow and vents against insects and animals, and clean it periodically. Rainwater and well water should be treated before drinking and tested regularly. Separate bottled emergency water, by contrast, should be replaced every 6 months if you filled it yourself (CDC). Use our Safe Water Storage Duration Calculator for precise treatment guidance.
Can I use this calculator for livestock or irrigation water needs?
Yes — simply adjust the "daily usage per person" to reflect litres per animal or litres per hectare per day, and set "number of people" to the number of animals or irrigation units. For example, Ontario's agriculture ministry (OMAFRA) puts a milking dairy cow at about 115 litres a day on average (68–155 L, rising with milk yield). For more precise agricultural calculations, see our Livestock Water Requirement Calculator and Farm Water Storage Calculator.
What is the best tank material for off-grid water storage?
Polyethylene (plastic) is the most common choice — lightweight, corrosion-resistant, food-safe, and competitively priced. Concrete is durable and keeps water cool but can leach pH-raising minerals if not sealed. Steel (powder-coated or stainless) is strong and long-lived but more expensive and can corrode. Fibreglass is light and durable but costly. For most off-grid residential applications, UV-stabilised HDPE (high-density polyethylene) tanks offer the best balance of cost, longevity, and safety.
✅ Sources & Review
Sources and review
The formula is simple arithmetic; the usage benchmarks behind the presets come from these sources:
Sphere Association — The Sphere Handbook (2018): at least 15 L per person per day for drinking, cooking and personal hygiene.
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.
