📖 How To Use
How to Use This Calculator
Find out exactly how many days your stored water will last in four quick steps:
-
Enter your tank capacity
Type the total volume of your tank in litres, US gallons, UK gallons, or cubic metres. Check the tank manufacturer's label or measure the internal dimensions — use our
Rectangular Volume Calculator if you need to work it out from dimensions.
-
Set daily water usage
Enter how many litres (or gallons) your household uses each day. Use the quick presets — WHO minimum (50 L/person), typical household (150 L/person), or comfort level (250 L/person) — or enter your own figure from a water meter reading.
-
Add household size (optional)
If you entered a per-person daily usage, type the number of people in your home. The calculator multiplies usage by the household count automatically.
-
Adjust the fill level
Drag the slider to show how full your tank is right now. If the tank is three-quarters full, set 75%. Leave at 100% to calculate from a completely full tank.
Tip: For a more conservative estimate, reduce the fill level by 10–15% to account for unusable sediment at the bottom of the tank and recommended headspace to prevent overflow.
📐 The Formula
How Long Will My Tank Last — Formula Explained
The calculation is straightforward division:
Duration (days) = Usable Volume ÷ Daily Consumption
Usable Volume (L) = Tank Capacity (L) × Fill Level (%)
Daily Consumption (L) = Usage per person (L) × Number of people
Duration (hours) = Usable Volume ÷ (Daily Consumption ÷ 24)
All unit inputs are converted to litres internally before the calculation runs, so you can freely mix units between tank capacity and daily usage.
Reference: Daily Water Usage by Scenario
| Scenario | Litres/person/day | US Gal/person/day | Source |
|---|
| Survival minimum | 3–5 L | 0.8–1.3 | FEMA / Red Cross |
| WHO basic needs | 50 L | 13.2 | WHO Guidelines |
| Developing world average | 80–100 L | 21–26 | UN-Water |
| Typical household (developed) | 130–200 L | 34–53 | National averages |
| High-usage / garden / pool | 300–500 L | 79–132 | Utility estimates |
Unit Conversions Used
| Unit | Litres Equivalent |
|---|
| 1 Litre (L) | 1 L |
| 1 US Gallon | 3.785 L |
| 1 UK / Imperial Gallon | 4.546 L |
| 1 Cubic Metre (m³) | 1,000 L |
❓ FAQ
Frequently Asked Questions
How do I calculate how long my water tank will last?
Divide the usable volume of water in your tank by your daily consumption. For example, a 1,000-litre tank at 75% fill holds 750 litres. At 150 litres per day for a family of 3 (450 L/day total), the tank lasts 750 ÷ 450 = 1.67 days, or about 40 hours. This calculator performs all unit conversions automatically.
How much water does a family of 4 use per day?
The WHO recommends a minimum of 50 litres per person per day for basic needs. Most developed-world households consume 100–200 litres per person per day for full comfort. A family of 4 therefore typically uses 400–800 litres per day. High-usage households with gardens or pools can exceed 1,000 litres per day.
What size water tank do I need for 3 days of supply?
Multiply your daily household usage by the number of days. At the WHO minimum of 50 L/person/day for a family of 4: 50 × 4 × 3 = 600 litres. At a typical 150 L/person/day: 150 × 4 × 3 = 1,800 litres. Add a 10–15% buffer for sediment and headspace. Use our Water Tank Size for Home Calculator to size a tank from scratch.
How do I make my tank water last longer?
Reduce daily consumption by fixing leaks, taking shorter showers, using a bucket for flushing, avoiding garden watering, and using collected water for non-drinking purposes. Reducing usage from 200 to 100 L/person/day doubles how long your tank lasts. Even small changes — like turning off the tap while brushing teeth — add up quickly during a shortage.
Does this calculator account for evaporation or leakage losses?
No — this calculator assumes all stored water is fully available for use. In practice, evaporation from outdoor tanks can add 1–3% monthly losses in hot climates, and minor pipe drips add further waste. For a conservative estimate, reduce the fill level by 5–10% to build in a safety margin for these losses.
How long is stored water safe to drink?
According to the CDC, commercially sealed water lasts indefinitely if stored properly. Tap water stored in clean, sealed containers is generally safe for 6–12 months. In tanks with UV exposure or warm temperatures, bacteria can multiply faster — so tank water should be treated with chlorine or UV disinfection for storage exceeding a few weeks. Use our Safe Water Storage Duration Calculator for treatment guidance.
Can I use this calculator for business or industrial water tanks?
Yes — the formula is the same regardless of scale. For commercial or industrial use, enter the tank's total capacity and the site's average daily water draw. For precise commercial sizing, also see our Commercial Water Tank Size Calculator.