Enter your tank capacity, fill level, and daily consumption
Tank capacity
Tank Capacity
Daily Usage
Quick usage presets
Household size (optional multiplier)
People
If entered, daily usage above is treated as per person and multiplied by the household count.
Refill & unusable water optional
Water Coming In / Day
L/day
Unusable Below Outlet
% of tank
Current Fill Level
100%
25%50%75%Full
⚠ Please enter a valid tank capacity and daily usage greater than zero.
WaterTankCalculator.com — Tank Duration Results
Your Results
Estimated Supply Duration
—
Days
Calculating...
Total Hours
— hrs
Usable Volume
— L
Daily Consumption
— L/day
Run-Out Date
—
Fill Level
— %
Household Size
— person(s)
Current Tank Fill Level
📖 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) are used each day. The best figure comes from your own water meter or bill: a month's use divided by 30. Otherwise use a preset — minimum (50 L/person, the WHO intermediate-access level), typical (150 L/person) or comfort (250 L/person).
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.
Add refill and unusable water (optional)
If water still trickles in — a weak mains supply, a borehole pump or rain — enter how much arrives per day for the whole household, in the same unit as your usage; the tank then empties at the difference. If the outlet sits above the tank floor, enter the share of the tank below it, since that water cannot be drawn off.
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: To read the fill level, measure the water depth (through the lid or with a dipstick) and divide by the full depth — for an upright cylinder or a box-shaped tank, depth percentage equals volume percentage. For a horizontal cylinder it does not; use our Horizontal Cylinder Tank Calculator instead.
📐 The Formula
How Long Will My Tank Last — Formula Explained
The calculation is a straightforward division, after allowing for water you cannot use and water still coming in:
Duration (days) = Usable Volume ÷ (Daily Consumption − Daily Inflow)
Usable Volume (L) = Tank Capacity × (Fill Level % − Unusable %)
Daily Consumption (L) = Usage per person (L) × Number of people
Duration (hours) = Duration (days) × 24
If inflow is equal to or more than consumption, the tank does not run dry.
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
Basis
Emergency drinking & sanitation
3.8 L
1
Ready.gov (FEMA) minimum
Low health risk
50 L
13.2
WHO "intermediate access" level
Water-careful home
80–100 L
21–26
Within UN/WHO 50–100 L basic-needs range
Typical piped home
130–200 L
34–53
Planning range
High use / garden / pool
300–500 L
79–132
US homes average ~310 L (US EPA)
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
💡 When To Use
When to Use a Tank Duration Calculator
🚰
Water Supply Interruptions
Plan for scheduled or unscheduled municipal supply outages. Know in advance whether your stored water will bridge the gap.
🏕️
Off-Grid Living
Manage harvested rainwater or borehole-fed storage tanks without a mains supply backup. Time your refill schedule accurately.
🌧️
Drought Contingency Planning
During dry seasons or hosepipe bans, calculate how long current reserves will last at normal and reduced consumption rates.
🆘
Emergency Preparedness
Verify that your emergency water stockpile meets your household's needs for a 72-hour, 7-day, or 30-day scenario.
🏗️
Construction Site Water
Estimate how long a site water bowser or storage tank will last at a given daily draw rate before a refill needs to be ordered.
🌱
Irrigation Planning
Know when your irrigation tank will run dry at a given sprinkler or drip-line flow rate, even without a mains connection.
🧮 Worked Example
Worked example: a supply cut with a weak trickle
A household of four has a 2,000-litre rooftop tank that is 70% full when the mains supply drops to a trickle that adds about 100 litres a day. Their bill works out at 120 litres per person per day, and the outlet sits a little above the floor, leaving about 5% of the tank unusable.
Usable water: 2,000 × (70% − 5%) = 1,300 L.
Net daily draw: 4 × 120 − 100 = 380 L/day.
Duration: 1,300 ÷ 380 ≈ 3.4 days (82 hours).
Without the trickle it would be 1,300 ÷ 480 ≈ 2.7 days. Cutting use to 80 litres per person (shorter showers, no garden watering, bucket flushing) stretches it to 1,300 ÷ 220 ≈ 5.9 days.
❓ 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?
It varies widely. The WHO links about 50 litres per person per day with low health risk; many piped urban homes use 100–200 litres per person, so a family of 4 typically uses 400–800 litres per day. US homes average about 310 litres (82 gallons) per person per day, according to US EPA WaterSense — over 1,200 litres a day for four — largely because of garden watering. Your water meter gives the real figure.
What size water tank do I need for 3 days of supply?
Multiply your daily household usage by the number of days. At 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. Then add a margin — commonly 15–25% — for the water below the outlet and busier days. 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 use: fix leaks first, take shorter showers, flush with a bucket of used water, stop garden watering, and reuse rinse water for cleaning. Halving use from 200 to 100 L/person/day doubles how long your tank lasts. Leaks matter more than most people think — US EPA estimates household leaks waste about 180 gallons (680 L) a week in an average home, nearly 100 L a day.
Does this calculator account for evaporation or leakage losses?
Not directly, but you can include them. A covered drinking-water tank loses very little to evaporation. Leaks are the bigger loss: add any known leak to your daily usage, or check by reading the water meter before and after two hours with no water being used. Use the "unusable below outlet" field for water the tank cannot deliver, and the "water coming in" field for any supply still trickling in.
How long is stored water safe to drink?
For bottled water, the CDC says to follow the expiration date; for tap water you store yourself in clean, sealed containers, it advises replacing it every 6 months. A household tank is different — water flows through it constantly, but warmth, sunlight and sediment let germs grow when water sits, and the chlorine from a treated supply fades over time. Keep the tank covered and shaded, clean it regularly, and if water has been standing for a long time or comes from an untreated source, boil or disinfect it before drinking. 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.
✅ Sources & Review
Sources and review
The duration formula is simple arithmetic; the usage benchmarks and guidance on this page come from these references:
US EPA WaterSense — Statistics and Facts: 82 gallons (about 310 L) per person per day at home, and household leaks of about 180 gallons a week; Fix a Leak Week for the meter leak test.
NIST SP 811 — unit conversions (1 US gallon = 3.785 L, 1 UK gallon = 4.546 L, 1 m³ = 1,000 L).
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.
