Water Tank Capacity Calculator & Litres, m³ & Days of Supply
Sizes overhead tanks
“One accepted answer here saves a hundred people an hour each.
Find your water tank's capacity — from the dimensions get the volume in litres, m³ and gallons, the usable volume at your fill level, and how many days it supplies.
Enter your tank
Next: size the tank to carry a few days' buffer — at 1,000 L/day this tank holds about 9.4 days when 100% full; add headroom for dry spells and pump downtime.
1 m³ = 1000 L = 264 US gal.
Water tank — key facts
- Cylinder
- π/4 × D² × H
- Rectangle
- L × W × H
- 1 m³
- = 1000 L = 264 US gal
- Use internal
- dimensions, not external
- Household use
- ≈ 100–150 L/person/day
- Dairy cow
- ≈ 40–100 L/day
- Days of supply
- usable ÷ daily demand
- Privacy
- Runs in your browser; nothing uploaded
Know your storage, plan your supply
Whether it's an overhead household tank, a farm sump, or a livestock trough, knowing the real capacity in litres — and how many days it covers — is the difference between confident planning and running dry. The volume is simple geometry, but it's easy to mis-measure (external instead of internal dimensions) or forget that you can't use every last litre below the outlet.
This tool computes the capacity of a cylindrical or rectangular tank in litres, m³ and gallons, the usable volume at your fill level, and the days of supply at your daily demand. Use it to size a new tank to a sensible reserve, check an existing one, or plan livestock and irrigation top-ups. Pair it with the Livestock Water, Rainwater Harvesting and Farm Pond tools to balance supply and demand.
Know the litres
Exact capacity in litres, m³ and gallons from the dimensions.
Size to days
Test a tank against your daily demand for a sensible reserve.
Use realistic volume
Fill level gives the usable water, not just the full figure.
Any tank
Works for round or rectangular tanks, sumps and troughs.
Frequently Asked Questions
How do I calculate a water tank's capacity?+
For a cylindrical tank, volume = π/4 × diameter² × height; for a rectangular tank, volume = length × width × height (all in metres) gives cubic metres, which you multiply by 1,000 for litres. A 2 m diameter × 3 m tall cylinder holds about 9,425 litres. This tool computes it from your dimensions.
How many litres is 1 cubic metre?+
1 m³ = 1,000 litres = 264.2 US gallons (≈ 220 imperial gallons). So a 5 m³ tank holds 5,000 litres. The tool reports capacity in m³, litres and US gallons so you can use whichever unit your supplier or pump rates use.
What is usable volume?+
It's the water you can actually draw — less than the full capacity because tanks are filled below the rim and the outlet sits above the floor. Set a fill level (e.g. 90–95%) and the tool gives the usable litres; allow extra dead storage below the outlet if your tank has a high draw-off.
How big a water tank do I need?+
Size it to a few days of demand plus a buffer for supply interruptions. Work out daily use (household, livestock or irrigation top-up) and multiply by the days of reserve you want. The tool's days-of-supply figure lets you test a tank size against your demand instantly.
How do I measure my tank?+
For a round tank, measure the internal diameter and the water height (or full height); for a rectangular tank, the internal length, width and height. Use internal dimensions, not external, so wall thickness doesn't inflate the volume. Enter them in metres.
How much water does a household use?+
Roughly 100–150 litres per person per day for all uses, less where water is scarce. For a family of five that's about 500–750 litres a day. Enter your real daily demand and the tool shows how many days your tank covers.
How much water do livestock need?+
It varies by animal — a dairy cow can drink 40–100+ litres a day, a goat 5–10, poultry a fraction of a litre each. Add up your herd's daily need (the Livestock Water tool helps) and use it as the daily demand here to size storage.
How do I convert tank volume to gallons?+
Multiply cubic metres by 264.2 for US gallons or by 220 for imperial gallons; or litres ÷ 3.785 for US gallons. The tool already reports US gallons alongside litres and m³, so you don't have to convert by hand.
Does tank shape affect capacity?+
Only through the volume formula — a cylinder and a box of the same footprint and height hold different amounts because the cylinder's area is π/4 (≈ 79%) of the enclosing square. The tool uses the correct formula for whichever shape you select.
Should I oversize the tank?+
A modest buffer is wise — for supply outages, peak demand and future growth — but very oversized tanks cost more, take longer to turn over (water can go stale) and need a strong stand if overhead. Size to your days-of-supply target plus a sensible margin, which this tool makes easy to test.
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