Water Tank Calculator — Capacity in Litres & Gallons
1 cu ft = 28.317 litres. 1 US gallon = 3.785 litres.
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How to Use This Water Tank Calculator
- Select the tank shape — rectangular (box-type concrete or plastic tanks) or cylindrical (round plastic/fibre tanks).
- Choose your unit — feet or metres.
- Enter the internal dimensions — length, width and height for rectangular; diameter and height for cylindrical. Use inside measurements for true capacity, not the outside of the walls.
- Read your results: capacity in litres, US gallons, cubic feet, and the weight of water the tank holds when full.
Practical note: Never fill a tank to the absolute brim. Leave 100–150 mm freeboard at the top for the float valve and to stop overflow splashing. On site we size for the usable volume, not the theoretical full volume.
Water Tank Capacity Formulas
Rectangular tank
Volume = Length × Width × Height
All in the same unit. The answer is in cubic units (cubic feet if you measured in feet).
Cylindrical tank
Volume = π × r² × h
where r = diameter ÷ 2, and h = height. Use π = 3.1416.
Unit conversions
- 1 cubic foot = 28.317 litres
- 1 litre = 0.0353 cubic feet
- 1 US gallon = 3.785 litres
- 1 cubic metre = 1,000 litres
- 1 litre of water weighs 1 kg (at normal temperature)
Worked Example — Rooftop Tank
A house has a rectangular rooftop tank, internal size 4 ft × 4 ft × 3 ft:
- Volume = 4 × 4 × 3 = 48 cubic feet
- Litres = 48 × 28.317 = 1,359 litres
- US gallons = 1,359 ÷ 3.785 = 359 gallons
- Water weight = 1,359 kg — that is nearly 1.4 tonnes sitting on your roof slab!
This is why the structural engineer must know the tank size before casting the roof. I have seen slabs crack because a “small” tank turned out to hold 2,000 litres nobody accounted for.
Standard Water Tank Sizes (Pakistan Market)
Ready-made plastic tanks are sold in standard capacities. Use this table to pick the right size instead of guessing:
| Capacity (litres) | Typical dimensions | Best for |
|---|---|---|
| 250 | 2.5 ft dia × 2 ft high | Small washroom / single storey top-up |
| 500 | 3 ft dia × 2.5 ft high | 1–2 person household backup |
| 1,000 | 3.5 ft dia × 3.5 ft high | Standard family rooftop tank |
| 1,500 | 4 ft dia × 4 ft high | Large family / double storey |
| 2,000 | 4.5 ft dia × 4.2 ft high | Commercial / apartment block floor |
| 5,000 | Underground RCC tank ~6×5×6 ft | Whole-house storage / tube-well backup |
How Much Water Does Your Family Need?
A quick sizing rule used on site:
| Use | Litres per person per day |
|---|---|
| Drinking + cooking | 10–15 |
| Bathing | 40–60 |
| Flushing + washing | 40–60 |
| Total per person | 100–135 |
A family of 6 needs roughly 700–800 litres per day. A 1,000-litre tank gives one full day of backup — the most common choice for houses in Pakistan.
Rectangular vs Cylindrical Tanks — Which to Choose?
| Factor | Rectangular (RCC/box) | Cylindrical (plastic) |
|---|---|---|
| Space use | Fits corners, better for tight roofs | Wastes corner space |
| Cost | Higher (formwork + labour) | Cheaper, ready-made |
| Cleaning | Corners trap sediment | Easier to scrub round walls |
| Strength | Needs proper RCC design | Self-supporting shape |
| Sizes | Any custom size | Fixed standard sizes |
On site: for underground storage, RCC rectangular is standard (custom-fit to the plot). For rooftops, ready-made cylindrical plastic tanks win on cost and speed — just confirm the slab design first.
Field Tips from Site
- Measure inside, not outside. A 4-ft tank with 3-inch walls holds far less than 4×4×4 suggests. Always deduct wall thickness.
- Check the slab. 1,000 litres = 1 tonne. Place tanks over beams or load-bearing walls, never mid-span on a thin slab.
- Underground tanks need waterproofing (waterproof plaster + chemical admixture) — a leaking tank wastes water and undermines the foundation.
- Black tanks beat white in sunlight — less algae growth inside.
- Overflow pipe is mandatory, led to a visible spot so you notice a stuck float valve before the roof floods.
- Clean the tank every 6 months. Sediment at the bottom breeds bacteria — drain fully, scrub with a brush, and refill. A tank you can’t open and clean is a tank you shouldn’t buy.
Frequently Asked Questions
How do I calculate water tank capacity in litres?
Measure the internal length, width and height in feet, multiply them (L × W × H) to get cubic feet, then multiply by 28.317. For round tanks use π × r² × h. This water tank calculator does it automatically.
How many litres in a 4×4×3 ft water tank?
48 cubic feet = 1,359 litres (359 US gallons).
What size water tank do I need for my house?
Allow 100–135 litres per person per day. For a family of 5–6, a 1,000-litre rooftop tank is the standard choice in Pakistan.
How heavy is a full 1000-litre water tank?
About 1,000 kg (1 tonne) of water, plus the tank’s own weight (~30–50 kg for plastic). Always confirm the roof can carry it.
What is the difference between US gallons and Imperial gallons?
1 US gallon = 3.785 litres; 1 Imperial (UK) gallon = 4.546 litres. This calculator shows US gallons — multiply litres by 0.2642 for US gallons, or 0.22 for Imperial gallons.
Should I fill my tank to the top?
No. Keep 100–150 mm freeboard below the rim for the float valve and to prevent overflow. Size your purchase on usable volume.
How do I calculate an underground water tank?
Same formula — internal L × W × H. Underground tanks are usually rectangular RCC; remember to subtract wall and base slab thickness from outside measurements.
Why does my new tank hold less than its rated capacity?
Rated capacities are nominal. Wall thickness, the unusable space below the outlet pipe, and freeboard at the top all reduce real storage by 5–10%.
Final Thoughts
A water tank calculator turns a guessing game into a one-minute check — capacity in litres, gallons, and the all-important weight on your structure. Size for your family’s daily use, keep freeboard, and never put a tonne of water where the slab wasn’t designed for it.
Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.