📖 How To Use
How to Use This Calculator
Getting your rectangular tank's exact capacity takes under 30 seconds:
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Enter your 3 measurements
Measure Length, Width, and Height from the inside of the tank. Internal dimensions give the actual water storage volume — external dimensions overstate it by 5–10%.
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Choose your unit per field
Use whatever units are on your tape measure — ft, in, cm, m, or mm. Each field has its own selector. You can even mix units (e.g., length in ft, height in cm).
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Set the fill level (optional)
Drag the slider if you want volume at a partial fill, e.g. 75% for current water level. Leave at 100% for full capacity.
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Hit Calculate
Results appear instantly — litres, gallons, cubic metres, weight in kg and lbs. Copy or print them with one click.
Tip: For tanks with internal fittings or baffles, subtract their volume from the result. A 50 mm pipe running 120 cm tall displaces roughly 2.4 litres — negligible for households, worth noting for industrial use.
📐 The Formula
Rectangular Tank Volume Formula
The math is simple — multiply three numbers:
Volume = Length × Width × Height
Litres (cm inputs): V (L) = (L × W × H) ÷ 1,000
Cubic metres (m inputs): V (m³) = L × W × H
Cubic feet (ft inputs): V (ft³) = L × W × H
This calculator converts all units to centimetres internally before calculating, so mixed-unit inputs always produce correct results.
Unit Conversion Reference
| Unit | Equals Litres | From Litres |
|---|
| 1 Litre (L) | 1 L | × 1 |
| 1 Cubic Metre (m³) | 1,000 L | ÷ 1,000 |
| 1 US Gallon | 3.785 L | ÷ 3.785 |
| 1 UK Gallon | 4.546 L | ÷ 4.546 |
| 1 Cubic Foot (ft³) | 28.317 L | ÷ 28.317 |
| 1 Cubic Inch (in³) | 0.01639 L | × 61.024 |
Water Weight
1 litre of fresh water = 1 kg (or 2.2046 lbs). This is the standard used for structural load calculations worldwide.
Weight (kg) = Volume (litres) × 1
Weight (lbs) = Volume (litres) × 2.2046
📏 Reference Data
Standard Rectangular Tank Sizes
Common manufactured dimensions and their calculated volumes:
| Capacity | L × W × H (cm) | L × W × H (ft) | Litres | US Gal | Full Weight |
|---|
| 250 L | 80 × 60 × 52 | 2.6 × 2 × 1.7 | 250 | 66 | 250 kg |
| 500 L | 100 × 70 × 72 | 3.3 × 2.3 × 2.4 | 504 | 133 | 504 kg |
| 750 L | 120 × 80 × 80 | 3.9 × 2.6 × 2.6 | 768 | 203 | 768 kg |
| 1,000 L | 120 × 100 × 85 | 3.9 × 3.3 × 2.8 | 1,020 | 269 | 1,020 kg |
| 1,500 L | 150 × 100 × 102 | 4.9 × 3.3 × 3.3 | 1,530 | 404 | 1,530 kg |
| 2,000 L | 160 × 120 × 106 | 5.2 × 3.9 × 3.5 | 2,035 | 537 | 2,035 kg |
| 5,000 L | 200 × 160 × 158 | 6.6 × 5.2 × 5.2 | 5,056 | 1,335 | 5,056 kg |
| 10,000 L | 250 × 200 × 200 | 8.2 × 6.6 × 6.6 | 10,000 | 2,642 | 10,000 kg |
Note: Actual manufacturer dimensions vary. Always verify against the supplier's data sheet.
⚠️ Common Mistakes
Common Measurement Mistakes
1. External Instead of Internal Dimensions
A 10 mm wall on each side of a 100 cm tank means the internal length is only 98 cm — compounds across all three dimensions. Always measure inside the tank.
2. Including Dead Space
Inlet/outlet zones, sediment sumps, and internal columns displace volume. Only measure space that can actually hold water.
3. Inconsistent Units Without Conversion
Measuring length in feet and height in inches then multiplying directly gives nonsense. This calculator handles mixed units correctly — just set the right unit per field.
4. Forgetting Fill Headspace
Most manufacturers recommend leaving 10–15% empty to prevent overflow from thermal expansion. Use the fill slider to plan for realistic operational levels.
5. Trusting the Label Over Measurement
A "1,000-litre tank" may hold 980 or 1,040 litres depending on actual internal geometry. Measure and calculate — don't rely on the label when precision matters.
❓ FAQ
Frequently Asked Questions
How do I calculate the volume of a rectangular water tank?
Multiply Length × Width × Height. If measuring in centimetres, divide the result by 1,000 to get litres. This calculator handles all conversions automatically.
Should I use internal or external tank dimensions?
Always use internal dimensions. External dimensions include wall thickness (5–100+ mm), which overstates actual water storage — sometimes by 10–15% on small thick-walled tanks.
What is the difference between US gallons and UK/Imperial gallons?
A US gallon equals 3.785 litres; a UK gallon equals 4.546 litres — about 20% larger. This calculator shows both simultaneously.
How heavy is a full rectangular water tank?
Water weighs 1 kg per litre. A 1,000 L tank carries 1,000 kg of water plus the tank shell (typically 30–60 kg for polyethylene). Always confirm your roof structure can handle the total weight before installation.
Can I use this for liquids other than water?
Yes — the volume results apply to any liquid. Only the weight estimate assumes water (1 kg/L). For other liquids, multiply the litre result by that liquid's density: diesel ≈ 0.85 kg/L, milk ≈ 1.03 kg/L, cooking oil ≈ 0.92 kg/L.
How many litres per day does a family of 4 need?
WHO estimates 50 litres per person per day for basic needs. Most developed-world households use 100–200 L/person/day for full comfort. A family of 4 typically needs 400–800 litres per day.
What tank size do I need for a 72-hour emergency supply?
At WHO's minimum (50 L/person/day) for 4 people over 3 days: 50 × 4 × 3 = 600 litres minimum. For full comfort (150 L/person/day): 1,800 litres.
How much does a full rooftop tank weigh — and is my roof safe?
Water weighs 1 kg per litre, so a full 1,000-litre tank puts 1,000 kg on your roof plus 30–80 kg for the tank shell. Most residential flat roofs are rated for 150–250 kg/m². A 1,000 L tank sitting on a 1.2 m × 1.0 m base concentrates roughly 870 kg/m² — well above the limit. Always use a load-distributing frame and have a structural engineer verify before installation. Use our Water Tank Weight Calculator for precise load figures.
Can I use this calculator for liquids other than water?
Yes — the volume results apply to any liquid. Only the weight estimate assumes fresh water (1 kg/L). For other liquids, multiply the litre result by that liquid's density: diesel ≈ 0.85 kg/L, milk ≈ 1.03 kg/L, cooking oil ≈ 0.92 kg/L, seawater ≈ 1.025 kg/L. This makes the calculator equally useful for fuel storage, food production, and industrial tank sizing.