📖 How To Use
How to Use This Water Weight Calculator
Two modes, one result — pick whichever is easier for your situation:
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Choose your input method
If you already know the volume (e.g. from a tank label), choose "Enter Volume." If you have physical measurements, choose "Enter Dimensions" for a rectangular tank.
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Enter the value(s) and select units
For volume mode, type the amount and pick litres, gallons, cubic metres, etc. For dimension mode, enter Length, Width, and Height with their respective units — mix freely (e.g. length in feet, height in cm).
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Select liquid type
Fresh water is selected by default (1.000 kg/L). Choose salt water for marine tanks, or select a custom density for other liquids like diesel, oil, or milk.
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Read your results
Weight is shown in kg, lbs, metric tonnes, and Newtons — along with volume in litres and US gallons. Copy or print the results with one click.
Structural tip: The weight calculated here is the water alone. Add the tank's own weight (typically 20–80 kg for plastic, 50–200 kg for steel) and the mounting frame for total rooftop load calculations. Use our Rooftop Load Bearing Calculator for complete structural analysis.
📐 The Formula
Water Weight Formula Explained
The weight of water follows a simple relationship between volume and density:
Weight (kg) = Volume (L) × Density (kg/L)
Fresh water: Weight (kg) = Volume (L) × 1.000
Salt water: Weight (kg) = Volume (L) × 1.025
Weight (lbs) = Weight (kg) × 2.20462
Weight (N) = Weight (kg) × 9.80665
For tanks measured in dimensions rather than volume, the calculator first computes volume in litres, then multiplies by the liquid density.
Water Density at Different Temperatures
Pure water is densest at 4°C (1.000 kg/L). It becomes slightly lighter as temperature rises — relevant for hot water tanks and industrial applications.
| Temperature | Density (kg/L) | Weight of 1,000 L | Difference vs 4°C |
|---|
| 4°C (39°F) | 1.00000 | 1,000.0 kg | — |
| 10°C (50°F) | 0.99970 | 999.7 kg | −0.30 kg |
| 20°C (68°F) | 0.99821 | 998.2 kg | −1.79 kg |
| 40°C (104°F) | 0.99222 | 992.2 kg | −7.78 kg |
| 60°C (140°F) | 0.98320 | 983.2 kg | −16.80 kg |
| 100°C (212°F) | 0.95840 | 958.4 kg | −41.60 kg |
For most household and commercial purposes, the difference is negligible. Engineers sizing structural supports for steam or hot-water systems should account for thermal expansion using accurate density figures.
Common Liquid Densities
| Liquid | Density (kg/L) | Weight of 1,000 L |
|---|
| Fresh water (4°C) | 1.000 | 1,000 kg |
| Seawater (avg.) | 1.025 | 1,025 kg |
| Drinking water (20°C) | 0.998 | 998 kg |
| Milk (whole) | 1.030 | 1,030 kg |
| Cooking oil | 0.920 | 920 kg |
| Diesel fuel | 0.850 | 850 kg |
| Petrol / Gasoline | 0.740 | 740 kg |
| Concrete slurry | 2.400 | 2,400 kg |
❓ FAQ
Frequently Asked Questions
How much does 1 litre of water weigh?
1 litre of fresh water weighs almost exactly 1 kilogram (1 kg) at 4°C — the temperature at which water reaches its maximum density. At typical room temperature (20°C), it is marginally lighter at 0.998 kg. For all practical household and engineering calculations, 1 litre = 1 kg is accurate enough.
How much does 1,000 litres (1 m³) of water weigh?
Exactly 1,000 kg (1 metric tonne) at 4°C. In pounds, that is 2,204.6 lbs. A standard 1,000-litre IBC tote full of water weighs approximately 1,025–1,060 kg when you include the container itself.
Does salt water weigh more than fresh water?
Yes. Seawater has an average density of about 1.025 kg/L, meaning 1,000 litres of seawater weighs 1,025 kg compared to 1,000 kg for fresh water. The extra weight comes from dissolved salts (mainly sodium chloride). Ocean salinity varies from around 33–38 g/kg depending on location, which affects density slightly.
How do I calculate water weight from a tank's dimensions?
For a rectangular tank: multiply Length × Width × Height (all in cm) to get cubic centimetres, divide by 1,000 to get litres, then multiply by the density (1 kg/L for fresh water) to get kilograms. This calculator does all of that automatically — just switch to "Enter Dimensions" mode and enter your measurements.
How much does a gallon of water weigh?
A US gallon of fresh water weighs approximately 8.34 lbs (3.785 kg). A UK/Imperial gallon is larger and weighs roughly 10.02 lbs (4.546 kg). Always clarify which gallon definition applies when working with US vs. international specifications.
How do I check if my roof can hold a water tank?
Calculate the total load: water weight + tank shell weight + frame weight. Divide the total load by the base area in square metres to get load per m². Compare this against your roof's rated capacity — most residential concrete slabs can handle 150–250 kg/m². A 1,000 L tank with a 1.0 m × 1.2 m base concentrates about 870 kg/m² of water alone — typically requiring a load-spreading frame and a structural engineer's sign-off. Use our Rooftop Load Bearing Calculator for a full assessment.
Why is this calculator useful for more than just water?
Any liquid stored in a tank has weight that impacts structural loads, pump sizing, and transportation planning. By selecting the correct density — whether for seawater, diesel, cooking oil, or a custom fluid — this calculator gives you accurate weight figures for any tank or container used in agriculture, industry, food production, or emergency storage.