Water density and specific weight: how much 1 liter and 1 m³ weigh

At a room temperature of about 20 °C, the density of pure water is approximately 998.2 kg/m³. This means that 1 liter of water has a mass of about 0.9982 kg, while 1 m³ has a mass of about 998.2 kg. For everyday and preliminary calculations, these values are usually rounded to 1 liter of water ≈ 1 kg and 1 m³ ≈ 1000 kg.

Water density, however, depends on temperature. Liquid water reaches its maximum density at about 4 °C, where it is approximately 999.975 kg/m³. Above this temperature, water expands as it warms, so the mass contained in a fixed volume gradually decreases.

It is important to distinguish mass, density, and specific weight. Kilograms measure mass. Density is expressed, for example, in kg/m³. In physics and engineering, specific weight is the weight force per unit volume, so it is measured in N/m³.

Water density at different temperatures

The table gives values for pure water between 0 and 40 °C. They are rounded for practical calculations. Laboratory, metrological, and other high-precision work must also account for water composition, dissolved gases, pressure, and the exact measurement conditions.

Using 1000 kg/m³ at 4 °C is convenient for quick calculations, but it is a rounded value. The more precise density of pure water near this temperature is slightly below 1000 kg/m³.

How much does 1 liter of water weigh?

One liter is 0.001 m³. The mass of water is calculated with:

m = ρ × V

where:

• m is mass, kg;
• ρ is water density, kg/m³;
• V is volume, m³.

For 1 liter of water at 20 °C:

V = 1 L = 0.001 m³

m = 998.2 × 0.001 ≈ 0.9982 kg

So at 20 °C, one liter of pure water has a mass of about 998 g. The difference from the familiar rule of thumb “1 liter = 1 kg” is small, so it is often ignored in everyday calculations.

How much does 1 m³ of water weigh?

For one cubic meter, the calculation is even simpler because the volume is already 1 m³:

m = ρ × 1

At 20 °C:

m ≈ 998.2 kg

Therefore, 1 m³ of pure water at 20 °C has a mass of about 998.2 kg, or approximately 0.998 metric tonne.

At about 4 °C, the mass of 1 m³ of water is practically one metric tonne.

How to calculate the mass of any volume of water

If the volume is given in liters, first convert it to cubic meters:

V(m³) = V(L) / 1000

Then use:

m = ρ × V

For example, suppose you need the mass of 50 liters of water at 20 °C.

1. Convert the volume: 50 L = 0.05 m³.
2. Use the density: ρ ≈ 998.2 kg/m³.
3. Calculate: m = 998.2 × 0.05 ≈ 49.91 kg.

So 50 liters of water at 20 °C have a mass of about 49.9 kg.

Using the same method, 200 liters at 20 °C have a mass of approximately:

998.2 × 0.2 ≈ 199.6 kg

Why temperature changes water density

As temperature changes, the average spacing between molecules changes, so the same volume of water can contain a slightly different mass.

Water has an unusual property: its maximum density occurs at about 4 °C. When it is cooled further toward its freezing point, its density decreases slightly again. That is why the value at 0 °C is lower than at 4 °C.

Above 4 °C, water expands as it warms. For example, one liter at 40 °C contains less mass than one liter at 20 °C.

A change in density should not be confused with a change in total mass. If a closed container holding a fixed amount of water is heated without losing any water, its mass remains the same while its volume changes.

Water density vs. specific weight

Density is mass divided by volume:

ρ = m / V

The SI unit of density is kg/m³.

Specific weight is weight force divided by volume:

γ = ρ × g

where:

• γ is specific weight, N/m³;
• ρ is density, kg/m³;
• g is gravitational acceleration, m/s².

Using the standard value g = 9.80665 m/s², for water at 20 °C:

γ ≈ 998.2 × 9.80665 ≈ 9790 N/m³

or approximately 9.79 kN/m³.

In everyday tables, “specific weight of water” is sometimes used loosely for density, with a value stated in kg/m³. Physically, these are different quantities: kg/m³ is density, while N/m³ is specific weight.

What else affects the actual density of water?

Temperature is not the only factor. The actual value is also affected by:

• dissolved salts and other substances;
• dissolved gases;
• pressure;
• chemical and isotopic composition;
• accuracy of temperature and volume measurements.

For this reason, values for pure water should not automatically be treated as exact values for seawater, mineral water, process water, or concentrated saline solutions.

For household tanks, containers, reservoirs, and rough load estimates, using 1 kg per liter is usually sufficient. If the result affects calibration, dosing, a technical process, or a design load, use density at the actual water temperature and composition.

How to convert kilograms of water to liters

If mass is known, volume is calculated with the inverse formula:

V = m / ρ

At 20 °C, one metric tonne of pure water occupies:

V = 1000 / 998.2 ≈ 1.0018 m³

That is approximately 1001.8 liters.

Near 4 °C, the same calculation gives a value very close to 1000 liters.

Comparison with other liquids

Water is a useful reference because its density is close to 1000 kg/m³. Many common liquids are either lighter or heavier.

For comparison, see how much 1 liter of gasoline weighs and how crude-oil density affects the mass of 1 liter.