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Is a Litre of Water 1 kg?

Published in Water Measurement 2 mins read

Yes, a litre of water is almost exactly one kilogram (1 kg). This is because the kilogram was originally defined based on the mass of a cubic decimeter of water (which is equal to one liter).

Why is this the case?

The relationship between liters and kilograms for water is not a coincidence. It's a consequence of how the metric system was initially defined. The gram was originally defined as the mass of one cubic centimeter of water at its maximum density (approximately 4°C). Since there are 1000 cubic centimeters in a liter, one liter of water has a mass of approximately 1000 grams, or 1 kilogram.

  • It's important to note: This relationship holds true approximately. The exact mass of a liter of water can vary slightly depending on temperature and pressure. However, at standard temperature and pressure (STP), the difference is negligible for most practical purposes.

  • Other liquids: It's crucial to understand that this 1:1 ratio applies specifically to water. Other liquids have different densities; therefore, one liter of another liquid will not weigh one kilogram. For example, one liter of oil will weigh less than one kilogram, while one liter of mercury will weigh significantly more.

Density and the 1:1 relationship:

The density of a substance is its mass per unit volume. Water has a density of approximately 1 kg/L. This means that for every liter of water, there is approximately one kilogram of mass. This high density is why the relationship between liters and kilograms for water is so convenient.

Examples:

  • 1 milliliter (mL) of water: Approximately 1 gram (g)
  • 1000 liters of water: Approximately 1000 kg (1 tonne or megagram)