**Metric System**This is a decimal system of weights and measures originally based on the meter as the unit of length and the kilogram as the unit of mass.

**SI System**The International system of units was adopted by the 11th General Conference of Weights and Measures in 1960. The SI units are widely used but they have not been fully accepted by the scientific community. In fact, metric system is still used in most countries.

**SI Base Units**

**Units of Length**

The SI unit of length is the meter (m).

The commonly used metric length units are listed below:

**Units of Volume**

- The derived SI unit of volume is Cubic meter (m3).
- This is the volume of a cube that is 1 meter on each edge. The related units of volume which are also used are: Cubic centimeter (cm3), Cubic decimeter (dm3)

In the metric system of measurement, the most common units of volume are milliliters (mL) and liters (L).

**Units of Temperature**

- The SI unit of temperature is the Kelvin (k).
- The metric unit of temperature is the Celsius (°C).

A degree on the Kelvin scale has the same magnitude as a degree on the Celsius scale but zero on the Kelvin scale equals – 273.15ºC. Thus the temperature (0 K) is often referred to as the absolute zero.

Celsius and Kelvin temperature are related as:

K = °C + 273.15,

°C = K – 273.15.

**Units of Mass and Weight**

The basic unit of mass in the metric system is gram (g) and in the SI system is kilogram (kg).

The British system of metric weights is also used by chemists or dispensing pharmacist in which

- 1 Pound (lb) = 16 Ounce (oz)
- 1 Pound (lb) = 453.6 gram (g)
- 2.205 Pound (lb) = 1 kilogram (kg)
- 1 ton = 2000 Pound (lb)
- Units of Force
- The SI unit for force is kg ms–2 (It’s also called Newton).

**Units of Work and Heat Energy**

- The SI units of work and energy is the newton-meter. It is also called Joule (J).
- The metric units of heat is the calories (cal).

One calorie is defined as exactly 4.184 joules.

Thus, 1 cal = 4.184 J

**Units of Pressure**

- The SI unit of pressure is Nm–2 is named pascal and given the symbol Pa.
- Three other units which have been traditionally used is atmosphere, symbol atm.

**Units of Density**

- SI unit for density is kgm-3.
- The other units of density commonly used are: gml-1, gL-1.

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