## Introduction

In the fields of civil engineering, unit conversion of various measurements is must have knowledge now-a-days. Therefore, we must know the basic conversion of different units. A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.

For example, the length is a physical quantity. A metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called “metre”. Measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind. (Source: Wikipedia)

## Importance of Unit

Any physical quantity has two parts- 1. Magnitude and 2. Unit. Magnitude is the numerical value of the measurement, while the unit is a standard for measurement of the same kind of quantity. No measurement can be expressed without a unit. For the above example, if we say the length of a wooden plank is 10 (without unit), we can not justify the actual length of the plank. It may be 10 metres, 10 feet or 10 inches. If we give the unit, we can understand the proper length. Therefore, it is necessary to include the unit of the measurement along with the magnitude. However, in some cases, where the quantity is expressed as the ratio of two similar quantities, that quantity has no unit. For example- Specific Gravity.

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## Systems of Unit Measurement

There are different standards and unit systems used in the world. Few common systems of measurements used in Engineering fields are:

### CGS Unit System

In the CGS unit system, the length is measured in centimetre, mass is measured in gram and time is measured in second.

Unit | Abbreviation | To Measure |

Centimetre | cm | Length |

Gram | g | Mass |

Second | s | Time |

### FPS Unit System

In the FPS system, the length is measured in feet, mass is measured in pound and time is measured in second.

Unit | Abbreviation | To Measure |

Feet or Foot | cm | Length |

Pound | g | Mass |

Second | s | Time |

### SI Unit System

Different units are used in different countries for the measurement of physical quantities. In the US, pound metrics is used commonly for indicating mass. But in India, the kilogram is used to measure the same physical quantity. To remove these differences, the SI (International System of units) system was standardized in 1960. In the SI unit, 8 physical quantity is assumed to be the Fundamental Unit or Basic Unit.

Unit | Abbreviation | To Measure |

Metre | cm | Length |

Kilogram | g | Mass |

Second | s | Time |

Ampere | A | electric current |

Kelvin | K | thermodynamic temperature |

Mole | mol | amount of substance |

Candela | Cd | luminous intensity |

Radian | rad or ^{c} | Angle |

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## Unit Conversion

### Conversion of Length

Imperial units of measurement | 1 foot (ft or ‘) = 12 inches (in or “) 1 yard (yd) = 3 feet 1 mile = 1,760 yards |

Metric units of measurement | 1 centimetre (cm) = 10 millimetres (mm) 1 metre (m) = 100 centimetres 1 kilometre (km) = 1,000 metres |

Converting from Imperial to metric measurements | 1 in = 25.4 mm 1 in = 2.54 cm 1 ft = 0.3048 m 1 yd = 0.9144 m 1 mile = 1.6093 km |

Converting from metric to Imperial measurements | 1 mm = 0.0394 in 1 cm = 0.3937 in 1 m = 3.2808 ft 1 m = 1.0936 yd 1 km = 0.6214 miles |

### Conversion of Area

Imperial units of measurement | 1 sq foot (ft^{2}) = 144 sq inches (in^{2})1 sq yard (yd ^{2}) = 9 sq feet1 acre = 4,840 sq yards 1 sq mile = 640 acres |

Metric units of measurement | 1 sq cm (cm^{2}) = 100 sq mm (mm^{2})1 sq metre (m ^{2}) = 10,000 sq cm (cm^{2})1 sq km (km ^{2}) = 100 hectares |

Converting from Imperial to metric measurements | 1 in^{2} = 6.4516 cm^{2}1 ft ^{2} = 0.0929 m^{2}1 yd ^{2} = 0.8361 m^{2}1 acre = 4046.9 m ^{2}1 mile ^{2} = 2.590 km^{2} |

Converting from metric to Imperial measurements | 1 cm^{2} = 0.1550 in^{2}1 m ^{2} = 10.7643 ft^{2}1 m ^{2} = 1.1960 yd^{2}1 m ^{2} = 0.0002 acres (not used)1 km ^{2} = 0.3861 mile^{2} |

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### Conversion of Volume/Capacity

Imperial units of measurement | 1 cu foot (ft^{3}) = 1,728 cu inches (in^{3})1 cu yard (yd ^{3}) = 27 cu feet (ft^{3})1 pint (pt) = 20 fluid ounces (fl oz) 1 gallon (gal) = 8 pints |

Metric units of measurement | 1 cu decimetre (dm^{3}) = 1,000 cu cm (cm^{3})1 cu metre (m ^{3}) = 1,000 cu decimetre1 litre (l) = 1 cu decimetre |

Converting from Imperial to metric measurements | 1 in^{3} = 16.387 cm^{3}1 ft ^{3} = 28.3286 dm^{3}1 ft ^{3} = 0.0283 m^{3}1 yd ^{3} = 0.7646 m^{3} |

Converting from metric to Imperial measurements | 1 cm^{3} = 0.0610 in^{3}1 dm ^{3} = 0.0353 ft^{3}1 m ^{3} = 35.3357 ft^{3}1 m ^{3} = 1.3080 yd^{3}1 ml = 0.0352 fl oz 1 l = 1.76 pints (UK) 1 l = 0.220 gallons (UK) 1 hl = 21.997 gallons (UK) |

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