Area

The square metre (m²) is the SI unit of area, the hectare is used for large areas.   The imperial system uses the square yard (yd²) and the acre for large areas.

Calorific Values

The joule per cubic metre (J/m³) is the SI unit of heat content (Calorific Value).   Many Gas or Fuel Engineers prefer the calorific value to be expressed in terms of heat units. the metric system uses the kilocalorie per cubic metre (kcal/m³) and the imperial system the British thermal unit per cubic foot (Btu/ft³)

Energy

The joule is the SI unit of energy. the joule is really a unit of work, being the work done when a force of 1 newton moves through a distance of 1 metre.   Many fuel and gas engineers prefer to use heat units for energy measurement.   The metric unit for heat energy being the calorie (cal) or kilocalorie (kcal) and the imperial unit being the British thermal unit (Btu).

Force

The Newton is the SI unit of force, and is defined as the force required on a mass of 1Kg to acheive an acceleration of 1m/sec.sec. (kg.m/s²)   In the metric (MKS) system, the Kg.force (kg.f) is used.   In the imperial system the basic unit of force is the poundal (lb.ft/s²) and the pound-force (lb.f) is also used.

Length

The metre is the SI unit of length. In the metric system the kilometre is used for long distances.   In the imperial system the basic unit is the yard (yd) and for longer distances the mile.   Both metric and imperials systems use other units which are multiples or fractions of the two basic units.

Mass

The Kilogram (kg) is the basic SI unit of mass. In the metric system the tonne (1000Kg) is used for large weights.   The basic unit of mass in the imperial system is the pound (lb), wich is itself sub-divided into ounces for smaller weights and multiiplied by various factors for larger weight units such as the stone , hundredweight and ton.   It should be noted that in the U.S.A. the ton and hundredweight are not the same as in the U.K. and are refered to as the short ton (2000lb) and short hundredweight (100lb).

Power

Power is the rate of work or energy useage. It is usually expressed in energy or work units per unit of time.
The watt (w) is the SI unit of power,being defined as a joule. per. second. (br/> It is common for electric heaters and gas burners to be sized in kilowatt, although many combustion engineers prefer burners to be rated in Btu. per.hour or Kcal.per hour.
Mechanical engineers use work units such as horsepower (hp) or foot pound-force per second (ft lbf/s).

Pressure

The pascal (Pa) is the SI unit of pressure. The pascal is a very small unit so usually pressures are expressed in Kilopascal (kPa) or megapascal(mPa).
Pressure is force per unit of area, many engineers prefer the more practical units of Pounds force per square inch or Kg force per square meter.
Some engineers prefer to use the very practical units of inches or cm. water or mercury gauge.

Volume

The cubic metre is the SI unit of volume. The unit of capacity is the litre, being the volume taken up by 1 Kilogram of water at specific temperature conditions, i.e. 1000 cubic centimetres or 1 cubic decimetre.
In the imperial system the gallon is the unit of capacity, being the volume taken up by 10lbs of water at specific temperature conditions