It is now the accepted practice in the Industry to use a Metric system based on the International Standard System of Units called "SI" (Système International). This system has now been adopted in the writing of the National and International Standards for all issues concerning fans and air performance. However, there is still enough documentation in use throughout the world for us to have to be aware of Imperial or "I-P" units and one or two other local deviations from the basic SI system. Some conversion factors are given in the following table. Multiply the "Alternate" unit by the conversion factor to obtain the "Preferred" unit.
Units commonly found in Fan or System Performance Specifications:
| Quantity
| Preferred Unit
| Alternate Unit
| Conversion Factor
| | Length (L)
| metre (m) | foot (ft) | 0.305
| | Mass (M)
| kilogram (kg) | pound (lb) | 0.4536
| | Temperature (t)
| degree Celsius (°C) | deg Fahrenheit (°F) | (t-32) × (5/9)
| | Velocity (v)
| metre per second (m/s) | foot per minute (fpm) | 0.00508
| | Volume Flow (Q)
| cubic metre per second (m³/s)
litre per second (l/s)
| cubic foot per minute (cfm)
cubic metre per hour (m³/hr)
| 0.000472
0.472
3.6
| | Pressure (P)
| Pascal (Pa or N/m²) | inch water gauge (in.wg)
millimetre of water (mm.H2O)
| 249
9.81
| | Speed (n)
| revs per second (rps) | revs per minute (rpm) | 0.01667
| | Electric Current (l)
| ampere (A) | - | -
| | Electromotive Force (V)
| volt (V) | - | -
| | Power (W)
| watt (W or J/s) | horse power (hp) | 746
| | Density (p)
| kg per cubic metre (kg/m³) | pound per cubic foot (lb/ft³) | 16.02
| | Power Factor (cos Ø)
| W : VA
| | Efficiency (n)
| QP / W
| | Air Change Rate
| air changes per hour (ac/h) | - | -
|
To avoid using very large or very small numbers, there are "Multiplying Prefixes".
| Prefix Name | Prefix Symbol | Factor by which the unit is multplied
| | tera
| T
| 10 12
| =
| 1 000 000 000 000
| | giga | G | 109 | = | 1 000 000 000
| | mega | M | 106 | = | 1 000 000
| | kilo | k | 103 | = | 1 000
| | hecto | h | 102 | = | 100
| | deca | da | 101 | = | 10
| | deci | d | 10-1 | = | 0.1
| | centi | c | 10-2 | = | 0.01
| | milli | m | 10-3 | = | 0.001
| | micro | u | 10-6 | = | 0.000 001
| | nano | n | 10-9 | = | 0.000 000 001
| | pico | p | 10-12 | = | 0.000 000 000 001
| | femto | f | 10-15 | = | 0.000 000 000 000 001
| | atto | a | 10-18 | = | 0.000 000 000 000 000 001
|
Examples:
gigahertz (Ghz), megawatt (MW), kilometre (km), centimetre (cm), microsecond (us), nanometre (nm), picofarad (pF)
| 
