Metric prefixes (SI)
An SI prefix in front of a unit multiplies it by a power of ten: 1 kW is 1000 W, 1 mm is 0.001 m. The prefixes are defined by the BIPM in the SI brochure. These are the ones from pico to tera, the range you meet on drive and motor datasheets.
| Prefix | Symbol | Factor | Multiplier | Example |
|---|---|---|---|---|
| tera | T | 1012 | 1,000,000,000,000 | TWh for yearly energy use of a plant or grid |
| giga | G | 109 | 1,000,000,000 | GΩ insulation resistance of a winding |
| mega | M | 106 | 1,000,000 | MW drives; MΩ insulation test readings |
| kilo | k | 103 | 1,000 | kW motor power, kNm gearbox torque, km |
| hecto | h | 102 | 100 | hPa air pressure |
| deca | da | 101 | 10 | daN force on some European datasheets |
| deci | d | 10-1 | 0.1 | dm, now rare in engineering |
| centi | c | 10-2 | 0.01 | cm, Ncm torque on small motors |
| milli | m | 10-3 | 0.001 | mm, mNm, mH winding inductance, ms |
| micro | µ | 10-6 | 0.000001 | µm positioning accuracy, µs cycle times |
| nano | n | 10-9 | 0.000000001 | nm encoder resolution, nF filter capacitors |
| pico | p | 10-12 | 0.000000000001 | pF/m capacitance of a motor cable |
Things that trip people up
- Case matters:
mis milli,Mis mega. A 1 mΩ and a 1 MΩ resistance are 109 apart. - Kilo is a lower-case
k: kW, not KW. - The micro sign is
µ. Where it can't be typed,uis a common stand-in (um, uF), but that is not SI. - A prefix belongs to the unit it is written against: mNm is millinewton metre, 0.001 N·m.
Every unit in the unit converter that uses a prefix converts to its base unit there.
Summarised from the standard named above. For a design decision, check the motor maker's datasheet and the current edition of the standard. More datasheet reference pages.