Engineering notation is scientific notation in which the powers of ten are limited to those where the exponent is a multiple of three, i.e, the powers are powers of 1000 (but written as e.g. 10⁶, not 1000²).

As an alternative for writing powers of 10, SI prefixes can be used. These also provides steps of a factor 1000, except if the unit has an exponent: in that case the SI prefixes provide only steps of a factor one million, one billion, or more.

A disadvantage both of using an SI prefix and of using engineering notation, compared with scientific notation, is that e.g. 500 µm and m do not show the accuracy distinction between 5 × 10⁻⁴, 5.0 × 10⁻⁴, and 5.00 × 10⁻⁴ m. That can be solved by not using a range 1–1000 for the number in front of the power, but a number in the range 0.01–10. Depending on the case this may be suitable, or impractical. In the example we can use 0.5, 0.50, and 0.500 mm.

Another example: when the speed of light is expressed as 3.00 × 10⁸ m/s or km/s then it is clear that it is between 299 500 and 300 500 km/s, but when using 300 × 10⁶ m/s, or 300 × 10³ km/s, 300 000 km/s, or the unusual but short 300 Mm/s, this is not clear. A possibility is using 0.300 Gm/s, convenient to write, but somewhat impractical in understanding (writing something large as a fraction of something even larger; in a context of larger numbers expressed in the same unit this could be convenient, but that is not applicable here).

See also

• SI
• Significant figures