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Engineering notation is a version of scientific notation in which the powers of ten must be multiples of three (i.e., they are powers of a thousand, but written as, for example, 10^{6} instead of 1000^{2}).^{[1]} As an alternative to writing powers of 10, SI prefixes can be used, which also usually provide steps of a factor of a thousand.^{[2]}

Compared to normalized scientific notation, one disadvantage of using SI prefixes and engineering notation is that significant figures are not always readily apparent. For example, 500 µm and 500 × 10^{−6} m cannot express the uncertainty distinctions between 5 × 10^{−4}, 5.0 × 10^{−4}, and 5.00 × 10^{−4} m. This can be solved by changing the range of the coefficient in front of the power from the common 1–1000 to 0.001–1.0. In some cases this may be suitable; in others it may be impractical. In the previous example, 0.5, 0.50, or 0.500 mm would have been used to show uncertainty and significant figures. It is also common to state the precision explicitly, such as "47 kΩ ±5%"

Another example: when the speed of light (exactly 299 792 458 m/s by the definition of the meter and second) is expressed as 3.00 × 10^{8} m/s or 3.00 × 10^{5} km/s then it is clear that it is between 299 500 and 300 500 km/s, but when using 300 × 10^{6} m/s, or 300 × 10^{3} 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).

Engineering notation, like scientific notation generally, can use the E notation, such that

3.0 × 10^{−9}

can be written as

3.0E−9 or 3.0e−9

The E or e should not be confused with the exponential e which holds a completely different significance. In the latter case, it would be shown that 3e^{−9} ≈ 0.000 370 23.