Engineering notation is a version of scientific notation in which the exponent in expressions of the form is chosen to always be divisible by 3. Numbers of
forms such as ,
,
340, and
therefore correspond to engineering notation, while numbers such as , , and do *not*.

16 of the 20 SI prefixes (excluding centi-, deci-, deka-, and hecto-) correspond to engineering notation.

Engineering notation is sometimes written using "E notation," which represents for example
as `3.0E9` or `3.0e-9`. Here, the E or e is shorthand for and should not be confused with the base of the natural
logarithm .

Care must be taken when attempting to infer significant digits from numbers expressed in engineering notation. For example, while unambiguously indicates two significant digits in scientific notation, the number of significant digits in is not clear, with the only obvious way to indicate significance being to write the number in the awkward form .