By default, results of conversions are shown to eight significant figures; this can be changed with the --exponential and --output-format options. The former sets an exponential format (i.e., scientific notation) like that used in the original Unix units program; the latter allows the format to be given as that of the printf function in the ANSI C standard.
The format recognized with the --output-format option is a
subset of that for printf(). Only a floating-point format of the form
is allowed: it must begin with ‘%’, and must end with a
floating-point type specifier (‘E’ or ‘e’ for scientific
notation, ‘f’ for fixed-point decimal, or ‘G’ or ‘g’ to
specify the number of significant figures). The format specification
may include one optional flag (‘+’, ‘-’, ‘#’, or a
space), followed by an optional value for the minimum field width, and
an optional precision specification that begins with a period (e.g.,
‘.6’). In addition to the digits, the field width includes the
decimal point, the exponent, and the sign if any of these are shown. A
width specification is typically used with fixed-point decimal to have
columns of numbers align at the decimal point; it normally is not
useful with units. Non-floating-point type specifiers make
no sense for units, and are forbidden.
The default format is ‘%.8g’; for greater precision, you could specify ‘-o %.15g’. The ‘G’ and ‘g’ formats use exponential format whenever the exponent would be less than −5, so the value 0.000013 displays as ‘1.3e-005’. If you prefer fixed-point display, you might specify ‘-o %.8f’; however, very small numbers may display very few significant figures, and for very small numbers, may show nothing but zeros.
See the documentation for printf() for more detailed descriptions of the format specification.