zettameter

A zettameter (symbol Zm) is a unit of length in the metric system, equal to 10^21 meters. It is part of the SI prefixes, with zetta representing 10^21 and followed by yotta at 10^24.

In terms of other units, 1 Zm equals 1 × 10^21 meters, or 1 × 10^18 kilometers. It corresponds to about 6.68 × 10^9 astronomical units, and about 32,400 parsecs (roughly 32.4 kiloparsecs). In light-years, 1 Zm is approximately 105,700 ly. This places a zettameter near the size of the Milky Way, whose diameter is around 30 kiloparsecs.

Usage and context: While zettameter is a valid SI unit, it is rarely used in routine measurements. Astronomical distances are typically expressed in parsecs, kiloparsecs, megaparsecs, or light-years. For cosmological scales, larger prefixes such as gigameters or units like parsecs per megaparsec are more common. The observable universe, at about 4.4 × 10^26 meters in radius, corresponds to roughly 4.4 × 10^5 Zm, illustrating the scale where such a prefix becomes conceptually convenient.

See also: related astronomical distance units such as parsec, light-year, astronomical unit, and the broader SI prefix sequence.