attolitre

An attolitre is a unit of volume in the metric system equal to 10-18 litres, or one quintillionth of a litre. In cubic centimetres, this equals 10-21 cm3. The unit sits at the extreme lower end of the SI scale, used only in specialized scientific and metrological work where extremely small fluid or liquid volumes must be quantified with formal precision.

The prefix atto derives from the Danish word atten, meaning eighteen, and denotes 10-18 in SI notation. It was formally adopted in 1964 alongside other extreme prefixes to support particle physics, quantum chemistry, and high-precision measurement. The attolitre itself remains primarily theoretical in practical metrology; actual laboratory work at such scales typically uses alternative language drawn from molecular or atomic contexts rather than named volume units.

The attolitre exists mainly in calibration standards, computational models of molecular systems, and academic literature on nanoscale phenomena. In industrial practice, metrologists and standards bodies maintain the term as part of the complete SI framework, even though direct measurement of attolitre quantities is beyond conventional instrumentation. When volumes at this scale arise in research, they are often expressed in scientific notation directly (10-18 L) rather than by invoking the unit name.

The British spelling attolitre and American attoliter are identical in meaning and value. Metrological institutions recognize both forms, though the choice depends on regional convention and organizational preference. Neither spelling is more correct or precise than the other.