A decametre (American spelling: decameter, symbol dam) is a very rarely used unit of length in the metric system, equal to ten metres, the current SI base unit of length. It can be written in scientific notation as (exponential notation) — meaning .

nanometre <<< micrometre <<< millimetre < centimetre < decimetre < metre < decametre < hectometre < kilometre

This measure is included mostly for completeness. One of the few practical uses of the decameter is for altitude of geopotential heights (circumscribing equal pressure) in meteorology. Interestingly, meteorologists also use another seldom encountered SI prefix: hecto- in hectopascal (hPa).

For surface the square decametre (dam²) is a common unit, be it by the name are (a). The are is a measurement of area the size of 1 decametre by 1 decametre — the square decametre — 10 metres by 10 metres, equalling 100 square metres (100 m²).

For volumes the cubic decametre (dam³) is also used, 10 m by 10 m by 10 m equalling 1,000 cubic metres (1,000 m³).