The ''rai'', ''ngan'', and ''tarang wa'' or square ''wa'' are customary
Thai units of
area
Area is the measure of a region's size on a surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an open surface or the boundary of a three-di ...
, used in the measurement of land. They are defined as exactly 1,600, 400, and 4
square metres
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter ( American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...
, respectively (17,222, 4,306, and 43 sq ft).
The ''tarang wa'' (square ''wa'', ''tarang'' meaning 'grid') is derived from the area of a square with sides of 1 ''
wa'' (the Thai
fathom
A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. The fathom is neither an international standard (SI) unit, nor an internationally accepted non-SI unit. H ...
). 1 ''ngan'' ('work') is equal to 100 square ''wa'', and 1 ''rai'' ('field' or 'plantation') equals 4 ''ngan'' or 1 square ''
sen''.
The units were standardised in square metres when Thailand (then Siam) adopted the
metric system
The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
in 1923, although the
Royal Survey Department was already reported in 1908 to be using the metre-based conversion for its
cadastral maps.
The units are commonly used for cadastre and property matters, and official and legal documents express areas of land in such units. They are sometimes notated in the abbreviated format ''rai''-''ngan''-''tarang wa'', e.g. "4-2-25 ''rai''", which means "4 ''rai'', 2 ''ngan'', and 25 ''tarang wa''", though this is discouraged by some government documents.
Notes
References
{{reflist
Thai units of measurement
Units of area