The rai,^[a] ngan,^[b] and tarang wa or square wa^[c] are customary Thai units of area, used in the measurement of land. They are defined as exactly 1,600, 400, and 4 square metres, respectively (17,222, 4,306, and 43 sq ft).^[1]

The tarang wa (square wa, tarang meaning 'grid') is derived from the area of a square with sides of 1 wa (the Thai fathom). One ngan ('work') is equal to 100 square wa, and one rai ('field' or 'plantation') equals 4 ngan or 1 square sen.^[1] The units were standardized in square metres when Thailand (then Siam) adopted the metric system in 1923,^[2][3] although the Royal Survey Department was already reported in 1908 to be using the metre-based conversion for its cadastral maps.^[4]

The units are commonly used for cadastre and property matters, and official and legal documents express areas of land in such units.^[5] 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.^[6]