Rai, ngan, and tarang wa

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]

"Ngan" redirects here. For other uses, see Ngan (disambiguation).

The tarang wa (square wa, tarang meaning 'grid') is derived from the area of a square with sides of 1 wa (the Thai fathom). 1 ngan ('work') is equal to 100 square wa, and 1 rai ('field' or 'plantation') equals 4 ngan or 1 square sen.^[1] The units were standardised 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]

Unit conversions

Unit	Derivation	Relative	SI (or other metric)	US or UK customary
Tarang wa	Square wa		4 m2	43 sq ft
Ngan		100 tarang wa	400 m2	4,306 sq ft
Rai	Square sen	4 ngan or 400 tarang wa	1,600 m2 (16 ares)	17,222 sq ft (0.4 acres)

Notes

1. Thai: ไร่, pronounced [râj].
2. Also spelled ngaan; งาน [ŋɑ̄ːn].
3. Also tarang wah or square wah; ตารางวา [tɑ̄ː.rɑ̄ːŋ wɑ̄ː].