A rai (Thai: ไร่, [râj]) is a unit of area, equal to 1 square sen or 1,600 square metres (40 m × 40 m), and is used in measuring land area for a cadastre or cadastral map. Its current size is precisely derived from the metre, but is neither part of nor recognized by the modern metric system, the International System (SI).
The rai equals four ngaan or 400 square wa. The rai equals 0.3954 acres.
It is commonly used in Thailand and equals 16 ares, another unit that is based on the metre and commonly used in several countries. Although recognized by the SI, its use is not encouraged. The word rai also means plantation.
References
Rai (unit) Wikipedia(Text) CC BY-SA