A rai (Thai: ไร่, pronounced [râj]) is a unit of area equal to 1,600 square metres (16 ares, 0.16 hectares, 0.3954 acres), 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).
- Giblin, R. W. (2008) . "Chapter Royal Survey Work.". In Wright, Arnold; Breakspear, Oliver T (eds.). Twentieth century impressions of Siam (PDF). London: Lloyds Greater Britain Publishing Company. Retrieved January 28, 2012.
It so happens that 40 metres or 4,000 centimetres are equal to one sen, so all cadastral plans are plotted, drawn, and printed to a scale of 1:4,000.
- Area metric conversion, British and U.S., Japanese, Chinese, Thai, old French, others.