A rai (Thai ไร่) is a unit of area, equal to 1,600 square metres (40 m × 40 m), used for measuring land area. 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 4 ngaan or 400 dtaaraangwaa ''aka'' square waa.

It is commonly used in Thailand and equals 16 are, another unit based on the metre and as commonly used in several countries recognized though not encouraged by the SI.

References

{{cite web

|publisher=Thailand Guru, a service of Export Quality Services Co., Ltd
|title=Units of Measurement
|url=http://www.thailandguru.com/infra-time.html#units
|accessdaymonth=15 April
|accessyear=2007}} (which uses the name 'waa' instead of 'dtaaraangwaa' for the 'square waa')

{{cite web

|publisher=Baan Thai Property Co. Ltd
|title=Information and advice when buying property in Thailand (Land Sizes)
|url=http://www.baanthaiproperty.com/info.php
|accessdaymonth=15 April
|accessyear=2007}} (with spelling variant 'data-raang waa' for 'dtaaraangwaa')

{{cite web

|publisher=www.PureThailand.com
|title=Thailand Information - Measurement
|url=http://www.purethailand.com/thailand_information/misc/measurement.htm
|accessdaymonth=15 April
|accessyear=2007}} (with spelling variants 'wah' and 'taraang wah' for 'waa' and 'dtaaraangwaa')