Generally speaking, the metric system is commonly used in Thailand for weights and measurements. However, especially in the case of surface measurements (only seldom with linear measurements) the old traditional Thai units of measure are still being used. These are in a symmetric correlation to western measures and can easily be converted.
On title deeds (landownership documents) the sizes of areas are exclusively expressed in Thai units of measure. Here we offer a little help to make the conversion easier for you:
Relaxed and low tech: measurements for a land survey in Thailand`s countryside.