How is land measured in Thailand?

In Thailand Land is measured in Rai, Ngan and Wah. Here is a rough conversion rate.

1 Rai = 4 Ngan or 400 Wah (or 1600 Sqm.)17109 square feet
1 Ngan = 100 Wah (or 400 Sqm. ) 4277 square feet
1 Wah = 4 Sqm. 42.77 square feet

Imperial conversion
1 Acre = 2.5 Rai (approx.)

Metric Conversion
1 Hectare = 6.25 Rai (approx.)

The land prices in the city is often quoted in Baht per Tanwang Wah as it is so small a lots.

Where in the country it is calculated in Price per Rai. If it is per Wah then maybe it is a little over priced. Thai farmer are often good traders.