Convert Rai (ไร่) to square metres

An exactly metricated traditional unit

Thailand metricated its traditional units in 1923 by defining them against the metre rather than replacing them. The wah became exactly 2 metres, so the square wah is exactly 4 square metres, the ngan is 100 square wah (400 m²), and the rai is 4 ngan (1,600 m²). Nothing in that chain is rounded.

This is why Thai land documents are unusually easy to work with: the traditional figure and the metric figure are both exact and convert without loss. A title deed area of "3 rai 2 ngan 45 square wah" is a precise 5,780 square metres.

Reading a chanote area

Land areas on Thai title deeds are written as a rai-ngan-wah triple, in that order, and must be read positionally like feet and inches. Converting the triple means multiplying each component by its own factor and summing: rai by 1,600, ngan by 400, square wah by 4.

The most common transcription failure is reading the third component as decimal fractions of a ngan. Because a ngan holds 100 square wah, a "45" in the third position is 180 square metres, not 45 per cent of a ngan, and the two readings differ by more than a factor of two.

How Thai property is priced

Agricultural and development land is quoted per rai; urban and residential plots are quoted per square wah, which is the granular unit that matters when a plot is small. Condominium units, by contrast, are sold in square metres, since they are governed by a separate legal regime.

A listing that gives a per-rai price for a small urban plot is usually converting upward from a per-wah price, and the rounding can be significant. For plots under one rai, ask for the per-square-wah figure and the exact triple from the deed.

Neighbouring units

The wah also exists as a linear unit of exactly 2 metres, used for frontage and for construction dimensions. A plot described as "20 wah frontage" is 40 metres wide; the same number in square wah would be 80 square metres. Thai documents rely on context to distinguish the two, so carry the dimension explicitly when transcribing.

Regionally, Cambodia and Laos use related but not identical traditional units, and Myanmar's land measures descend from a different tradition entirely. Do not apply the Thai rai to documents from neighbouring countries without checking the local definition.

Thai agricultural policy, land taxation bands and foreign-ownership restrictions on agricultural land are all expressed in rai, so the unit carries legal as well as commercial weight. Statistical comparisons with international agricultural data, which is almost universally per hectare, require the conversion: one hectare is 6.25 rai exactly, so a yield per rai is 6.25 times smaller than the same yield per hectare. Because that factor is an exact and memorable number, it is worth carrying explicitly in any spreadsheet that mixes Thai and international sources rather than embedding it in a formula where a later reader cannot see it.

Common mistakes to avoid

• Rai-ngan-wah triples are positional, not decimal: 1 ngan = 100 square wah, not 100 hundredths.
• The wah is both a linear unit (2 m) and, squared, an area unit (4 m²) — carry the dimension.
• Condominium areas are in square metres by law, not in wah.
• Neighbouring countries' units of similar name are not the same size.