Thai Land Units Explained: Rai, Ngan, Square Wah and Western Units
Thailand uses a traditional land-measurement system alongside modern metric units. This guide explains where Rai, Ngan and Square Wah come from, how the hierarchy works today, and how to translate Thai land areas into square meters, hectares, acres and square feet.
Quick legend: the Thai land-unit system
The system is hierarchical: a small unit builds into the next larger unit. The metric values below are exact under Thailand's modern measurement law.
How the Thai land units developed
Rai, Ngan and Square Wah are customary Thai units that predate Thailand's modern metric framework. Their structure reflects older Thai measures of length and area rather than the decimal metric ladder.
The Wa is a traditional Thai length measure. Square Wah is the area of a square one Wa on each side.
100 Square Wah make 1 Ngan, and 4 Ngan make 1 Rai. This creates a practical mixed-unit system for plots of different sizes.
Thailand adopted metric measurement law in the twentieth century. The current Measurement Act keeps Rai, Ngan and Square Wah as recognised units with exact square-meter values.
The 1999 Thai Measurement Act explicitly defines 1 Rai as 1,600 m², 1 Ngan as 400 m² and 1 Square Wah as 4 m². The Act also repealed and replaced the earlier Measurement Act of 1923, which was part of Thailand's move toward metric standardisation.
Why Thailand still uses Rai, Ngan and Square Wah
Metric units are standard for science, engineering and international comparison, but Thai land practice still commonly expresses plot size in Rai, Ngan and Square Wah. The Department of Lands uses the three fields together in land-area terminology and official land-related forms.
That is why a property listing, survey discussion or title-deed reference may show a mixed area such as 2 Rai 1 Ngan 50 Square Wah while a building plan or an international listing may use square meters.
Thai units compared with Western and metric units
| Thai / international unit | Square meters | Rai | Useful comparison |
|---|---|---|---|
| 1 Square Wah | 4 m² | 0.0025 Rai | ≈ 43.0556 ft² |
| 1 Ngan | 400 m² | 0.25 Rai | ≈ 0.098842 acre |
| 1 Rai | 1,600 m² | 1 Rai | 0.16 ha · ≈ 0.395369 acre |
| 1 hectare | 10,000 m² | 6.25 Rai | ≈ 2.47105 acres |
| 1 acre | 4,046.8564224 m² | ≈ 2.529285 Rai | ≈ 10.11714 Ngan |
| 1 km² | 1,000,000 m² | 625 Rai | 100 hectares |
Rounded international comparisons are for practical reference. ThailandCalc performs calculations from square meters as the common base unit.
Worked example: 2 Rai 1 Ngan 50 Square Wah
A mixed Thai land measurement is not a decimal. Convert each component separately:
- 2 Rai = 3,200 m²
- 1 Ngan = 400 m²
- 50 Square Wah = 200 m²
Total: 3,800 m². That is also 2.375 Rai, 950 Square Wah, 0.38 hectare, or approximately 0.939 acre.
For more examples of mixed notation, see How to read Rai–Ngan–Square Wah.
Which unit should you use?
| Situation | Usually easiest unit |
|---|---|
| Thai land documents or local property listings | Rai, Ngan and Square Wah |
| Small residential plots | Square Wah or m² |
| Building plans and dimensions | m² |
| Large farms or international metric comparison | Rai or hectare |
| Comparing with US/UK-style listings | Acre or square foot, then convert back to Rai/m² |
Common mistakes when converting Thai land units
- Wah is not Square Wah. Wah measures length; Square Wah measures area.
- Mixed notation is not decimal notation. 1 Rai 2 Ngan equals 1.5 Rai, not 1.2 Rai.
- 1 Rai is not 1 acre. 1 Rai is 1,600 m², while 1 acre is 4,046.8564224 m².
- Do not compare asking prices before normalising the area unit. Convert both plots to the same unit first.
- A unit conversion is not a boundary or ownership check. Legal area, boundaries and title rights should be verified from official documents and qualified professionals.
Sources and measurement standard
ThailandCalc uses the exact Thai metric relationships defined in Thailand's measurement law and cross-checks land terminology against Department of Lands material.
- Thailand Measurement Act B.E. 2542 (1999), Department of Internal Trade — defines Rai = 1,600 m², Ngan = 400 m² and Square Wah = 4 m².
- Department of Lands terminology — shows land area expressed as Rai, Ngan and Square Wah in official land terminology.
ThailandCalc is an independent conversion tool and is not a government website.