Tsubo to Square Metres: How to Convert Japan's Area Unit

Converts tsubo to square metres. One tsubo is 400/121 ≒ 3.3058 m², the area of two tatami mats.

Explanation

The tsubo is the traditional Japanese unit of area, and it still runs the property market. Agents advertise in it and builders price in it, but the land registry, the drawings and the building permit are all metric. Converting between the two is a daily chore in Japan, and this calculator does it in one step.

1\ \text{tsubo} = \dfrac{400}{121}\ \text{m}^2 \approx 3.3058\ \text{m}^2

Write $T$ for the number of tsubo and $S$ for the area in square metres.

S = T \times \dfrac{400}{121}

The awkward fraction is a fossil of the old shakkanho measures. One tsubo is a square one ken (6 shaku) on a side, and Japanese law fixes the shaku at exactly 10/33 of a metre. Six shaku is 20/11 m, and squaring that gives 400/121 m². The decimal never terminates, so 3.3058 m² is a rounding rather than the definition.

Example

Take the default, 30 tsubo.

S = 30 \times \dfrac{400}{121} = \dfrac{12000}{121} = 99.17355\ldots

So 30 tsubo is about 99.17 m², or roughly 1,067 sq ft. A 30 tsubo house is an utterly ordinary Japanese family home. For a rough figure in your head, multiply by 3.3.

Notes

Tsubo is not a legal unit for transactions in Japan. Contracts, deeds and permit applications all state the area in square metres, with tsubo printed alongside as a convenience.
People say one tsubo is two tatami mats. That is the historical origin, but mats differ by region, and the 1.62 m² mat used in Japanese property advertising gives 3.24 m² for two, close to but not the same as 3.3058 m².
Always ask which area is being quoted. A 30 tsubo house normally means total floor area, while a 30 tsubo plot is land. Builders sometimes quote a third figure, the construction floor area, which is larger again.