Convert square metres to Tsubo (坪)

A unit derived from the room, not the ruler

The tsubo is one square ken, the ken being six shaku, and the shaku was fixed in 1891 as exactly 10/33 of a metre. That chain makes the tsubo an exact rational quantity rather than a rounded convenience: it is the square of 1.81818… metres. Traditional Japanese construction is modular on the ken, so rooms, mats and structural bays all resolve into whole tsubo.

This is why the unit refuses to die. A floor plan drawn on a 910 mm module — the modern metric rounding of half a ken — is still fundamentally a tsubo plan, and construction cost estimates are quoted per tsubo because the module is what determines material quantities.

Legal metrication and commercial reality

Japan's Measurement Act made metric units mandatory for official transactions in 1966, so registered land and building areas appear in square metres. Advertising and negotiation, however, continued in tsubo, and listings typically show both: the square-metre figure for the register and the tsubo figure for the buyer.

Appraisals and construction contracts follow the same pattern. A quoted tsubo-tanka — unit price per tsubo — is the standard comparator for construction cost, and comparing two builders means comparing their tsubo-tanka, not their totals.

Tatami sizing is regional

The "two tatami mats" definition is a useful mental image but not a constant. Kyoto-standard (Kyōma) mats are about 0.955 by 1.91 m; Nagoya (Chūkyōma) and Tokyo (Edoma) mats are progressively smaller, with the Edoma around 0.88 by 1.76 m. The tsubo itself does not change — it is fixed to the ken — but a room described as "six mats" occupies different areas in different regions.

So a listing that gives both a mat count and a tsubo figure is not redundant. The mat count describes the room's traditional proportion, the tsubo figure describes its area, and only the latter converts cleanly.

Related units and Korean parallels

Above the tsubo sits the tan (300 tsubo) and the chō (10 tan, about 9,917 m²) for agricultural land. Korea's pyeong and Taiwan's ping are the same size as the tsubo, all three descending from the same East Asian ken-based module, which makes cross-market property comparisons within the region unusually direct.

Korea formally banned pyeong in official transactions in 2007 and Japan restricts tsubo to non-official use, yet both units remain dominant in listings. When comparing regional property data, convert to square metres first: the units are equal in size, but the legal areas they describe may be measured to different standards.

Land, as distinct from buildings, is also traded per tsubo, and in dense urban districts the per-tsubo land price published in the annual government land-price survey is the reference point for negotiation. Those published figures are per square metre; converting them to tsubo is the routine first step for anyone reading the survey as a market participant rather than as an official.

Common mistakes to avoid

• Registered areas are in square metres; tsubo figures in listings are advertising, not the legal record.
• Tatami mat sizes vary by region — a "six-mat room" is not a fixed area, though the tsubo is.
• Construction quotes per tsubo (tsubo-tanka) may or may not include the same scope; compare inclusions, not just rates.
• Balconies and shared areas may be counted differently in the tsubo figure than in the registered square-metre figure.