Convert feet to Ken (Japanese)

A module, not just a length

A ken is six shaku, and with the shaku fixed at exactly 10/33 m the ken is 20/11 m — 1.81818 m. Ten ken make a chō of 109.09 m, the unit that sets the Japanese urban grid, and the same word gives the chō of area (about 0.9917 ha) used for farmland.

What makes the ken different from an ordinary length unit is that Japanese construction treats it as a spacing rule. A room is described by how many ken across it is, and everything inside — mats, doors, ceiling panels — is sized to fit that grid.

Why Japanese rooms come in mat counts

A tatami mat is half a ken by one ken, so a square ken holds two mats and is one tsubo. Rooms are then described as four-and-a-half, six or eight mats, which is simultaneously a size, a proportion and a construction specification.

Because the mat derives from the module rather than the other way round, changing the module changes the mat. Kyōma (Kansai) mats are larger than Edoma (Kantō) mats because the two traditions measured the ken between different points on the columns — inside face versus centre. Both are "one ken"; the rooms differ by several percent.

The module in modern building

Contemporary Japanese housing still uses a 910 mm half-ken grid, which is why Japanese plywood and plasterboard come in 910 × 1820 mm sheets rather than the 1200 × 2400 mm standard used elsewhere. The traditional module survives inside an entirely industrialised supply chain.

For imports and renovations this matters practically: foreign-standard sheet goods do not land on Japanese stud spacing, and fittings sized to a metric grid will not align with an existing ken-based frame.

Working with ken dimensions

Convert using the exact 20/11 m rather than 1.82 m; across a ten-ken span the rounding error is over a centimetre, enough to matter where new work meets old timber.

When reading a historical or regional drawing, establish whether the ken is measured centre-to-centre or face-to-face before computing room areas. That single convention explains most of the discrepancies between nominal and measured sizes in Japanese buildings.

Module of Japanese building

The ken of about 1.818 metres — six shaku — is the module Japanese traditional architecture is laid out on. Column spacing, sliding door widths and tatami dimensions are all derived from it, which is why Japanese rooms are sized in tatami count rather than in square metres and why the conversion between the two is regionally variable.

Two ken standards persist. The Kyoto-ma of the Kansai region uses a larger inter-column spacing than the Edo-ma of the Kantō region, so a six-mat room in Kyoto is measurably larger than a six-mat room in Tokyo. A room described in mats therefore converts to a range rather than a single area unless the regional standard is known, and property listings that quote both mats and square metres are stating the resolution explicitly.

The ken also appears as a unit of frontage in older land and tax records, where a shop's street width was recorded in ken while its depth ran back an unrecorded distance. Converting such a frontage figure gives a real measurement, but it describes only one edge of the property and should never be squared into an area.

Common mistakes to avoid

• Kyōma and Edoma measure the ken differently (face-to-face versus centre-to-centre), so identical mat counts give different room areas.
• Rounding the ken to 1.82 m accumulates over a building span; use the exact 20/11 m.
• Japanese sheet materials follow the 910 mm half-ken grid — metric-standard sheets do not align with a ken-based frame.

Questions people ask about the Ken (Japanese)

How long is one ken in metres?

Exactly 20/11 m, about 1.81818 m — six shaku of 10/33 m each.

How does the ken relate to tatami and tsubo?

A tatami mat is half a ken by one ken. A square ken is one tsubo (about 3.3058 m²) and holds two mats, which is why Japanese rooms are described in mat counts.

Why is Japanese plywood 910 × 1820 mm?

Because Japanese framing uses a 910 mm half-ken grid inherited from traditional carpentry, so sheet goods are sized to land on it.