Converts between the traditional sun, shaku and ken and the metre. One shaku is defined by Japanese law as exactly 10/33 m; a sun is a tenth of that and a ken is six times it. Timber houses are laid out in multiples of 910 mm and 1820 mm because those are the half-ken and the ken.
Japanese timber construction is still dimensioned in traditional units. Drawings may be marked in metres, but the spacing of posts and the sizes of fittings trace back to multiples of the shaku.
Japanese law defines one shaku as exactly 10/33 m. That fraction is not an approximation; it is the definition, chosen on adopting the metric system as the neatest fraction close to the shaku then in use.
A sun is a tenth of a shaku and a ken is six shaku. A bu is a tenth of a sun again, readable here as the decimal part of the sun figure.
The default input is one ken. That is 6 shaku, or 60 sun, about 1.8182 m, or about 1818.18 mm.
Entering half a ken gives about 909.09 mm. The 910 mm that appears everywhere on Japanese house plans is that half-ken rounded up. Post spacing, the short side of a tatami mat and the width of a plywood sheet all come in 910 mm and 1820 mm steps because the traditional units underlie them.
Going the other way, one metre comes out as exactly 3.3 shaku. Since the definition is 10/33, its reciprocal of 3.3 is exact.
Laying out on 910 mm is called the shaku module; laying out on 1000 mm is the metre module.
The metre module makes corridors and stairs about ten percent wider, which matters for wheelchair access. The same floor plan then produces a larger building, which costs both site area and money. When reading the plans of an existing house, establishing which module it uses makes all the dimensions fall into place.
These are the kanejaku used in construction. The kujirajaku used for kimono is a different unit entirely, 1.25 times as long at 25/66 m. Confusing them puts every dimension out by 25 percent.
The tsubo and the tatami are units of area, not length, and are not handled here. Separate tools cover them.