Convert inches to Shaku (Japanese)

Ten thirty-thirds of a metre

When Japan reconciled the shakkanhō system with the metre, the shaku was defined as exactly 10/33 m. The odd-looking fraction was chosen because it matched the existing standard closely enough to leave the building trades undisturbed, and it makes every derived unit exact: the sun is one-tenth of a shaku (30.303 mm), the ken is six shaku (1.81818 m), and the jō is ten shaku.

Because the conversion is a rational fraction rather than a rounded decimal, Japanese traditional dimensions convert without accumulated error — an advantage over most historical foot units, which survive only as measured approximations.

The module that shapes Japanese buildings

Japanese residential construction is modular, and the module is the ken. Column spacing, tatami sizes, fusuma and shōji openings and standard plywood sheets all derive from it, which is why Japanese room sizes are quoted in tatami mats rather than square metres in everyday speech.

Regional mat standards differ — Kyōma in the Kansai region is larger than Edoma in Kantō — because the module was applied differently: some regions set the ken between column centres and others between column faces. The shaku is exact; the room built from it is not standard nationwide.

Where the shaku still appears

Carpentry squares (sashigane), joinery drawings, shrine and temple restoration specifications, and the sizing of traditional tools and instruments all use shaku and sun. Renovation of an older Japanese house is essentially impossible without them, because the existing fabric was cut to that module.

A separate kujira-jaku, the "whale shaku" used in textiles at 25/66 m (about 378.8 mm), coexisted with the carpenter's shaku. Kimono patterns and older cloth measurements may use it, and confusing the two overstates a length by a quarter.

Converting for restoration and design

Work in the exact fraction rather than 0.303 m. Over the span of a traditional building the rounding difference reaches several millimetres, which matters when new joinery has to meet century-old timber.

When a drawing gives dimensions in shaku and sun, convert the whole dimension string at once and keep the original notation on the drawing. Japanese conservation practice expects both, and a metric-only drawing loses the modular logic that explains why the dimensions are what they are.

The 10/33 metre definition

The shaku was fixed at exactly 10/33 of a metre — 30.303 millimetres short of a foot — during the Meiji standardisation, which is why every Japanese traditional length converts exactly today. The fraction looks arbitrary but it was chosen to preserve the existing shaku as closely as possible while tying it to the metre.

Two shaku variants matter in practice. The kanejaku, the carpenter's shaku, is the standard 10/33 metre value used in construction and in tatami and timber sizing. The kujirajaku, the whale-scale shaku used in textiles, is one and a quarter times longer at about 37.9 centimetres, and kimono measurements written in shaku follow it. Converting a garment specification with the carpenter's value shortens every dimension by twenty percent.

Sub-divisions follow the same decimal logic: ten sun make a shaku and ten bu make a sun, so a timber dimension written in shaku-sun-bu converts by simple place value once the correct shaku is identified. Japanese lumber yards still stock stock sizes on this grid, which is why imported metric timber often needs re-sawing to fit a traditional frame.

Common mistakes to avoid

• The textile kujira-jaku (about 378.8 mm) is not the carpenter's shaku (303.03 mm) — check which trade the document comes from.
• Tatami sizes vary by region (Kyōma, Chūkyōma, Edoma); "six mats" is not a fixed area.
• Rounding the shaku to 0.303 m accumulates visible error across a building; use the exact 10/33 fraction.

Questions people ask about the Shaku (Japanese)

How long is one shaku in metres?

Exactly 10/33 m, which is 0.30303… m or about 303.03 mm — very close to, but not equal to, an imperial foot.

What is a ken and how does it relate to the shaku?

A ken is six shaku, about 1.818 m, and it is the module Japanese traditional buildings are laid out on — column spacing, tatami and sliding-door openings all follow it.

Is the shaku legal for trade in Japan?

No. The Measurement Act removed the shakkanhō units from legal commercial use, but they remain in daily use in carpentry, restoration and traditional crafts.