Convert metres to Cun (Chinese Inch)

The metrological cun

As a length, the cun is defined by its position in a decimal chain: ten fen to the cun, ten cun to the chi, ten chi to the zhang. Once the People's Republic fixed the market chi at one third of a metre, the cun became exactly one thirtieth of a metre, or 33.33 millimetres. No regional variant of the modern market cun exists.

It appears wherever small dimensions are described in traditional terms: joinery and furniture, the width of a fabric border, the diameter of a teapot, the dimensions of seals and calligraphy. Because it is close in size to the imperial inch, English translations routinely render it as "inch", which is convenient prose and a systematic four per cent error if the number is then used.

The proportional cun of acupuncture

In traditional Chinese medicine the cun — often written b-cun or "body cun" in English texts — is a proportional unit used to locate acupuncture points. It is defined against the patient: the width of the thumb at the knuckle is one cun, the combined width of the four fingers is three cun, and the distance from the elbow crease to the wrist crease is twelve cun regardless of the patient's actual size.

This is deliberate. Anatomical landmarks scale with the individual, so a fixed metric distance would mislocate points on a tall or short patient while a proportional unit does not. It also means that converting an acupuncture cun into millimetres is a category error: the correct answer depends on whose body is being measured, and any single number is wrong for almost everyone.

Telling the two apart

Context resolves it cleanly. A cun in a furniture specification, a fabric width or a classical description of an object is a length and converts metrically. A cun in a clinical text, a point-location table or an acupuncture chart is proportional and does not.

The confusion is common enough in translated medical literature that reputable sources now write "body cun" or "proportional cun" explicitly. Where a text does not, the tell is that the figure is being used to locate something on a person rather than to describe the size of an object.

Historical values

The cun inherits every historical variation of the chi, because it is defined as one tenth of it. A Han dynasty cun is about 2.31 centimetres; a Tang cun is around 3.1 centimetres. Pre-modern measurements described in cun therefore need the same dynastic sourcing as measurements in chi, and the same caution about false precision.

Hong Kong retains a traditional cun of 37.1475 millimetres, one tenth of its statutory chi, occasionally seen in older property and construction documents. As with the chi, jurisdiction must be established before a factor is applied.

Japanese and Korean texts add a further layer. The sun, one tenth of the Japanese shaku, is about 3.03 centimetres and is written with the same character the Chinese cun uses. Carpentry manuals, joinery specifications and antique furniture descriptions from Japan therefore look identical to Chinese ones on the page while carrying a value about nine per cent smaller. When a document is translated without its country of origin, that difference is invisible, so the safest habit is to record the source language and jurisdiction next to any figure in cun before converting it at all.

Common mistakes to avoid

• The acupuncture cun is proportional to the patient and has no fixed metric value.
• Translating cun as "inch" and then using the number introduces a systematic error of about four per cent.
• Historical cun values follow the dynastic chi and are as low as 2.3 cm in Han-era texts.

Questions people ask about the Cun (Chinese Inch)

How many centimetres is one cun?

In modern mainland China, exactly one thirtieth of a metre — 3.333 centimetres — because the cun is one tenth of the market chi of one third of a metre.

Why does acupuncture use a different cun?

Point locations must scale with the patient, so the medical cun is defined proportionally against body landmarks such as thumb width or the elbow-to-wrist distance. It deliberately has no fixed metric equivalent.

Is a cun the same as an inch?

No. A cun is 33.33 millimetres and an imperial inch is 25.4 millimetres. Translations often use "inch" loosely, which is a systematic error if the figure is then converted.