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The Chi (尺) is a traditional Chinese unit of length, often translated as the "Chinese foot". It served as a fundamental base unit within the traditional Chinese system of weights and measures (度量衡, dùliànghéng).

The Chi is traditionally subdivided and multiplied as follows:

• 1 Zhang (丈) = 10 Chi
• 1 Chi (尺) = 10 Cun (寸 - Chinese inch)
• 1 Cun (寸) = 10 Fen (分 - Chinese line)

While sometimes called the "Chinese foot", its length has historically varied significantly and is generally longer than the imperial foot. The modern standardized value is exactly 1/3 of a meter.

The Chi has been used in China for millennia, with its length undergoing numerous changes across different dynasties and regions. Archaeological evidence suggests its use dates back to the Shang Dynasty (c. 1600–1046 BC).

• Early Dynasties: Lengths varied, often based on standards set by the ruling government. For example, the Chi during the Han Dynasty (206 BC – 220 AD) was around 23.1 cm, while during the Tang Dynasty (618–907 AD), it was closer to 30.7 cm.
• Later Dynasties: Variations continued. During the Qing Dynasty (1644–1912), different standards existed for different trades, such as a gongchéng chi (工尺) for engineering (approx. 32 cm) and a cáiféng chi (裁縫尺) for tailoring (approx. 35.5 cm).
• 20th Century Standardization: In the Republic of China era (1912–1949), efforts were made to standardize traditional units with the metric system. In 1929, the Chi was officially defined as exactly 1/3 of a meter (33.33... cm). This standard was largely retained by the People's Republic of China after 1949.

Unlike some Western units (like the foot or digit), the Chi's connection to direct anthropomorphic measurement (like the length of a forearm or foot) is less clear and likely became conventionalized early in its history.

Historically, the Chi was ubiquitous in Chinese society. Its modern usage is less common but persists in specific areas:

• Historical Measurement: Essential for understanding dimensions in ancient and imperial Chinese architecture, engineering projects (like canals and walls), land surveying (often in conjunction with the Bu 步 and Li 里), and everyday objects.
• Textile Measurement: Used for measuring cloth lengths, similar to the Western ell or yard. The tailor's chi (裁縫尺) was specifically used for this.
• Traditional Crafts: Still used in some traditional arts and crafts, such as calligraphy, painting, furniture making, and seal carving, where traditional proportions are valued.
• Traditional Chinese Medicine (TCM): The cun (1/10th of a chi) is used as a proportional body measurement unit (known as the body inch) to locate acupuncture points. The patient's own body proportions determine the length of their cun.
• Cultural Context: Appears frequently in historical texts, literature, idioms (e.g., 得寸進尺, dé cùn jìn chǐ - "gain an inch, advance a foot," meaning to become greedy), and cultural practices.
• Informal Measurement: In some contexts, particularly among older generations or in rural areas, informal measurements might still reference the chi or cun.