Si () is a traditional Chinese unit of length. One si is equal to 1/10 of a hao, 1/100 000 of a chi, or 3+1⁄3 μm. In the field of engineering, one si equals 10 μm.https://en.wiktionary.org/wiki/絲
Present law on Chinese length units
This law of length measurement was issued by the Chinese government in 1929, and has been effective since 1 January 1930. The base unit chi is defined to be 1/3 meter.《中华人民共和国国务院公报》1959年第16号（总号：180）第316页《統一公制計量單位中文名称方案》（"Gazette of the State Council of the People's Republic of China" No. 16, 1959 (Total No.: 180) p. 316 "Proposal for Unifying the Chinese Names of Metric Measurement Units" ）
+ Table of Chinese length units effective since 1930PinyinCharacterRelative valueMetric valueImperial valueNotessī μmháo μmChinese millí (T) or (S) mm0.0131 inChinese calibrefēn mm0.1312 inChinese linecùn cm1.312 inChinese inchchǐ1 cm13.12 inChinese footzhàng10 m3.645 ydChinese yardyǐn100 m36.45 ydChinese chainlǐ1500500 m546.8 ydChinese mile.
Metric length units These units are based on the metric system. The Chinese word for metre is mǐ, which can take the Chinese standard SI prefixes (for "kilo-", "centi-", etc.). In the engineering field, traditional units are rounded up to metric units. 1981 Gazette of the State Council of the People's Republic of China , No. 365 , page 575, Table 7: SI prefixes
+ Table of Chinese length units in engineeringPinyinCharacterRelative valueMetric valueImperial value Noteshū1 μmAuthorized name: sī (T) or (S)10 μmAuthorized name: háo100 μmAuthorized name: (T) or (S)lí (T) or (S)1 mmAuthorized name: fēn10 mmAuthorized name: (T) or (S)cùn100 mmAuthorized name: chǐ11 mAuthorized name: Zhàng1010 mAuthorized name: yǐn100100 mAuthorized name: lǐ10001000 mthis li is not the small li above, which has a different character and tone
Compounds 絲毫 (sīháo) 一絲一毫 (yīsīyīháo) 一絲不苟 一絲 希望／一丝 希望 ― yī sī xīwàng ― a glimmer of hope.
See also Hao (length) Chinese units of measurement
References