单位换算是指通过乘以换算系数实现不同计量单位间的等量换算。
单位换算过程与用户具体情况和预期目的相关。可能受法令、合同、技术规格或其它出版物技术标准的限制。工程判断可能包括以下因素:
某些从一个单位系统需要不增减精度地向另一个单位体系严密转换第一个量度。有时被称为“软转换”。它不涉及更改被计量的物理配置。
与之相比，“硬转换”或“适应转换”可能不完全相等。它更改计量以适合在新体系内的有效数字与单位。它有时涉及项目的稍微不同的配置或尺寸替代。有时允许并使用标称值。
此条目内不列出在公制单位国际单位制词头间的转换（例如：1 千克 = 1000 克, 1 毫克 = 0.001 克）。除非单位还有其他常见名称。
各表内的单位以其英文字词的字母顺序列出，突出显示SI单位（基本单位或导出单位）。
此条目给出列于索引内的各个物理量数值转换系数列表。各物理量不同单位的数值以相应的SI术语表示并显示（某些仅有历史意义）。
Notes:
A velocity consists of a speed combined with a direction; the speed part of the velocity takes units of speed.
See also: Conversion between weight (force) and mass
Often, information entropy is measured in shannons, whereas the (discrete) storage space of digital devices is measured in bits. Thus, uncompressed redundant data occupy more than one bit of storage per shannon of information entropy. The multiples of a bit listed above are usually used with this meaning. Other times the bit is used as a measure of information entropy and is thus a synonym of shannon.
The candela is the preferred nomenclature for the SI unit.
Please note that although becquerel (Bq) and hertz (Hz) both ultimately refer to the same SI base unit (s−1), Hz is used only for periodic phenomena, and Bq is only used for stochastic processes associated with radioactivity.
The roentgen is not an SI unit and the NIST strongly discourages its continued use.
Although the definitions for sievert (Sv) and gray (Gy) would seem to indicate that they measure the same quantities, this is not the case. The effect of receiving a certain dose of radiation (given as Gy) is variable and depends on many factors, thus a new unit was needed to denote the biological effectiveness of that dose on the body; this is known as the equivalent dose and is shown in Sv. The general relationship between absorbed dose and equivalent dose can be represented as
where H is the equivalent dose, D is the absorbed dose, and Q is a dimensionless quality factor. Thus, for any quantity of D measured in Gy, the numerical value for H measured in Sv may be different.
Home and office computers come with converters in bundled spreadsheet applications or can access free converters via the Internet. Units and measurements can be easily converted using these tools, but only if the units are explicitly defined and the conversion is compatible (e.g., cmHg to kPa).
When a measure is converted to another unit in a fiction work like a novel or film, the value often loses meaning (as in numerology) or gets too many significant digits ("40 miles" turns into "64 km"). For this reason, authors sometimes choose numbers that can be converted easily. Most notably, distances measured in miles often begin with 3 or 5, so the value ghjcfugffyhfdfgjvcgconverted to km also has one significant digit (5 or 8).