Sign Convention

Rules for writing units are international scientific conventions used to record physical quantities clearly and consistently without causing confusion. Following these standard guidelines ensures that scientific measurements and research papers can be understood properly by scientists everywhere.

Writing names and symbols

• Full unit names must always begin with lowercase letters, even when they are named after famous scientists, such as newton, henry, ampere, and watt.
• Unit symbols named after scientists are written with capital letters, such as N for newton and A for ampere.
• Unit symbols that are not named after people must be written in lowercase letters, such as m for meter and kg for kilogram.
• Only officially accepted unit symbols may be used, so ampere must be written as a capital A rather than invented abbreviations like amp or amps.

Punctuation, spacing, and plurals

• Punctuation marks such as dots or commas must never be placed in the middle or at the end of unit symbols, meaning fifty meters cannot be written with a dot after the symbol.
• A clear space must always be kept between the numerical value and the unit symbol when writing any physical quantity.
• Units must never be written in plural forms, so fifty kilograms is written with the singular kg rather than kgs, because the letter s already represents the unit of seconds.

Temperature and division rules

When writing temperatures, the unit kelvin does not take a degree symbol, so a value is written simply as one hundred kelvin. In contrast, temperatures in Celsius and Fahrenheit must always include the degree symbol. When units are divided, a single slash may be used, but using more than one slash in a unit expression is incorrect. For example, a quantity like joule per kelvin per mole cannot use multiple slashes and must instead be written using negative powers.

Scientific notation and prefixes

Numerical values are written in scientific notation with powers of ten to avoid writing long rows of zeros for very large or very small measurements. For example, a density value like 13,600 kilograms per meter is written as 1.36 into 10 to the power of 4 kilograms per meter, and the speed of light is expressed as 3 into 10 to the power of 8 meters per second. Standard prefixes and symbols replace these powers, including peta for 10 to the power of 15, tera for 10 to the power of 12, giga for 10 to the power of 9, mega for 10 to the power of 6, kilo for 10 to the power of 3, hecto for 10 to the power of 2, and deca for 10 to the power of 1. Smaller numbers use negative power prefixes such as deci for 10 to the power of minus 1, centi for 10 to the power of minus 2, and milli for 10 to the power of minus 3.