The International System of Units (SI), commonly known as the metric system, is a logical, universal, and precise way to express physical quantities. But sloppy usage and unawareness of SI rules threaten to cause confusion and destroy the uniformity of the system.

Here is a list of common mistakes in using the metric system, with examples of incorrect usage and how to make them correct. Each subtopic on the left column is a link to a longer explanation at the bottom of the page.

In addition to the mandatory rules above, the following table contains common usages that are not ideal. They may be popular enough to resist change or they are only minor deviations, but should be improved on when there is an opportunity to do so.

Ad hoc abbreviations (mandatory)

Don’t make up your own abbreviations for units. Metric already defines short, reasonable abbreviations, and introducing non-standard ones brings unnecessary confusion. The most common violation is sec for second, which should be s. Also, never put a period after a metric symbol unless it comes at the end of a sentence.

Plural of symbols (mandatory)

For a unit symbol (abbreviation), never add an s to the end of it. Unit symbols do not change spelling to indicate plural. Furthermore, s already means second, and juxtaposition means multiplication (for example, N s means newton times second). However, full unit names have their own set of rules for pluralization (e.g. metre vs. metres).

Capitalization of symbols (mandatory)

Metric notation is case-sensitive. For example, the prefix M means mega (106) and the prefix m means milli- (10−3); the prefix k means kilo- and the unit K means kelvin; the unit s means second and the unit S means siemens. Forsaking the capitalization destroys useful distinctions in the metric system.

Capitalization of unit names (mandatory)

When spelled out in full, all prefix names and all unit names are in lowercase (e.g. millivolt, kiloohm, megapascal), except that degree-person must capitalize the person’s name (e.g. degree Celsius, degrees Fahrenheit). This point is a matter of convention; it won’t cause ambiguity if disobeyed.

Degree sign/word (mandatory)

The units of degree Celsius, degree Fahrenheit, and degree Rankine always contain the word degree(s) (but dropped in informal contexts). The unit kelvin must not be written with the word degree (although it did in the past). Regarding plain unit symbols without °, C means coulomb and F means farad – hence including the degree sign is critical to indicate degrees Celsius.

p standing for per (mandatory)

Unlike the imperial system which uses p to denote per (e.g. mph , mpg , APM ), p stands for the pico- prefix in SI, and the role of per is covered by division and negative powers. For example, kilograms per litre can be expressed as kg/L or kg L−1.

Power-of-1024 prefixes (mandatory)

SI prefixes always mean the same multiplier in every context. Kilo- is always 1000, even when applied to bytes. Treating kilo- as 1024 is an abuse of notation perpetuated by the computer industry, which ended up harming and confusing consumers everywhere (e.g. Why does my 1000 GB HDD show up as “909 GB” in Windows?). Always use SI prefixes to denote powers of 1000 (e.g. 1 MB = 1000000 bytes), and instead use IEC binary prefixes to denote powers of 1024 (e.g. 1 MiB = 1048576 bytes).

Multiple prefixes (mandatory)

Never use multiple prefixes for metric quantities. Either write out the full number, or adjust the single prefix to equal the product of multiple prefixes.

Bare prefix (mandatory)

Never write a quantity with a prefix but no unit. It is unacceptable to imply a unit (e.g. She drove 30 k vs. She drove 30 km).

Mass/weight distinction (mandatory)

Grams (g) and prefixes thereof are measures of mass, not weight or force. Newtons (N) measure weight and force. This distinction is made clear when you consider that if you take a trip up to the International Space Station, your body mass in kilograms will stay the same, but your weight in newtons will be zero (weightlessness in orbit).

Deprecated units (mandatory)

The standard symbol for cubic centimetre is cm3, not the commonly used cc. Moreover, a millilitre (mL) is exactly equal to a cubic centimetre and is more appropriate for indicating volumes. Micron (μ) is the old way of expressing micrometre (μm). The unit’s name and symbol both break the spelling pattern and is an unwanted wart. An ångström equals 0.1 nanometre, and is needless field-specific jargon (particularly in optical physics) that can be easily converted to nanometres by dividing 10. Don’t use units like these.

Substitutions for Greek mu (mandatory)

Almost all SI prefixes, SI units, and non-SI units are written in Latin letters, with the exception of the prefix micro (μ), the unit ohm (Ω), and the unit ångström (Å). Typing and transmitting non-Latin characters can pose technical difficulties, and sometimes writers work around this in haste by substituting μ with u or mc. This can be okay for informal private communications, but is unacceptable for published works with wide dissemination.

Spurious conversion precision (mandatory)

Unit conversions, such as from imperial to metric, should follow the rules for significant figures. Just because the definition of a unit has numerous digits doesn’t mean you need to keep that many digits after a calculation. Having too many significant figures in metric numbers makes them harder to understand and reduces their chances for wide acceptance. For example, a road speed limit posted as 40 mph is equal to exactly 64.37376 km/h by definition, but should be written as 65 km/h because the precision is superfluous in such a context. The long number makes the metric system look arbitrary and incomprehensible.

Additive mixed units (mandatory)

In other systems it is customary to express quantities in mixed units, for example “4 foot 9 3/ 8 inches” or “11 stone, 2 lb, and 5 oz”. However, the metric system is decimal-based and does not need this kind of phrasing. Splitting the units makes no improvement to clarity, but adds needless work when handling numbers in calculations.

Multiple quantities (mandatory)

When more than one physical quantity participates in a calculation (e.g. addition, multiplication), every quantity must have its own units, or appropriate parentheses must be used. If we say that a film frame has the dimensions “36 × 24 mm”, this arithmetic expression evaluates to 864 mm, a quantity which makes no sense in this context. Instead, we say that it has dimensions “36 mm × 24 mm”, which correctly evaluates to its area of 864 mm2.

Space between number and unit (suggested)

For readability, there should be a space between the number and the unit(s). This is especially important for numbers with decimal places and compound units, e.g. 4.567 N·m2·s−2. Also, the space should be non-breaking (U+A0), to avoid splitting the number and units on different lines. However, there is no space before the angle units of degrees, minutes, and seconds (e.g. 12° 34′ 56″).

Capital L for litre (suggested)

The lowercase letter l looks like the number 1 in many fonts. As such, the unit symbol for litre should be written with an uppercase L. There are other writers and manufacturers who use script lowercase ℓ, but it seems to be non-standard.

Pronunciation of kilometre (suggested)

The English word kilometre should be pronounced with stress on the first syllable, not the popular way where stress is on the second syllable. The former style matches the pattern of how kilo- is pronounced in front of every other unit, and also respects the fact that every other prefix in front of metre is also pronounced with stress on the first syllable. The latter style is self-inconsistent – why not extend it to pronounce centimetre as [sɛn.ˈtɪ.mɪ.tɚ], millimetre as [mə.ˈlɪ.mɪ.tɚ], et cetera? One danger of the extension is that the word micrometre means the unit when stress is on the first syllable, and the measurement device when stress is on the second syllable. Hence, other than popularity, it makes no sense whatsoever to pronounce kilometre with stress on the second syllable.

Avoid semi-SI units (suggested)

Just putting a prefix like kilo- or milli- in front of a non-SI unit doesn’t make it compatible with SI. This gives the unit pseudo-legitimacy like SI units, but adds another unit for a quantity that can already be expressed by SI units. For example, a kiloparsec is a measure of distance that should be expressed in metres instead; a megaelectronvolt is a unit of energy that should be expressed in joules instead.

Avoid centi/deca/etc. (suggested)

Most SI prefixes are based on powers of 1000, except for centi-, deci-, deca-, hecto-, myria-, and dimi-. Out of these six oddball prefixes, centi- is by far the most popular, essentially only used in the unit centimetre. These prefixes, spaced apart by multiples of 10 instead of 1000, are too near each other and can increase cognitive load by creating too many subunits. For example, in a typical supermarket you can buy various liquid goods that are between 1 to 10 millilitres, centilitres, decilitres, and litres. It is arguably easier to have a sharp divide between quantities expressed in millilitres and litres.

Avoid common non-SI units (suggested)

Some units like hour (3600 seconds) are not in SI but are commonly used anyway (e.g. km/h). For everyday purposes like driving, it is better to keep this customary usage. For serious science and engineering, SI units like metres per second should be used instead. Also, some non-SI units are related to SI units by some power of 10, for example 1 bar = 100000 pascals. These units create needless jargon for quantities that are already covered by SI units.

Short/long scale to prefixes (suggested)

In some languages and countries, big number words like billion and trillion can have unexpected or disputed meanings. SI prefixes like tera-, peta-, etc. always mean the same number everywhere. Leverage this fact to cut down on ambiguity and confusion.

Scientific temperatures in kelvins (suggested)

Measurements and calculations for scientific and engineering purposes should be expressed in kelvins. For example, working with cryogenic technology near absolute zero will yield small numbers on the kelvin scale, but ugly −200-something numbers on the Celsius scale. Conversely, when talking about high temperatures plasmas and stars above a few thousand kelvins, the fact that kelvin and Celsius are offset by 273.15 K makes no practical difference. The lack of negative kelvin temperatures removes a needless psychological distinction – for example, it’s not really special that nitrogen boils at 77 K (−196 °C) and gold boils at 3243 K (2970 °C); they are both temperatures on a uniform scale.

Everyday temperatures in °C (suggested)

There is no need to report everyday temperature measurements in kelvins. For outdoor weather, room temperature, body temperature, and cooking, it is customary to use degrees Celsius, with typical numbers from −50 °C to 250 °C. Using kelvins would be strange because ambient temperature is around 300 K, and the range of useful values would range from 250 K to 500 K.

Try unpopular prefixes (suggested)