Common mistakes when using the metric system
Mandatory rules
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.
Type of error  Incorrect example  Correct example 

Ad hoc abbreviations  394 Mtr below sea level  394 m below sea level 
66 sqm downtown apartment  66 m^{2} downtown apartment  
29 gr of sugar  29 g of sugar  
15 msec network latency  15 ms network latency  
Plural of symbols  Distance of 70 kms  Distance of 70 km 
Mass of 86 kgs  Mass of 86 kg  
Capitalization of symbols  2.4 ghz wireless network  2.4 GHz wireless network 
15 mb picture file  15 MB picture file  
1 MM pen tip  1 mm pen tip  
Capitalization of unit names  60Watt light bulb  60watt light bulb 
3 milliKelvins  3 millikelvins  
402 KiloHertz  402 kilohertz  
170 Degrees celsius  170 degrees Celsius  
Degree sign/word  25 C weather is warm  25 °C weather is warm 
98 F is body temperature  98 °F is body temperature  
STP is defined as 273.15 °K  STP is defined as 273.15 K  
Helium boils at four degrees Kelvin  Helium boils at four kelvin  
p standing for per  The car went 100 kph  The car went 100 km/h 
Fuel consumption of 9.4 LPK  Fuel consumption of 9.4 L / 100 km  
Powerof1024 prefixes  1 KB = 1024 bytes  1 KB = 1000 bytes 
512 MB of RAM  512 MiB of RAM  
Multiple prefixes  Odometer reading of 30 k km  Odometer reading of 30000 km 
6 M kg of coal shipped  6 Gg of coal shipped  
3 M μF capacitor  3 F capacitor  
Bare prefix  500 G hard disk drive  500 GB hard disk drive 
2 kilos of rice  2 kilograms of rice  
Mass/weight distinction  He weighs 70 kg  His mass is 70 kg 
She weighs 50 kg  She weighs 490 N on Earth  
The bridge can hold up 3000 kg  The bridge can hold up 29 kN  
Deprecated units  30 cc of saline solution  30 mL of saline solution 
10 μ thick sheet of paper  10 μm thick sheet of paper  
6800 Å red laser  680 nm red laser  
50 cps (cycles per second)  50 Hz (hertz)  
Substitutions for Greek mu  430 mcg of medicine  430 μg of medicine 
81 uL of catalyst  81 μL of catalyst  
Spurious conversion precision  Add 1 ounce (28.349523125 g) of butter  Add 1 ounce (30 g) of butter 
Additive mixed units  The tower is 630m 24cm 1mm tall  The tower is 630.241 m tall 
Mass of 15 kilograms and 700 grams  Mass of 15700 grams 
Suggested rules
In addition to the mandatory rules above, the follow 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 opportunity to do so.
Type of situation  Bad example  Good example 

Space between number and unit  Mains voltage of 120V  Mains voltage of 120 V 
700m racetrack  700 m racetrack  
Capital L for litre  4 l plastic milk jug  4 L plastic milk jug 
355 ml soda drink  355 mL soda drink  
Pronunciation of kilometre  40 kuhLAWmihter journey  40 KILomeeter journey 
90 [kə.ˈlɑ.mɪ.tɚz] per hour  90 [ˈkɪl.oʊ.mi.tɚz] per hour  
Avoid semiSI units  120 mmHg systolic blood pressure  16.0 kPa systolic blood pressure 
938 MeV gamma ray  150 pJ gamma ray  
The Milky Way spans 50 kiloparsecs  The Milky Way spans 1.5 zettametres  
Avoid centi/deca/etc.  A4 paper is 21 cm × 29.7 cm  A4 paper is 210 mm × 297 mm 
Square field with 1 hm side length  Square field with 100 m side length  
Shotglass containing 4 cL of vodka  Shotglass containing 40 mL of vodka  
Avoid common nonSI units  Air pressure at 1020 mbar  Air pressure at 102.0 kPa 
Magnetic field of 38000 gauss  Magnetic field of 3.8 tesla  
6 tons of wheat  6000 kg of wheat  
Wind speed of 35 km/h  Wind speed of 9.7 m/s  
280 kWh monthly electrical energy consumption  1008 MJ monthly electrical energy consumption  
Short/long scale to prefixes  1 billion (short scale) litres of oil  1 gigalitres of oil 
1 billion (long scale) litres of oil  1 teralitres of oil  
Scientific temperatures in kelvin  Tank of liquid nitrogen at −196 °C  Tank of liquid nitrogen at 77 K 
Copper melts at 1085 °C  Copper melts at 1358 K  
Color temperature of 5730 °C  Color temperature of 6000 K  
Everyday temperatures in °C  Rainy and 276 K outside  Rainy and 3 °C outside 
Set the oven to 450 K  Set the oven to 175 °C  
Try unpopular prefixes  1000 kg is a metric tonne  1 Mg (megagram) is a metric tonne 
150 million km to reach the Sun  150 Gm (gigametres) to reach the Sun  
Compressed to 0.123 bits per symbol  Compressed to 123 millibits (mb) per symbol  
Frequency error of only 0.000045 Hz  Frequency error of only 45 μHz (microhertz) 
Explanation of rules
 Ad hoc abbreviations (mandatory)

Don’t make up your own abbreviations for units. Metric already defines short, reasonable abbreviations, and introducing nonstandard 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 casesensitive. For example, the prefix M means mega (10^{6}) 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 degreeperson 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}.
 Powerof1024 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. I drove 30 k vs. I 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 cm^{3}, 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 fieldspecific 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 nonSI 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 nonLatin 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 decimalbased 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.
 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·m^{2}·s^{−2}.
 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 nonstandard.
 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 selfinconsistent – 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 semiSI units (suggested)

Just putting a prefix like kilo or milli in front of a nonSI unit doesn’t make it compatible with SI. This gives the unit pseudolegitimacy 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 nonSI 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 nonSI 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 kelvin (suggested)

Measurements and calculations for scientific and engineering purposes should be expressed in kelvin. For example, working with cryogenic technology near absolute zero yield small numbers on the kelvin scale, but ugly −200something numbers on the Celsius scale. Conversely, when talking about high temperatures plasmas and stars above a few thousand kelvin, 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 kelvin. 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 kelvin 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)

For some reason, certain prefixes are not used in normal communication even though they are technically suitable. For example, car odometers report in kilometres (e.g. 12345.0 km), and car owners will colloquially mention that number rounded to thousands, like 10 thousand kilometres. This situation is undesirable because stacking prefixes is disallowed (e.g. 10 k km), and because using mega is both correct and more concise (e.g. 10 Mm). Similarly, large amounts of mass (e.g. commercial shipping, industrial mining) are reported in thousands or millions of tons instead of using large SI prefixes on gram, like gigagram (Gg) or teragram (Tg).
One consequence of this suggestion is that respecting the casesensitivity of metric notation is critical. Mm means megametre, while mm means millimetre, 9 orders of magnitude smaller. Gg means gigagram, while gg is a meaningless typo.
More info
 NIST: Writing with Metric Units
 Wikipedia: International System of Units  General rules
 UK Metric Assocation: Measurement units style guide