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a chemical element, atomic number 14, atomic weight 28.086, symbol Si. (See Appendix 6.)
silicon dioxide silica.

SI units

the units of measurement generally accepted for all scientific and technical uses; together they make up the International System of Units. (See also metric system.) The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements, and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in the accompanying table.

SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m3). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.

The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g., nanometer (nm) = 10−9 m. The units of mass are specified in terms of the gram, e.g., microgram (μg) = 10−9 kg.

Only one prefix is used with a unit; the use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g., a cubic millimeter (mm3) = 10−9 m3. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g., kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 103 are normally used. Hecto-, deka-, deci-, and centi- are usually attached only to the metric system units gram, meter, and liter.

Owing to the force of tradition, one noncoherent unit, the liter, equal to 10−3 m3, or 1 dm3, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1, especially in typescript. For this reason, the capital letter L is also used as a symbol for liter. The lower case letter is generally used with prefixes, e.g., dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.


Abbreviation for International System of Units (Système International d'Unités).


Symbol for silicon.


Abbreviation for 6-mercaptopurine ribonucleoside (or 6-thioinosine).


Système International d'Unités, or International System of Units. See SI unit, under unit.




symbol for the element silicon.


abbreviation for Systèeme International d'Unités, the French name for the International System of Units.


Abbreviation for:
International System of Units 
safety index
sagittal index
saline infusion
satiety index
saturation index
secondary infertility
seed implantation
selectivity index
sensitivity index
serious illness
serious injury
seriously ill
sexual intercourse
sexually inactive
sexually inexperienced
serum iron
severity index
shock index
signal intensity
silent infarction
silent ischaemia
Singh Index
sinus irregularities
situs inversus
small intestine
smoke inhalation
smoking index
snoring index
social interaction
Special Investigator
speech impaired
spinal injury
stability index
stratum intermedium
statutory instrument 
stress incontinence
stroke index
Structured Interview 
suicidal ideation
survival index
systolic index


Lab medicine Systeme International d'unites, Intl System of units The international system for standardization of units of measurement; the SI is a refinement and extension of the metric system
1. Sacroiliac.
2. Saline injection
3. Saturation index.
4. School inventory–psychological test.
5. Sex inventory–psychological evaluation.


Abbreviation for International System of Units; sensory integration.


Symbol for silicon.

SI units

; the metric system

Système International (SI) units

At an international convention in 1960, the General Conference of Weights and Measures agreed to an International System of Units: SI or Système International, the name for the current version of the metric system, first introduced in France at the end of the 18th century.

In any system of measurement, the magnitude of some physical quantities must be arbitrarily selected and declared to have unit value. These magnitudes form a set of standards and are called base units. All other units are derived units.

The SI measurement system is used for medical, scientific and technical purposes in most countries and comprises seven base units with several derived units. Each unit has its own symbol and is expressed as a decimal multiple or submultiple of the base unit by use of the appropriate prefix; for example, a millimetre is one thousandth of a metre.

SI base units
Name of unitSymbolQuantity
MolemolAmount of substance
AmpereAElectric current
Kelvin°KThermodynamic temperature
CandelacdLuminous intensity

SI derived units (obtained by dividing or multiplying two or more base units)
JouleJWork, energy, heat
VoltVElectrical potential, potential difference, electromotive force
Other values expressed in terms of base units
Square metrem2Area
Cubic metrem3Volume
Metres per seondm/s m.s-1Speed, velocity
Metres per second per secondm/s2 m.s-2Acceleration
Kilogram per cubic metrekg/m3 kg.m-3Density
Mole per cubic metremol/m3 mol.m-3Concentration

Decimal multiples and submultiples

The metric system uses multiples of 10 to express number. Multiples and submultiples of the base unit are expressed as decimals, with appropriate prefix to the name of the unit. The most widely used prefixes are kilo, milli and micro: e.g. 0.000 001 g = 10-6 g = 1 microgram.

Rules for using units and numbers

  • The symbol for a unit is unaltered in the plural and should not be followed by a full stop except at the end of a sentence: 5 cm not 5 cm. or 5 cms.

  • The decimal point between digits is indicated by a full stop. Commas are not used to divide large numbers into groups of three: a space is left after every third digit. If the numerical value of the number is less than 1, a zero should precede the decimal point: 0.123 456 not .123,456.

  • The SI symbol for 'day' (i.e. 24 hours) is 'd' but excretion of substances, fluid intake or output, for example, should preferably be expressed as amount or mass or volume 'per 24 hours'.

  • 'Squared' and 'cubed' are expressed as numerical powers and not by abbreviation: square centimetre is cm2 not sq cm.

Multiples and submultiples of units
Multiplication factorPrefixSymbol
1 000 000 000 0001012teraT
1 000 000 000109gigaG
1 000 000106megaM
0.000 00110-6microμ
0.000 000 00110-9nanon
0.000 000 000 00110-12picop
0.000 000 000 000 00110-15femtof
0.000 000 000 000 000 00110-18attoa

Units used for common measurements

Temperature The SI base unit is the kelvin but temperature is normally expressed in degrees Celsius (°C).

1° Celsius = 1° Centigrade

Energy The SI unit joule replaces the calorie.

1 calorie = 4.2 J

1 kilocalorie (calorie) = 4.2 kilojoules

Energy requirement, expenditure and content of food are expressed in kilojoules (kJ), but the kilocalorie (kcal) remains in common use.

Amount of substance in SI units is expressed in moles (mol), and concentration (amount per unit volume) in moles per litre (mol/L) or millimoles per litre (mmol/L). This replaces milliequivalents per litre (mEq/L). There are some exceptions: grams per litre (g/L), e.g. for haemoglobin and plasma proteins; international units (IU, U or iu) for enzyme activity.

Pressure The SI unit is the pascal (Pa); the kilopascal (kPa) replaces millimetres of mercury pressure (mmHg) for arterial blood pressure and partial pressure (or tension) of oxygen and carbon dioxide (PO2 and PCO2) in gas or in blood.

1 mmHg = 133.32 Pa

1 kPa = 7.5006 mmHg

Arterial blood pressure is, however, still widely measured in mmHg, cerebrospinal fluid pressure in mmH2O and central venous pressure in cmH2O.

Volume is calculated as length × width × depth. The metre(m) is the SI unit for length but a cubic metre is not a practical unit of volume for most purposes. The litre (L or l - the volume of a 10 cm cube) and the millilitre (mL, ml) are therefore used (1 L = 10-3 m3 = 1 dm3).

Weights and measures

Linear measure
1 kilometre (km)= 1000 metres (m)
1 metre (m)= 100 centimetres (cm) or 1000 millmetres (mm)
1 centimetre (cm)= 10 millimetres (mm)
1 millimetre (mm)= 1000 micrometres (μm)
1 micrometre (mm)= 1000 nanometres (nm)
1 metre (m)= 39.370 inches (in)
1 centimetre (cm)= 0.3937 inches (in)
30.48 centimetres (cm)= 1 foot (ft)
2.54 centimetres (cm)= 1 inch (in)
1 litre (L)= 1000 millilitres (mL)
1 millilitre (mL)= 1000 microlitres (mL)
1 litre (L)= 1.76 pints (pt)
568.25 millilitres (mL)= 1 pint (pt)
28.4 millilitres (mL)= 1 fluid ounce (fl oz)
1 kilogram (kg)= 1000 grams (g)
1 gram (g)= 1000 milligrams (mg)
1 milligram (mg)= 1000 micrograms (μg)
1 microgram (mg)= 1000 nanograms (ng)
1 kilogram (kg)= 2.204 pounds (lb)
1 gram (g)= 0.0353 ounce (oz)
453.59 grams (g)= 1 pound (lb)
28.34 grams (g)= 1 ounce (oz)



  • Fahrenheit (9/5× x °C) + 32

  • Centigrade = °Celsius = ( x °F-32)×5/9

where x is the temperature to be converted.

Conversion scales

Figure 1.

For Body Temperature: Figure 2.

Conversion scales

Figure 1: Conversion scales.

Figure 2: Conversion scale for body temperature showing the normal and abnormal ranges.


n small intestine channel; an acupuncture channel running from the hand to the face along the ulnar surface of the arm and associated with the heart (HT) channel. See also HT.
Enlarge picture


n See sacroiliac joint.
Enlarge picture


Abbreviation for International System of Units.


Symbol for silicon.


chemical symbol, silicon.