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Prefixes

A prefix may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth; hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.

Base Units

The International System of Units (SI) defines seven SI base units. For a set of physical quantities of measure, or dimensions, that are used to define all other SI units, known as SI derived units.

Physical Measure

Physical measure

Other Measure

Physical measure

Length        Converter

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. This may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length. In the physical sciences and engineering, the word "length" is typically used synonymously with "distance", with symbol l or L or letter-like symbol l. In the International System of Units (SI), the basic unit of length is the meter and is now defined in terms of the speed of light.

Area        Converter

Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved then the definition of arc length of a curve.

Volume        Converter

The volume of any solid, liquid, gas, plasma, theoretical object, or vacuum is how much three-dimensional space it occupies, often quantified numerically. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space. Volume is commonly presented in units such as cubic meters, cubic centimeters, litres, or millilitres.

Plane angle        Converter

In geometry and trigonometry, an angle (in full, plane angle) is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. The magnitude of the angle is the \"amount of rotation\" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide with the other. Where there is no possibility of confusion, the term \"angle\" is used interchangeably for both the geometric configuration itself and for its angular magnitude (which is simply a numerical quantity).

Mass        Converter

In the physical sciences, the weight of an object is the magnitude, W, of the force that must be applied to an object in order to support it (i.e. hold it at rest) in a gravitational field. The weight of an object equals the magnitude of the gravitational force acting on the object, less the effect of its buoyancy in any fluid in which it might be immersed. Near the surface of the Earth, the acceleration due to gravity is approximately constant; this means that an object´s weight near the surface of the Earth is roughly proportional to its mass.

Density        Converter

The density of a material is defined as its mass per unit volume.

Time        Converter

In physics as well as in other sciences, time is considered one of the few fundamental quantities. Time is used to define other quantities - such as velocity - so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life.

Frequency        Converter

Number of cycles per period.

Speed or velocity        Converter

Speed is a scalar quantity with dimensions length/time; the equivalent vector quantity to speed is velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.

Flow (volume)        Converter

Flow (volume).

Acceleration        Converter

In physics, and more specifically kinematics, acceleration is the change in velocity over time. Because velocity is a vector, it can change in two ways: a change in magnitude and/or a change in direction. In one dimension, acceleration is the rate at which something speeds up or slows down. However, as a vector quantity, acceleration is also the rate at which direction changes. Acceleration has the dimensions L T-2. In SI units, acceleration is measured in metres per second squared (m/s2).

Force        Converter

In physics, a force is any agent that causes a change in the motion of a free body, or that causes stress in a fixed body. It can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. Force has both magnitude and direction, making it a vector quantity. Newton´s second law states that an object with a constant mass will accelerate in proportion to the net force acting upon and in inverse proportion to its mass. Equivalently, the net force on an object equals the rate at which its momentum changes.

Pressure or mechanical stress        Converter

Pressure (symbol: p or P) is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Torque or moment of force        Converter

Torque, also called moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist. In more basic terms, torque measures how hard something is rotated. For example, imagine a wrench or spanner trying to twist a nut or bolt. The amount of \"twist\" (torque) depends on how long the wrench is, how hard you push down on it, and how well you are pushing it in the correct direction.

Energy, work, or amount of heat        Converter

In physics, is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law. Different forms of energy include kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy. The forms of energy are often named after a related force. Any form of energy can be transformed into another form, but the total energy always remains the same. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether´s theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.

Power or heat flow rate        Converter

In physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power is: P = W\t , where P is power, W is work and t is time.

Dynamic viscosity        Converter

Dynamic viscosity (or absolute viscosity) determines the dynamics of an incompressible Newtonian fluid. Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. Viscosity describes a fluid´s internal resistance to flow and may be thought of as a measure of fluid friction.

Kinematic viscosity        Converter

Kinematic viscosity is the dynamic viscosity divided by the density for a Newtonian fluid.

Electric current        Converter

Electric current can mean, depending on the context, a flow of electric charge (a phenomenon) or the rate of flow of electric charge (a quantity). The electric charge that flows is carried by, for example, mobile electrons in a conductor, ions in an electrolyte or both in a plasma. The SI unit for rate of flow of electric charge is the ampere. Electric current is measured using an ammeter.

Electric charge        Converter

Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces. The electric charge on a body may be positive or negative. Two positively charged bodies experience a mutual repulsive force, as do two negatively charged bodies. A positively charged body and a negatively charged body experience an attractive force.

Voltage, electromotive force        Converter

Voltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt (symbol: V, not italicized). Electric potential is a hypothetically measurable physical dimension, and is denoted by the algebraic variable V (italicized). The voltage between two (electron) positions \"A\" and \"B\", inside a solid electrical conductor (or inside two electrically-connected, solid electrical conductors), is denoted by (VA - VB). This voltage is the electrical driving force that drives a conventional electric current in the direction A to B. Voltage can be directly measured by an voltmeter.

Magnetic flux        Converter

Magnetic flux, represented by the Greek letter F (phi), is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field. The SI unit of magnetic flux is the weber (in derived units: volt-seconds).

Magnetic flux density        Converter

Magnetic flux density, is the weber per square meter, or tesla.

Temperature        Converter

In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics. If no net heat flow occurs between two objects, the objects have the same temperature; otherwise heat flows from the hotter object to the colder object. This is the content of the zeroth law of thermodynamics. On the microscopic scale, temperature can be defined as the average energy in each degree of freedom in the particles in a system. Because temperature is a statistical property, a system must contain a few particles for the question as to its temperature to make any sense. For a solid, this energy is found in the vibrations of its atoms about their equilibrium positions. In an ideal monatomic gas, energy is found in the translational motions of the particles; with molecular gases, vibrational and rotational motions also provide thermodynamic degrees of freedom.

Information entropy        Converter

In information theory, entropy is a measure of the uncertainty associated with a random variable. The term by itself in this context usually refers to the Shannon entropy, which quantifies, in the sense of an expected value, the information contained in a message, usually in units such as bits.

Luminous intensity        Converter

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

Luminance        Converter

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). A non-SI term for the same unit is the nit. The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.

Illuminance        Converter

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In SI derived units, these are both measured in lux (lx) or lumens per square metre (cd×sr×m-2). In the CGS system, the unit of illuminance is the phot. One phot is equal to 10,000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.

Radioactivity        Converter

Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of TBq (terabecquerel) or GBq (gigabecquerel) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 * 1010 Bq. The use of Ci is presently discouraged by the SI.

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