Measurement Glossary
Essential terms and definitions for units of measurement, from SI standards to traditional systems.
SI Units 22
Meter
The SI base unit of length, defined as the distance light travels in 1/299,792,458 of a second.
Kilogram
The SI base unit of mass, defined by the Planck constant.
Second
The SI base unit of time, defined by the cesium-133 atom.
Kelvin
The SI base unit of temperature, starting at absolute zero.
Ampere
The SI base unit of electric current.
International System of Units (SI)
The modern metric system of measurement, accepted worldwide for scientific and commercial use.
Base Unit
A fundamental unit from which other units in a system are derived.
Derived Unit
A unit of measurement expressed as a combination of base units.
Pascal
The SI unit of pressure, equal to one newton per square meter.
Newton
The SI unit of force, equal to the force needed to accelerate 1 kg at 1 m/s².
Watt
The SI unit of power, equal to one joule per second.
Hertz
The SI unit of frequency, equal to one cycle per second.
Mole
The SI base unit for amount of substance, defined as exactly 6.02214076×10²³ elementary entities (atoms, molecules, ions, etc.).
Candela
The SI base unit of luminous intensity, defined by fixing the luminous efficacy of 540 THz monochromatic radiation to 683 lm/W.
Joule
The SI unit of energy, equal to the work done by a force of one newton acting over a distance of one meter (1 J = 1 N·m = 1 kg·m²/s²).
Volt
The SI unit of electric potential difference (voltage), defined as one joule of energy per coulomb of charge (1 V = 1 J/C).
Ohm
The SI unit of electrical resistance, defined as the resistance between two points when a potential difference of one volt drives a current of one ampere (1 Ω = 1 V/A).
Lumen
The SI derived unit of luminous flux, measuring the total quantity of visible light emitted by a source per second (1 lm = 1 cd·sr).
Coulomb
The SI unit of electric charge, defined as the charge transported by a current of one ampere in one second (1 C = 1 A·s), equal to the charge of approximately 6.242×10¹⁸ elementary charges.
Becquerel
The SI unit of radioactivity, equal to one nuclear disintegration (decay event) per second (1 Bq = 1 s⁻¹).
Lux
The SI unit of illuminance, measuring luminous flux received per unit area: one lux equals one lumen per square meter (1 lx = 1 lm/m²).
Farad
The SI unit of electrical capacitance, defined as the capacitance of a capacitor that stores one coulomb of charge when a potential difference of one volt is applied (1 F = 1 C/V).
Imperial/US 24
Imperial System
A system of measurement used primarily in the United States, based on historical English units.
Troy Weight
A system of weights used for precious metals and gemstones.
Avoirdupois
The standard system of weights based on the 16-ounce pound, used for everyday goods.
Horsepower
A unit of power. Mechanical horsepower equals approximately 745.7 watts.
BTU (British Thermal Unit)
The amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit.
PSI (Pounds per Square Inch)
A unit of pressure expressing force in pounds applied over one square inch.
Acre
An imperial unit of area equal to 43,560 square feet or about 4,047 square meters.
Stone
A British unit of weight equal to 14 pounds (6.35 kg).
Fathom
A unit of length equal to 6 feet (1.8288 m), used to measure water depth.
Furlong
A unit of length equal to 1/8 of a mile (201.168 m).
Grain
A unit of mass equal to 1/7000 of a pound (64.799 mg).
Slug
An imperial unit of mass where 1 slug accelerates at 1 ft/s² when 1 pound-force is applied.
Mile
An imperial unit of length equal to 5,280 feet, 1,760 yards, or exactly 1.609344 kilometers.
Foot
An imperial unit of length equal to 12 inches or exactly 0.3048 meters, subdivided into 12 inches.
Yard
An imperial unit of length equal to 3 feet, 36 inches, or exactly 0.9144 meters.
Inch
An imperial unit of length defined as exactly 2.54 centimeters or 1/12 of a foot.
Gallon
A volume unit with two definitions: the US gallon equals 3.785411784 liters; the imperial (UK) gallon equals 4.54609 liters.
Pint
A volume unit equal to 1/2 quart: the US liquid pint is 473.176 mL; the UK imperial pint is 568.261 mL.
Quart
A volume unit equal to one quarter of a gallon: the US liquid quart is 946.353 mL; the UK imperial quart is 1,136.52 mL.
Ounce
A dual-use imperial unit: as a mass unit (avoirdupois), 1 oz = 28.3495 g (1/16 lb); as a volume unit, 1 US fluid ounce = 29.5735 mL.
Pound
An imperial unit of mass equal to exactly 453.59237 grams (16 avoirdupois ounces), also used as a unit of force (pound-force) equal to 4.44822 newtons.
Ton
Three distinct units: the short ton (US) = 2,000 lb (907.185 kg); the long ton (UK) = 2,240 lb (1,016.05 kg); the metric tonne = 1,000 kg (2,204.62 lb).
Bushel
A dry volume unit used for agricultural commodities: the US bushel equals 35.2391 liters (2,150.42 cubic inches); the UK bushel equals 36.3687 liters.
Hand
A unit of length equal to exactly 4 inches (10.16 cm), used internationally to measure the height of horses.
Metric System 11
Metric Prefix
A prefix that indicates a decimal multiple or fraction of a unit (kilo-, mega-, milli-, micro-, etc.).
Hectare
A metric unit of area equal to 10,000 square meters or 2.471 acres.
Bar
A unit of pressure equal to 100,000 pascals, slightly less than standard atmospheric pressure.
Metric System
A decimal-based international system of measurement built on seven SI base units, used officially by every country in the world except the United States, Myanmar, and Liberia.
Decimal System
A base-10 numbering system in which each place value is ten times the next smaller value, forming the mathematical foundation of the metric system.
CGS System
A metric unit system based on the centimeter, gram, and second as fundamental units, used in some branches of physics and electromagnetism.
MKS System
A metric unit system based on the meter, kilogram, and second, which served as the direct predecessor of the modern SI system adopted in 1960.
Kilometer
A metric unit of length equal to 1,000 meters or approximately 0.621371 miles.
Gram
A metric unit of mass equal to one-thousandth of a kilogram (10⁻³ kg), originally defined as the mass of 1 cubic centimeter of water at 4°C.
Liter
A metric unit of volume equal to one cubic decimeter (1 dm³ = 1,000 cm³ = 1,000 mL), approximately 0.264 US gallons or 1.057 US liquid quarts.
Celsius
A metric temperature scale where 0°C is the freezing point of water and 100°C is the boiling point at standard atmospheric pressure (101.325 kPa).
Measurement 33
Conversion Factor
A numerical multiplier used to convert a quantity from one unit to another.
Precision
The degree of refinement in a measurement, indicated by the number of significant digits.
Accuracy
How close a measurement is to the true value of the quantity being measured.
Dimensional Analysis
A method of converting units by multiplying by conversion factors that equal 1.
Nautical Mile
A unit of distance used in sea and air navigation, equal to 1,852 meters.
Significant Figures
The meaningful digits in a number that contribute to its measurement precision.
Rounding
The process of reducing the number of digits in a value while keeping it close to the original.
Knot
A unit of speed equal to one nautical mile per hour (1.852 km/h).
Carat
A unit of mass for gemstones, equal to 200 milligrams (0.2 grams).
Barrel (Oil)
A unit of volume for crude oil, equal to 42 US gallons (158.987 liters).
Pyeong
A Korean unit of area equal to approximately 3.3058 square meters.
Point (Typography)
A unit of type size. The desktop publishing point equals 1/72 of an inch.
Metrication
The process of adopting the metric system as a country's standard system of measurement.
Orders of Magnitude
A way of expressing and comparing quantities using powers of 10, where each order represents a tenfold difference from the previous.
Scientific Notation
A method of expressing numbers as a coefficient between 1 and 10 multiplied by a power of 10 (e.g., 6.022 × 10²³).
Error Margin
The range above and below a measured or estimated value within which the true value is expected to fall, typically expressed at a stated confidence level.
Calibration
The process of comparing and adjusting a measuring instrument against a known reference standard to ensure its readings are accurate.
Tolerance
The permissible range of variation in a measurement or manufactured dimension, defined as the difference between the upper and lower acceptable limits.
Standard Conditions (STP)
The IUPAC reference state for gas measurements: 0°C (273.15 K) and 101.325 kPa (1 atm), at which one mole of an ideal gas occupies 22.414 liters.
Unit Analysis
A problem-solving technique that verifies equations and performs conversions by treating units as algebraic quantities that multiply, divide, and cancel.
Proportionality
A mathematical relationship between two quantities where their ratio remains constant (direct proportionality) or their product remains constant (inverse proportionality).
Scaling
The process of multiplying or dividing measurements by a consistent factor to convert between sizes, representations, or units while preserving relative relationships.
Interpolation
The estimation of a value between two known data points by assuming a functional relationship (most commonly linear) between them.
Systematic Error
A consistent, repeatable deviation of measurements from the true value, caused by a flaw in the instrument, method, or experimental design.
Random Error
Unpredictable, statistical variation in measurements caused by uncontrolled factors such as electronic noise, vibration, or environmental fluctuations.
Reproducibility
The closeness of agreement between independent measurement results obtained under changed conditions, such as different operators, instruments, laboratories, or time periods.
Traceability
An unbroken, documented chain of calibrations linking a measurement result to a national or international measurement standard, each with stated uncertainties.
Measurement Uncertainty
A parameter, associated with a measurement result, that characterizes the dispersion of values reasonably attributable to the measurand, expressed as a standard uncertainty or expanded uncertainty.
Reference Standard
A measurement standard with the highest metrological quality available at a given location, used to calibrate working standards and other instruments in that organization.
Dimensional Homogeneity
The requirement that all terms in a physically valid equation must have identical dimensions, so that quantities of different kinds are never added or equated.
Derived Quantity
A physical quantity defined by a mathematical combination of SI base quantities (length, mass, time, electric current, thermodynamic temperature, amount of substance, luminous intensity).
Unit Symbol Rules
The BIPM conventions governing how SI unit symbols are written: upright (roman) type, case-sensitive, no plural forms, no trailing periods, and separated from the numeral by a space.
Resolution
The smallest change in the quantity being measured that causes a detectable change in the indication of a measuring instrument.
Scientific 19
Absolute Zero
The lowest possible temperature: 0 K, -273.15°C, or -459.67°F.
Calorie
A unit of energy. The small calorie (cal) heats 1g of water by 1°C; the food Calorie (kcal) is 1000 small calories.
Mach Number
The ratio of an object's speed to the speed of sound in the surrounding medium.
Light-Year
The distance light travels in one year in vacuum: approximately 9.461 trillion kilometers.
Electronvolt
A unit of energy equal to the kinetic energy gained by an electron accelerated through 1 volt.
Astronomical Unit
The average distance from Earth to the Sun: 149,597,870,700 meters.
Parsec
A unit of distance equal to about 3.26 light-years or 3.086×10¹⁶ meters.
Speed of Light
The exact speed at which light travels in vacuum: 299,792,458 meters per second, which since 1983 has been used to define the meter.
Planck Constant
A fundamental physical constant with the exact value 6.62607015 × 10⁻³⁴ J·s, which since 2019 has been used to define the kilogram.
Avogadro's Number
The exact number of elementary entities (atoms, molecules, or ions) in one mole of a substance: 6.02214076 × 10²³ per mole.
Boltzmann Constant
A fundamental physical constant with the exact value 1.380649 × 10⁻²³ J/K, relating the average thermal kinetic energy of particles to the absolute temperature.
Standard Gravity
The nominal gravitational acceleration at Earth’s surface, defined exactly as 9.80665 m/s², used to define the kilogram-force and standardize weight measurements.
Standard Atmosphere
A reference unit of pressure defined as exactly 101,325 pascals (101.325 kPa), representing the approximate mean atmospheric pressure at sea level.
Specific Gravity
The dimensionless ratio of a substance’s density to the density of a reference substance, typically water at 4°C (999.97 kg/m³) for liquids and solids.
Specific Heat Capacity
Specific heat capacity is the amount of energy required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
Wavelength
Wavelength is the distance between two consecutive identical points (such as crests) of a periodic wave, typically measured in meters or nanometers.
Entropy
Entropy is a thermodynamic quantity measuring the degree of disorder or randomness in a system, expressed in joules per kelvin (J/K).
Half-Life
Half-life is the time required for exactly half of the radioactive atoms in a sample to undergo nuclear decay.
Refractive Index
The refractive index (n) of a medium is the ratio of the speed of light in a vacuum (c) to its speed in that medium: n = c/v.
Cooking & Kitchen 11
Cup (Measurement)
A cooking volume unit. The US cup equals 236.588 ml; the metric cup is 250 ml.
Tablespoon
A cooking volume unit. The US tablespoon equals approximately 14.787 ml.
Teaspoon
A cooking volume unit. The US teaspoon equals approximately 4.929 ml.
Fluid Ounce
A fluid ounce is a unit of liquid volume, with the US fluid ounce equal to 29.5735 mL and the imperial (UK) fluid ounce equal to 28.4131 mL.
Dash & Pinch
A dash is an informal culinary measure of approximately 1/8 teaspoon (0.6 mL), and a pinch is approximately 1/16 teaspoon (0.3 mL).
Weight vs Volume in Cooking
Weight measurement in cooking uses mass units (grams, ounces) rather than volume units (cups, tablespoons) to specify ingredient amounts.
Baker's Percentage
Baker's percentage is a notation system where every ingredient in a bread recipe is expressed as a percentage of the total flour weight, with flour always set at 100%.
Metric Cup
The metric cup is a unit of volume equal to exactly 250 mL, used as the standard cup measure in Australia, New Zealand, and Canada.
Jigger
A jigger is a bartending measure equal to 1.5 US fluid ounces (44.36 mL), used as the standard single shot of spirits in the United States.
Stick of Butter
A stick of butter is a US baking measure equal to 1/2 cup, 8 tablespoons, 1/4 pound, or approximately 113 grams.
Proof (Alcohol)
Proof is a measure of alcoholic strength; in the US system, proof equals exactly twice the alcohol by volume (ABV) percentage.
Digital & Data 8
Bit and Byte
A bit is the smallest unit of digital information, representing a 0 or 1; a byte is a group of 8 bits, capable of representing 256 distinct values.
Binary Prefix
Binary prefixes (KiB, MiB, GiB) denote powers of 1024, in contrast to metric SI prefixes (KB, MB, GB) which denote powers of 1000.
Bandwidth
Bandwidth is the maximum data transfer capacity of a communication channel, typically measured in bits per second (bps, Mbps, Gbps).
Latency
Latency is the time delay between a data packet being sent from a source and received at its destination, typically measured in milliseconds (ms).
Throughput
Throughput is the actual rate at which data is successfully transferred over a network or system in a given time period, measured in bits or bytes per second.
Clock Speed
Clock speed is the frequency at which a processor executes cycles, measured in hertz (Hz), with modern CPUs operating in the gigahertz (GHz) range.
Pixel
A pixel (picture element) is the smallest individually addressable point of color on a digital display or in a digital image.
DPI & PPI
DPI (dots per inch) measures print output resolution; PPI (pixels per inch) measures screen display resolution—both describe spatial density of image elements.
Engineering 8
Torque
Torque is the rotational equivalent of force, equal to the applied force multiplied by the perpendicular distance from the pivot point, measured in newton-meters (N·m) or pound-feet (lb·ft).
Stress
Stress is the internal force per unit area within a material resulting from externally applied forces, measured in pascals (Pa) or pounds per square inch (psi).
Strain
Strain is the dimensionless ratio of a material's deformation (change in length) to its original length under applied stress: ε = ΔL/L₀.
Viscosity
Viscosity is a fluid's resistance to deformation or flow, quantifying internal friction between fluid layers; dynamic viscosity is measured in pascal-seconds (Pa·s) or centipoise (cP).
RPM
RPM (revolutions per minute) is a unit of rotational speed measuring how many complete turns an object makes in one minute.
Tensile Strength
Tensile strength is the maximum stress a material can withstand while being stretched before it fractures, measured in megapascals (MPa) or pounds per square inch (psi).
Thermal Expansion
Thermal expansion is the tendency of matter to change its volume in response to temperature changes, characterized by the coefficient of thermal expansion (CTE), measured in units of 1/°C or 1/K.
Reynolds Number
The Reynolds Number is a dimensionless quantity that predicts whether fluid flow will be laminar or turbulent, defined as Re = ρvL/μ (density × velocity × length / dynamic viscosity).
Historical Units 8
Cubit
The cubit is an ancient unit of length based on the distance from the elbow to the tip of the middle finger, typically ranging from 45 to 52 cm depending on the civilization.
Span
The span is an ancient unit of length equal to the width of an outstretched hand from the tip of the thumb to the tip of the little finger, standardized at approximately 23 cm (9 inches).
League
The league is a unit of distance historically defined as the distance a person or horse can walk in one hour, most commonly standardized at approximately 4.8 km (3 miles) in English-speaking countries.
Talent
The talent was a unit of weight and currency in the ancient world, varying from approximately 26 kg in Hebrew usage to 60 kg in the Attic (Athenian) standard.
Chi (尺)
The chi (尺) is a traditional Chinese unit of length now standardized in the People's Republic of China at exactly 1/3 meter (33.333... cm).
Shaku (尺)
The shaku (尺) is a traditional Japanese unit of length equal to 10/33 meters, approximately 30.303 cm, historically derived from the Chinese chi.
Tsubo (坪)
The tsubo (坪) is a Japanese unit of area equal to the area of two tatami mats, approximately 3.306 m² (35.58 sq ft), and is equivalent to the Korean pyeong (평).
Stadion
The stadion (or stade) is an ancient Greek unit of length approximately equal to 185 meters (600 Greek feet), originally defined as the length of a footrace track.
Typography 5
Em
The em is a relative typographic unit equal to the current element's font size; in a 16 px context, 1 em equals 16 px.
Pica
The pica is a typographic unit equal to 12 points or exactly 1/6 of an inch (approximately 4.233 mm), used primarily to measure column widths and other layout dimensions.
Twip
A twip (twentieth of a point) is a typographic unit equal to 1/20 of a printer's point, or 1/1440 of an inch (approximately 17.64 micrometers).
Cap Height
Cap height is the typographic measurement of the height of a flat uppercase letter (such as 'H' or 'I') from the baseline to the top of the letter, expressed as a fraction of the font's em square.
X-Height
X-height is the typographic height of a typeface's lowercase letters, specifically the height of a flat lowercase 'x' measured from the baseline, typically expressed as a fraction of the em size.
Everyday Measurements 3
BMI (Body Mass Index)
The Body Mass Index (BMI) is a screening metric calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m²).
Blood Pressure Units
Blood pressure is measured in millimeters of mercury (mmHg), expressed as two values: systolic pressure (heart contracting) over diastolic pressure (heart at rest), e.g., 120/80 mmHg.
Tire Size Numbers
Tire size is encoded in the format Width/AspectRatio R Diameter (e.g., 205/55 R16), where width is in millimeters, aspect ratio is the sidewall height as a percentage of width, and diameter is the wheel rim size in inches.