This article provides an overview of the definition of the International System of Units (SI), the basic units and their extended units, and introduces the legal units of measurement in CHINA and their conversion relationship with non-SI units. The importance of following standard units and normative methods for measurement accuracy is emphasized to ensure data consistency and reliability in scientific research and industrial applications.
Unit of measure
The International System of Units (SI) is a widely adopted measurement system around the world that aims to provide a uniform standard of measurement for science, industry, and everyday life.
Definition and historical background of the International System of Units
The International System of Units (SI) originated during the France Revolution in the 1790s and was originally intended to create a scientific metrology system. The original "metric system" was refined several times and was finally officially defined as the International System of Units by the International Conference on Weights and Measures in 1960. The SI was established to provide a standardized measurement system to ensure uniformity and accuracy in science and industry worldwide.
The basic unit of the International System of Units
The International System of Units contains seven basic units
Length - m (m)
Mass - Kilograms (kg)
Time - Second(s)
Current - Amps (A)
Thermodynamic Temperature - Kelvin (K)
Amount of Matter - Moles (mol)
Light Intensity - Candela (CD)
An auxiliary unit of the International System of Units
An auxiliary unit (also known as a non-SI unit) is a unit that is closely related to the basic unit and is widely used:
Angle units: radians (rad), second arcs (sr)
Frequency: Hertz (Hz)
Force: Newton (N)
Pressure: Pascal (Pa)
SI derived units with specialized names
These units are derived from the base units and have specialized names:
Power: Watts (W) = 1 kg · m²/s³
Energy: Joule (J) = 1 N·
Voltage: Volts (V) = 1 W/A
Resistance: Ohm (Ω) = 1 V/A
Charge: Coulomb (C) = 1 A ·
Magnetic flux: Weber (Wb) = 1 V·s
Magnetic induction intensity: Tesla (T) = 1 Wb/m²
Prefix of the International System of Units
Prefixes are used to denote multiples and fractions of units to simplify the representation:
1000:(kilo-,10³)
Million: (mega-,10⁶)
Billion: (giga-,10⁹)
milli-, 10⁻³
Micro-,10⁻⁶
Na:(nano-,10⁻⁹)
Units used in conjunction with the International System of Units
The units used in conjunction with the SI are those that are not directly included in the SI system, but are compatible with the SI units and are widely used.
The unit of temperature: Celsius (°C), although the SI unit is Kelvin, there is a fixed conversion relationship between Celsius and Kelvin.
Units of length: centimeters (cm), kilometers (km), these units are interoperable with meters (SI units) in daily use.
Pressure unit: Standard atmospheric pressure (atm), commonly used in meteorology and chemistry, which has a definite conversion relationship with the SI unit Pascal (Pa).
Units that are temporarily used in conjunction with the International System of Units
Some Authorities, although not fully incorporated into the SI system, are still used in specific areas:
Inch: Inches are still widely used in some engineering and construction fields, especially in the United States. 1 inch is equal to 25.4 mm, and there is a fixed conversion relationship with meters.
Pound: Used to describe mass or force, especially in the United States. 1 pound is equal to 0.45359237 kilograms.
Fahrenheit: Used for temperature measurements in the United States and some other countries, it has a fixed conversion formula with degrees Celsius (SI units).
These units remain useful in different contexts and domains and are used in conjunction with the International System of Units to meet a variety of metrology needs.
The legal unit of measurement in our country
The legal unit of measurement in CHINA is the standard unit of measurement used in science, industry and daily life as prescribed by national law. These units are designed to ensure uniformity and accuracy of measurements across the country.
1984 Issuance and Transition of Legal Units of Measurement
In 1984, CHINA officially issued a new legal unit of measurement, a move that marked the full adoption of the International System of Units (SI). This transition process brought CHINA's metrology system into line with international standards and promoted the harmonization of domestic and foreign trade and scientific research. Some of the traditional units previously used, such as the city ruler and the city weight, were phased out or converted to auxiliary means to ensure consistency and accuracy of measurements.
Composition of legal units of measurement
CHINA's legal system of units of measurement includes the SI base unit, auxiliary units, derived units, and some country-specific non-SI units. Together, these units form the national system of measurement standards.
Non-SI systems of units selected by the country
In CHINA, although the SI is widely adopted, certain country-specific non-SI units are still used, especially in certain industries:
The units of length: centimeters (cm) and kilometers (km) are still used in some cases.
Unit of mass: tons (t) are still commonly used in engineering and transportation.
Temperature unit: Celsius (°C) is commonly used in daily life and meteorology.
These units are generally compatible with the International System of Units and are used uniformly with SI units through defined conversion relationships.
Conversion of legal units of measurement to non-statutory units of measurement
| category | Legal unit of measurement | Non-statutory units of measurement | Conversion relationships |
| Units of length | Meters (m) | Centimeter (cm), millimeter (mm), kilometer (km) | 1 m = 100 cm,1 cm = 10 mm,1 km = 1000 m |
| Area units | Square meters(m²) | Square centimeter (cm²), hectare (ha) | 1 m² = 10,000 cm²,1 ha = 10,000 m² |
| Volume and volume units | Cubic meters (m³) | liters (L), cubic centimeters (cm³), cubic kilometers (km³) | 1 m³ = 1000 L,1 L = 1000 cm³,1 km³ = 1,000,000,000 m³ |
| Units of mass | Kilogram (kg) | Grams (g), tons (t) | 1 kg = 1000 g,1 t = 1000 kg |
| Pressure units | Pascal (Pa) | bar, millimeters of mercury (mmHg) | 1 Pa = 0.01 mbar,1 bar = 100,000 Pa,1 mmHg ≈ 133.322 Pa |
| Mass Flow Units | Kilograms per second (kg/s) | Grams per second (g/s), tons per hour (t/h) | 1 kg/s = 1000 g/s,1 t/h = 1000 kg/h |
| Volume flow unit | Cubic meters per second (m³/s) | liters per second (L/s), cubic meters per hour (m³/h) | 1 m³/s = 1000 L/s,1 m³/h = 1/3600 m³/s |
| Function, energy, and calorific units | Joule (J) | Calories (cal), kilowatt hours (kWh) | 1 J = 0.239 cal,1 kWh = 3,600,000 J |
| Power units | Watt(W) | Kilowatt (kW), Horsepower (hp) | 1 kW = 1000 W,1 hp ≈ 745.7 W |
| Thermal conductivity unit | Watts per meter per Kelvin (W/(m · K)) | Thermal conductivity (Btu/(hr · ft · °F)) | 1 W/(m·K) = 0.5779 Btu/(hr·ft·°F) |
| Units of heat transfer coefficient | watts per square meter per kelvin (W/(m² · K)) | Thermal conductivity (Btu/(hr · ft² · °F)) | 1 W/(m²· K) = 0.3168 Btu/(hr·ft²·°F) |
| Temperature unit | Kelvin (K), degrees Celsius (°C) | Fahrenheit (°F) | T(℉) = (T(℃) × 9/5) + 32,T(K) = T(℃) + 273.15 |
| Specific heat capacity units | Joules per kilogram per Kelvin (J/(kg · K)) | Calories per gram per degree Celsius (cal/(g · °C)) | 1 J/(kg· K) = 0.239 cal/(g·℃) |
| Magnetic field strength unit | Amps per meter (A/m) | Gauss (G), Tesla (T) | 1 T = 10,000 G,1 G = 0.0001 T |
| Magnetic flux density unit | Tesla(T) | Gaussian (G) | 1 T = 10,000 G |
| A unit of electromagnetic energy | Coulombs (C) | Ampere-hour (Ah), milliampere-hour (mAh) | 1 C = 1 A·s,1 Ah = 3600 C |
| Optical units | Candela (CD) | Nits (NT) | 1 cd = 1 nt |
| A unit of measurement for a radioisotope | Becquerel (Bq) | Coulombs per second (C/s), Curie (Ci) | 1 Bq = 1 s⁻¹,1 Ci = 3.7 × 10¹⁰ Bq |
| Physical quantities and their units commonly used in chemical experiments | Various (e.g. volume (mL), concentration (mol/L), pressure (Pa), etc.) | These units are compatible with legal units of measurement, such as milliliters (mL) for volume and moles per liter (mol/L) for concentration |
