O que é um/uma Kilogram (kg)?
Formal Definition
The kilogram (symbol: kg) is the base unit of mass in the International System of Units (SI). Since 20 May 2019, it has been defined by fixing the numerical value of the Planck constant h to exactly 6.62607015 × 10⁻³⁴ joule-seconds (J·s), which equals kg·m²·s⁻¹. This definition links the kilogram to the second and the meter, both of which are themselves defined by fundamental physical constants. The redefinition ensures that the kilogram can be realized in any properly equipped laboratory in the world without reference to a physical object.
The kilogram is the only SI base unit whose name includes a prefix ("kilo-"). One kilogram is equal to 1000 grams. In everyday use, the kilogram is widely understood as the mass of one liter of water at approximately 4 °C, though this relationship is not part of the formal definition. The close correspondence between 1 kg and 1 liter of water is a deliberate feature of the metric system's original design, intended to make conversions between mass and volume intuitive for common substances.
Role in the SI System
The kilogram is used to measure the mass of physical objects across virtually every scientific, industrial, and commercial domain. It serves as the foundation for derived SI units such as the newton (kg·m·s⁻²), the pascal (kg·m⁻¹·s⁻²), and the joule (kg·m²·s⁻²). In practice, the kilogram is used for everything from measuring body weight and food portions to quantifying chemical reagents and specifying rocket propellant loads.
Among the seven SI base units, the kilogram occupies a unique position because of its historical reliance on a physical artifact — the International Prototype of the Kilogram — for more than 130 years. The 2019 redefinition made the kilogram the last of the base units to be freed from dependence on a material standard, completing a decades-long program to anchor the entire SI on invariant constants of nature.
Etymology
Ancient Roots
The word "kilogram" derives from the French "kilogramme," which was coined during the creation of the metric system in the 1790s. The prefix "kilo-" comes from the Greek word "chilioi" (χίλιοι), meaning "thousand." The root "gram" traces back to the Late Latin "gramma," meaning a small weight, which itself was borrowed from the Greek "gramma" (γράμμα), originally meaning something written or a small unit. In the context of weights, "gramma" referred to 1/24 of an ounce in the late Roman system.
Entry into Modern Languages
The French Revolutionary government assembled a commission of scientists, including Antoine Lavoisier and the Marquis de Condorcet, to devise a rational system of measurement. They chose the prefix system based on Greek and Latin roots: Greek-derived prefixes (kilo-, hecto-, deca-) for multiples and Latin-derived prefixes (milli-, centi-, deci-) for fractions. The "gramme" was defined as the mass of one cubic centimeter of water, and the "kilogramme" — one thousand grammes — became the practical standard because a one-gram mass was too small to serve as a reliable physical reference.
The decision to make the kilogram rather than the gram the base unit of mass has had a lasting quirk in scientific nomenclature. Unlike every other SI base unit, the kilogram carries a prefix, which creates an oddity when adding further prefixes: for instance, one millionth of a kilogram is called a milligram (not a microkilogram). The SI resolves this by applying additional prefixes to the gram rather than the kilogram, so we have micrograms, nanograms, and so forth.
Precise Definition
The Planck Constant Definition
The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 × 10⁻³⁴ when expressed in the unit J·s, which is equal to kg·m²·s⁻¹, where the meter and the second are defined in terms of the speed of light c and the cesium-133 hyperfine transition frequency ΔνCs. In algebraic form, the definition can be written as: 1 kg = (h / 6.62607015 × 10⁻³⁴) × s × m⁻². This means that anyone who can measure the Planck constant with sufficient precision can independently realize the kilogram.
Experimental Realization Methods
The two principal experimental methods for realizing the kilogram under this definition are the Kibble balance (formerly called the watt balance) and the X-ray crystal density method (the Avogadro project). The Kibble balance equates electrical power to mechanical power: it balances the gravitational force on a test mass against an electromagnetic force generated by a current-carrying coil in a magnetic field, linking mass to the Planck constant through measurements of voltage and current traceable to quantum electrical standards. The Avogadro project uses highly enriched silicon-28 spheres to count atoms and relate mass to the Planck constant via the Avogadro constant and atomic masses.
Practical Calibration
For practical calibration, national metrology institutes such as NIST (United States), PTB (Germany), and NPL (United Kingdom) maintain primary mass standards that are periodically verified against Kibble balance measurements. The relative standard uncertainty of kilogram realizations via the Kibble balance is on the order of 10⁻⁸ (about 10 micrograms per kilogram), which is sufficient for all known scientific and commercial needs. The BIPM in Sèvres, France, continues to coordinate international comparisons to ensure consistency among national standards.
História
Origins in the French Revolution
The kilogram was first defined in 1795 during the French Revolution as the mass of one cubic decimeter (one liter) of water at the temperature of melting ice. This practical definition was part of the broader effort to create a rational, decimal-based system of measurement that could serve as a universal standard. The revolutionary government sought to replace the chaotic patchwork of feudal measurement systems — France alone had over 800 different units of measure — with a single coherent framework based on nature.
In 1799, the Kilogramme des Archives, a solid platinum artifact, was fabricated by the French goldsmith and instrument maker Nicolas Fortin under the direction of the chemist Antoine Lavoisier and the mathematician Louis Lefèvre-Gineau. This cylinder, measuring 39 mm in both height and diameter, was deposited in the Archives de la République and served as the definitive standard of mass for 90 years.
The International Prototype
In 1875, the Metre Convention was signed by 17 nations, establishing the International Bureau of Weights and Measures (BIPM) in Sèvres, near Paris. The Convention mandated the creation of new, more precise international prototypes. In 1889, the First General Conference on Weights and Measures (CGPM) replaced the Kilogramme des Archives with the International Prototype of the Kilogram (IPK), a cylinder made of 90% platinum and 10% iridium, standing 39.17 mm tall with a diameter of 39.17 mm. The IPK — informally known as "Le Grand K" or "Big K" — and its six official copies were stored under three nested bell jars in a climate-controlled vault at the BIPM.
Forty replicas of the IPK were manufactured and distributed to signatory nations as national prototypes. These replicas were periodically returned to Sèvres for comparison with the IPK in a process known as a "periodic verification." The third verification, completed in 1988-1992, revealed troubling results: the masses of the national copies had diverged from the IPK by up to 50 micrograms over 100 years. Even more troublingly, it was impossible to determine whether the copies had gained mass or the IPK had lost mass — or some combination of both.
The Need for Redefinition
This instability was deeply problematic because the kilogram, as the last SI unit defined by a physical artifact, introduced fundamental uncertainty into all measurements that depended on it. Since the newton, pascal, joule, watt, and many other derived units depend on the kilogram, any drift in the IPK's mass propagated throughout the entire system. By the early 2000s, the metrology community agreed that the kilogram must be redefined in terms of a fundamental constant of nature.
The effort to redefine the kilogram took more than two decades and involved two independent experimental approaches. Bryan Kibble at the UK's National Physical Laboratory invented the watt balance (later renamed the Kibble balance in his honor after his death in 2016) in 1975. This device equates mechanical and electrical power to link mass to the Planck constant. Meanwhile, the International Avogadro Project, led by PTB in Germany, produced the world's most perfect sphere — a 1-kg ball of isotopically enriched silicon-28 — to determine the Avogadro constant with unprecedented precision. Both methods converged on consistent values of the Planck constant by 2017.
The 2019 Redefinition
On 16 November 2018, at the 26th General Conference on Weights and Measures held at the Palais des Congrès in Versailles, representatives from 60 nations voted unanimously to redefine the kilogram by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ J·s. The new definition took effect on 20 May 2019 — World Metrology Day. The IPK remains in its vault at the BIPM but is now a museum piece rather than a defining standard.
The word "kilogram" derives from the French "kilogramme," which was constructed from the Greek "chilioi" (thousand) and the Late Latin "gramma" (a small weight). Despite the prefix "kilo-," the kilogram, not the gram, was chosen as the SI base unit of mass because the one-kilogram platinum standard was far more practical to manufacture and maintain than a one-gram artifact would have been.
Uso atual
In Trade and Commerce
The kilogram is the standard unit of mass used in nearly every country in the world. It is the legal unit of measurement for trade, commerce, and labeling in all countries that have adopted the metric system, which includes virtually every nation on Earth. Grocery items, body weight, shipping parcels, and industrial materials are all commonly measured in kilograms. In the European Union, all packaged goods must display their mass in grams or kilograms, and price labels at markets and supermarkets reference kilograms as the standard unit.
In Science and Engineering
In science and engineering, the kilogram is fundamental and ubiquitous. It appears in calculations involving force (newtons = kg·m/s²), pressure (pascals = kg/m·s²), energy (joules = kg·m²/s²), and power (watts = kg·m²/s³). Medical dosages are calculated based on patient mass in kilograms — for example, many drugs are prescribed at milligrams per kilogram of body weight. Chemical formulations specify reagent quantities in grams and kilograms. Aerospace engineering relies on kilogram-based measurements for fuel loads, payload capacities, and thrust calculations. The kilogram is also essential in material science, where density is expressed as kilograms per cubic meter.
In the United States
The United States is one of the few countries that does not use the kilogram as its primary unit of mass in everyday commerce, preferring the pound instead. However, the US pound has been legally defined in terms of the kilogram since 1959 (1 lb = 0.45359237 kg exactly), and kilograms are widely used in American scientific, military, and medical contexts. US pharmaceutical labeling, for instance, uses metric units. The US military adopted the metric system for logistics and operations, and NASA uses metric units for all spacecraft design and mission planning — a policy reinforced after the loss of the Mars Climate Orbiter in 1999, which was caused by a unit conversion error between pounds-force and newtons.
In international trade and shipping, the kilogram is the universal standard. Air freight charges are calculated per kilogram worldwide, and the International System of Units serves as the common measurement language for global commerce. Even in countries that use non-metric units domestically, the kilogram is used for international transactions, customs declarations, and scientific collaboration.
Everyday Use
In the Kitchen
In the kitchen, the kilogram and its submultiples are indispensable. Recipes worldwide specify ingredients in grams and kilograms — a standard loaf of bread requires about 500 g of flour, a kilogram bag of sugar is a pantry staple, and butter typically comes in 250 g blocks in metric countries. Digital kitchen scales calibrated in grams provide the precision needed for baking, where accurate measurements directly affect the outcome. Professional chefs and bakers almost universally prefer weighing ingredients over measuring by volume because weight is more consistent and reproducible.
Health and Fitness
For health and fitness, the kilogram is the standard unit for monitoring body weight in most of the world. Body Mass Index (BMI), the most widely used screening tool for weight classification, is calculated using mass in kilograms and height in meters: BMI = mass (kg) / height² (m²). A BMI of 18.5 to 24.9 is considered healthy for adults. Nutritional labels on food products in metric countries express energy per 100 g or per serving, and dietary guidelines specify recommended daily intakes of macronutrients in grams. Gym equipment in metric countries — dumbbells, barbells, weight plates, and machines — is calibrated in kilograms, with standard Olympic plates weighing 25 kg, 20 kg, 15 kg, 10 kg, 5 kg, 2.5 kg, and 1.25 kg.
Shopping and Travel
When shopping, consumers encounter kilograms constantly. Fresh produce, meat, and seafood are priced per kilogram at supermarkets and farmers' markets throughout Europe, Asia, Africa, and South America. Packaged goods display net weight in grams or kilograms. Luggage weight limits for air travel are specified in kilograms — typically 23 kg for checked baggage on economy class international flights and 7 to 10 kg for carry-on bags. Postal services worldwide calculate shipping costs based on weight in kilograms.
In travel and daily logistics, the kilogram is ever-present. Vehicle payload capacities, elevator weight limits, and bridge load ratings are all specified in kilograms (or metric tonnes). A standard bag of cement weighs 25 kg or 50 kg depending on the country. Laundry machines specify their capacity in kilograms of dry clothes — a typical household washing machine handles 7 to 9 kg. Even newborn babies are weighed in kilograms in most countries, with average birth weight being approximately 3.5 kg.
In Science & Industry
Physics and Fundamental Constants
In physics, the kilogram is central to the definition of force, energy, and power. Newton's second law, F = ma, defines force in newtons, where one newton is the force required to accelerate a one-kilogram mass at one meter per second squared (1 N = 1 kg·m/s²). Pressure is measured in pascals (1 Pa = 1 kg/m·s²), energy in joules (1 J = 1 kg·m²/s²), and power in watts (1 W = 1 kg·m²/s³). The kilogram thus permeates the entire edifice of physical measurement. In gravitational physics, the masses of planets and stars are expressed in kilograms (the Earth's mass is approximately 5.972 × 10²⁴ kg), and Einstein's famous equation E = mc² relates mass in kilograms to energy in joules through the speed of light.
Metrology
In metrology — the science of measurement itself — the kilogram holds a special place. The 2019 redefinition was one of the most significant achievements in the history of measurement science. National metrology institutes around the world, including NIST (USA), PTB (Germany), NPL (UK), NRC (Canada), and NMIJ (Japan), maintain primary kilogram standards traceable to the Planck constant through Kibble balance experiments. A Kibble balance is a complex apparatus that typically costs several million dollars to build and operate, requiring ultra-precise measurements of electrical current and voltage referenced to quantum standards (the Josephson effect for voltage and the quantum Hall effect for resistance). The relative uncertainty of these measurements is approximately 1 × 10⁻⁸, corresponding to about 10 micrograms per kilogram.
Chemistry and Medicine
In chemistry and pharmacology, the kilogram and its submultiples (grams, milligrams, micrograms) are the standard units for quantifying reagents, products, and dosages. Molar mass is expressed in grams per mole (g/mol), linking the macroscopic world of kilograms to the atomic scale. Drug dosages are typically specified in milligrams or micrograms per kilogram of patient body mass — for example, the common anesthetic propofol is administered at 1.5 to 2.5 mg/kg for induction. In forensic toxicology, blood alcohol concentration and drug levels are measured in milligrams per deciliter or micrograms per liter, all ultimately traceable to kilogram-based mass standards.
Engineering
In engineering, the kilogram is essential for structural analysis, manufacturing quality control, and process optimization. Civil engineers calculate dead loads and live loads in kilograms (or kilonewtons) when designing buildings and bridges. Aerospace engineers must know the mass of every component to within grams to optimize fuel efficiency — the Space Shuttle's dry mass was approximately 78,000 kg, and every kilogram saved in structure allowed an additional kilogram of payload. In manufacturing, statistical process control uses mass measurements in grams or kilograms to ensure product consistency, from pharmaceutical tablets (typically 100 to 500 mg each) to automotive components.
Multiples & Submultiples
| Name | Symbol | Factor |
|---|---|---|
| Microgram | μg | 0.000000001 |
| Milligram | mg | 0.000001 |
| Gram | g | 0.001 |
| Kilogram | kg | 1 |
| Metric tonne (megagram) | t | 1000 |
Interesting Facts
The International Prototype of the Kilogram (IPK), also known as "Le Grand K," was used as the world's mass standard from 1889 to 2019 — a reign of exactly 130 years. It is a platinum-iridium cylinder just 39 mm tall and 39 mm in diameter, roughly the size of a golf ball.
During the third periodic verification (1988-1992), scientists discovered that the IPK and its copies had diverged in mass by up to 50 micrograms — roughly the mass of a fingerprint. Since the IPK was the definition of the kilogram, it was technically impossible to say whether it had gained or lost mass; by definition, it was always exactly one kilogram.
A Kibble balance — the instrument used to realize the kilogram from the Planck constant — typically costs between $1 million and $3 million to build and requires a vibration-isolated, temperature-controlled laboratory to operate. As of 2024, fewer than a dozen Kibble balances exist worldwide.
The Avogadro Project created the world's most perfect sphere: a 1-kg ball of isotopically enriched silicon-28, polished to within 0.3 nanometers of a perfect sphere. If this sphere were scaled up to the size of Earth, its tallest mountain would be only 2.4 meters high.
The kilogram is the only SI base unit whose name contains a prefix. This historical quirk means that SI prefix rules are applied to the gram (milligram, microgram) rather than the kilogram, making it unique among all seven base units.
On Earth, a one-kilogram mass weighs about 9.81 newtons. On the Moon, the same kilogram would weigh only about 1.62 newtons — roughly one-sixth of its Earth weight — but its mass remains exactly one kilogram regardless of location.
The International Space Station has a mass of approximately 420,000 kg (420 metric tonnes), making it the most massive human-made object ever assembled in orbit. Its mass has been measured using the Space Acceleration Measurement System, which tracks how the station responds to known forces.
Before the 2019 redefinition, the kilogram was the only SI base unit still defined by a physical artifact. All other base units had been redefined in terms of fundamental constants by 1983, when the meter was linked to the speed of light. The kilogram held out for 36 more years.
A standard Olympic weightlifting barbell weighs exactly 20 kg for men and 15 kg for women. The heaviest single lift in competition history is the 263.5 kg clean and jerk by Lasha Talakhadze of Georgia in 2021.
The human body is roughly 60% water by mass. For a 70 kg adult, this means approximately 42 kg of water, distributed among blood plasma, interstitial fluid, and intracellular fluid.
Regional Variations
Global Metric Adoption
In the vast majority of countries worldwide, the kilogram is the sole legal unit of mass for commerce, science, and daily life. The European Union, China, India, Japan, Brazil, Russia, Australia, and virtually all nations in Africa, Asia, and South America use the kilogram exclusively. In these countries, body weight is discussed in kilograms, produce is sold per kilogram, and industrial specifications reference metric mass units. The global standardization of the kilogram has been one of the great successes of the SI system.
The United States
The United States is the most prominent exception. Americans measure body weight in pounds, buy groceries in pounds and ounces, and use tons (short tons of 2000 pounds) for large quantities. However, the US pound is legally defined as exactly 0.45359237 kg, and metric units are used in American science, medicine, the military, and international trade. US food labels are required to show both customary and metric units. Myanmar and Liberia are the only other countries that have not fully adopted the metric system, though both are in the process of transitioning.
Traditional Units in Asia and the UK
Several countries maintain traditional mass units alongside the kilogram for cultural or commercial purposes. In China, the jin (市斤) equals exactly 500 grams (0.5 kg), and the liang (两) equals 50 grams. Chinese markets often price goods per jin rather than per kilogram. In Southeast Asia, the kati (also spelled catty) is a traditional unit that varies by country: it equals 604.79 grams in Malaysia and Singapore, but 600 grams in mainland China and Taiwan. In Japan, the traditional kan (貫) equals 3.75 kg and the momme (匁) equals 3.75 grams, though these are now used mainly in specific contexts such as pearl weight (momme) and traditional crafts. In the United Kingdom, the stone (14 pounds, approximately 6.35 kg) remains the preferred unit for stating personal body weight in everyday conversation, even though metric units are used for most other purposes.