Frequently Asked Questions

Why is measurement important?
Measurement affects our daily lives…

  •  When our medical care depends critically on measurements – of concentrations of chemicals
    in blood, or the intensity of X-rays
  • When a satellite navigation system guides us along a road, and it depends on time measured
    by ultra-precision clocks on satellites
  • When we buy a part that ‘just fits’: a nut fits a bolt, or a Lego® brick sticks perfectly to
    another brick

In all these situations, and thousands more, we are enjoying the benefits of a global system of
Measurement is the quantitative comparison of something against a reference. A measurement
result is expressed as a value (a number), together with one or more units of measurement, for
… a car travelling at a speed of 10.4 metres per second (m/s).
When a measurement result is expressed by a measured value, this tells us the dimension (such as
mass, length, or time) and the scale of the measured quantity.

What is the SI?

The International System of Units (SI) is a globally-agreed system of measurements. The SI has seven
base units and a number of derived units defined in terms of the base units. The SI units express
measurement results for any quantity, like physical size, temperature or time.
This International System of Units is necessary to ensure that our everyday units of measurement,
whether of a metre or a second, remain comparable and consistent worldwide. Being inaccurate by a
fraction of a second might not matter for cooking pasta, but it becomes very important for
determining who won the 100 metres at the Olympics or in high-frequency stock market trading.
Standardising such measurements not only helps to keep them consistent and accurate, but also
helps society to build confidence. For instance, the kilogram is used every day, and defining this unit
helps to outline how much food a shop is selling, and means that consumers can trust that the shop
is really providing the amount they say they are. This consistency is also relied on to ensure the
correct dosage of medicine is taken, even when measurements are very small.

When did the SI start?

The creation of the decimal Metric System, the ancestor to the SI, was considered to be on 22 June
1799, when two platinum standards representing the metre and the kilogram were deposited in the
Archives de la République in Paris.
In 1869, Emperor Napoleon III approved the creation of an international scientific commission to
propagate the new metric measurement to facilitate trade. On 16 November, the French
government invited countries to join the International Metre Commission – around 30 countries
joined and in 1870 they held their first meeting.

It was this community that eventually led to the signing of the Metre Convention on 20 May 1875, by
17 countries. Although it was called the ‘Metre Convention’, they actually agreed on three units: the
metre and the kilogram – defined by the physical artefacts that had been created; and the second
which would be based upon astronomical time.
In 1889, the international prototypes for the metre and the kilogram, together with the astronomical
second as the unit of time, were units constituted as the base units metre, kilogram, and second, the
original measurement system. In 1946, the scope of this was extended to adopt the ampere, giving
the four-dimensional system based on the metre, kilogram, second, and ampere.
The name International System of Units, with the abbreviation SI, was given to the system in 1960.

Why is the SI important?

The SI units form a foundation for measurement across the world to ensure consistency and
reliability. They are the basis of trading, manufacturing, innovation and scientific discovery around
the world.
SI units can provide new opportunities for innovation. Some examples where greater accuracy is
supporting better methods and understanding with a positive impact on society include:

  •  The accurate measurement of temperature: This will support the ability to identify and
    measure reliably very small changes across large time periods with greater accuracy.
    Therefore, it will allow for precise monitoring and better predictions for climate change.
  •  The accurate administration of drugs: The pharmaceutical industry needs to use a standard
    for very small amounts of mass in order to make dosages of medication even more
    appropriate for patients.

SI units can help us support innovation into the future. As our ability to measure properties
improves, the standards we have for measurement will need to keep up. The accuracy of services like
the Global Positioning System (GPS) are limited by our ability to use standard units, in this case the
second to measure time. We can track our locations effectively because we can establish time using
the SI definition of a second, which can be realized by an atomic clock. This advancement was made
possible because society had defined the second more accurately well before we had even
discovered what it could be used for. The atomic clock was made before computing really took off.
Now, accurate timing is a fundamental part of the industry; without it, the internet, mobile phones
and other technologies could not work reliably.

How are the units of measurement defined?

Originally, measurement units were defined by physical objects or properties of materials. For
example, the metre was originally defined by a metal bar exactly one metre in length.
However, these physical representations can change over time or in different environments, and are
no longer accurate enough for today’s research and technological applications. Over the last century,
scientists measured natural constants of nature, such as the speed of light in a vacuum and the
Planck constant, with increasing accuracy. They discovered that these are more stable than physical
objects, and fixed numerical values to the constants. These natural constants do not vary, so are at
least one million times more stable.

It has been the aspiration of the measurement community to move to a complete measurement
system redefined without physical artefacts. This definition marks the end of the process and an
historic moment as the last artefact, the International Prototype of the Kilogram, will be retired and
the kilogram defined in terms of Planck’s constant.

Why do we need more accurate definitions?

As science advances, ever more accurate measurements are both required and achievable. The
standard and definition must reflect this increasing accuracy. The kilogram has been based on a
physical object certified in 1889 using industrial revolution, consisting of a cylinder of platinumiridium,
and it is the last unit to be based on an actual object. Its stability has been a matter of
significant concern, resulting in recent proposals to change the definition to one derived from
constants of nature.
We are at the beginning of the quantum revolution. By defining measurement units in terms of
constants means that the definitions of the units are fit for purpose for this next generation of
scientific discovery.

What are the seven base units?

The kilogram (kg) – the SI base unit of mass
The metre (m) – the SI base unit of length
The second (s) – the SI base unit of time
The ampere (A) – the SI base unit of electric current
The kelvin (K) – the SI base unit of thermodynamic temperature
The mole (mol) – the SI base unit of amount of substance
The candela (cd) – the SI base unit of luminous intensity
Further information on how to use SI units:

What is the SI redefinition?

The global metrology community anticipates that a revision to the SI units will be agreed in 2018,
when the General Conference on Weights and Measures (CGPM) meets from 13–16 November.
This decision is expected to mean a more practical definition of the SI. All of the units would be
expressed in terms of constants that can be observed in the natural world (for example, the speed of
light in a vacuum, the Planck constant and the Avogadro constant). Using these unchanging
standards as the basis for measurement will mean that the definitions of the units will remain
reliable and unchanging into the future.
Information on the constants the SI units:

Why do we need this change?

It has been the aspiration of the measurement community since the Age of Enlightenment to have a
universally-accessible system. The use of physical artefacts for this has always been a practical

approach, but one that the community wanted to move away from as soon as possible. Physical
artefacts can be unstable, as they are vulnerable to both human and environmental damage.
This revision of the SI will, for the first time, see all base units in the SI defined by the constants of
physical science that we use to describe nature. Using the constants we have found in nature as our
universal basis for measurement allows not only scientists, but also industry and society, to have a
measurement system that is more reliable, consistent, and scalable across quantities, from very large
to very small.
There are two key ways the SI will change to create a more stable and future-proof basis for

  •  It will take physical artefacts out of the equation: the kilogram is still defined by a physical
    object equal to the mass of the International Prototype of the Kilogram (IPK), an artefact
    stored at the International Bureau of Weights and Measures (BIPM) in France. This revision
    will finally remove the need for this last artefact.
  • For the first time, all the definitions will be separate from their realizations: instead of
    definitions becoming outdated as we find better ways to realize units, definitions will remain
    constant and future-proof. For example, the ampere is currently defined as “the magnetic
    force between two wires at a certain distance apart”, which means that it uses the
    realization of a measurement to define it. However, advancements like the advent of the
    Josephson and quantum Hall effects, have revealed better ways of realizing the ampere,
    making the original approach obsolete.

Will the seven units change as part of the Revised SI?
No. The seven base units (second, metre, kilogram, ampere, kelvin, mole and candela) and their
corresponding base quantities (time, length, mass, electric current, thermodynamic temperature,
amount of substance and luminous intensity) remain unchanged.

Which are changing?

The kilogram (kg), ampere (A), kelvin (K), and mole (mol) will have new definitions.
The new definitions affect four of the base units:
The kilogram in terms of the Planck constant (h)
The ampere in terms of the elementary charge (e)
The kelvin in terms of the Boltzmann constant (k)
The mole in terms of the Avogadro constant (NA)

Defining the kilogram in terms of fundamental physical constants will ensure its long-term stability,
and hence its reliability, which is at present in doubt.

What about the definitions of the other units?

The definitions of the second (s), metre (m), and candela (cd), will not change, but the way the
definitions are written will be revised to make them consistent in form with the new definitions for the kilogram (kg), ampere (A), kelvin (K), and mole (mol). These new wordings are also expected to
be approved at the 26th CGPM in November 2018 and to come into force on 20 May 2019.

What about the 22 coherent derived units with special names and symbols?
These will remain unchanged in the Revised SI.

When will the proposed change come into effect?
Redefinition, if agreed, will come into practice on World Metrology Day, 20 May 2019.

What does this mean in practice?

On the surface, it will appear that not much has changed. In the same way that if you replaced the
decaying foundations of a house with robust new ones, it may not be possible to identify the
difference from the surface, but some substantial changes would have taken place to ensure the
longevity of the property.
These changes will ensure that the SI definitions will stand the test of time, despite advancements in
technology, but will continue to remain robust.
Information for users of the SI:

What and when is World Metrology Day?

World Metrology Day is an annual event on 20 May during which more than 80 countries celebrate
the impact measurement has on our daily lives.
On this day, the international metrology community, which works to ensure that accurate
measurements can be made across the world, raises awareness of the impact and importance of
having reliable measurements. The theme for World Metrology Day 2018 was ‘Constant Evolution of
the International System of Units (SI)’.

The date marks the beginning of a formal international collaboration in metrology in 1875, when the
first international measurement treaty, the Metre Convention, was signed by representatives from
17 nations to agree on the coordination of measurement.

This treaty saw the creation of organisations to oversee the running of the BIPM including the
General Conference on Weights and Measures (CGPM).
Information on World Metrology Day:

Who agrees on the SI and any proposed changes?

The signing of the Metre Convention in 1875 saw the creation of the International Bureau of Weights
and Measures (BIPM). It operates under the supervision of the International Committee for Weights
and Measures (CIPM), which is itself set under the authority of the General Conference on Weights
and Measures (CGPM).
The CGPM meets every three to six years. With delegates from all of the 60 member states, this body
discusses and chooses to endorse changes to the SI, after taking on board advice from the CIPM.
The CIPM is a committee made up of 18 individuals, each of a different nationality, nominated by the
CGPM for their high level of understanding in the field. The CIPM is still to this day responsible for

decisions about the SI, with the goal of creating a reliable basis for measurement that can be used
now and into the future. The international community now includes 60 Member States, and 42
Associate States and Economies.

The BIPM is based in Sèvres where it has laboratories that provide metrology services for the
Member States. It also carries out coordination and liaison activities and houses the secretariat for
the CIPM and its consultative committees. The BIPM’s original purpose was to create and house the
international prototypes defining units of what is now known as the SI, and as such it is where the
International Prototype of the Kilogram resides.

What is the proposal for the change?

The full resolution:
In short, at the 26th meeting of the CGPM, delegates will be asked to vote to accept the revision of
the SI. A resolution which invited the CIPM to propose a revised SI was put forward in 2011 at the
24th CGPM. The resolution laid out, in detail, a new way of defining the SI based on a set of seven
defining constants, drawn from the fundamental constants of physics and other constants of nature.
Since the meeting in 2011, the conditions of the resolution have been met and confirmed, and so the
proposal can now go ahead.

What impact does the redefinition have on the realization of the kilogram?

The kilogram will be defined in terms of the Planck constant, guaranteeing long-term stability of the
SI mass scale. The kilogram can then be realized by any suitable method (for example the Kibble
(watt) balance or the Avogadro (X-ray crystal density) method). Users will be able to obtain
traceability to the SI from the same sources used at present (the BIPM, national metrology institutes
and accredited laboratories). International comparisons will ensure their consistency.
The value of the Planck constant will be chosen to ensure that there will be no change in the SI
kilogram at the time of redefinition. The uncertainties offered by NMIs to their calibration customers
will also be broadly unaffected.

What impact does the redefinition have on the realization of the ampere?

The ampere and other electrical units, as practically realized at the highest metrological level, will
become fully consistent with the definitions of these units. The transition from the 1990 convention
to the revised SI will result in small changes to all disseminated electrical units.
For the vast majority of measurement users, no action need be taken as the volt will change by about
0.1 parts per million and the ohm will change by even less. Practitioners working at the highest level
of accuracy may need to adjust the values of their standards and review their measurement
uncertainty budgets.

What impact does the redefinition have on the realization of the kelvin?

The kelvin will be redefined with no immediate effect on temperature measurement practice or on
the traceability of temperature measurements, and for most users, it will pass unnoticed. The
redefinition lays the foundation for future improvements. A definition free of material and
technological constraints enables the development of new and more accurate techniques for making
temperature measurements traceable to the SI, especially at extremes of temperature.

After the redefinition, the guidance on the practical realization of the kelvin will support its worldwide
dissemination by describing primary methods for measurement of thermodynamic
temperature and equally through the defined scales ITS-90 and PLTS-2000.

What impact does the redefinition have on the realization of the mole?

The mole will be redefined with respect to a specified number of entities (typically atoms or
molecules) and will no longer depend on the unit of mass, the kilogram. Traceability to the mole can
still be established via all previously employed approaches including, but not limited to, the use of
mass measurements along with tables of atomic weights and the molar mass constant Mu.
Atomic weights will be unaffected by this change in definition and Mu will still be 1 g/mol, although
now with a measurement uncertainty. This uncertainty will be so small that the revised definition of
the mole will not require any change to common practice.

Will there be any change to the realization of the second, the metre and the candela.
The second will continue to be defined in terms of the hyperfine transition frequency of the caesium
133 atom. The traceability chain to the second will not be affected. Time and frequency metrology
will not be impacted.
The metre in the revised SI will continue to be defined in terms of the speed of light, one of the
fundamental constants of physics. Dimensional metrology practice will not need to be modified in
any way and will benefit from the improved long-term stability of the system.
The candela will continue to be defined in terms of Kcd, a technical constant for photometry and will
therefore continue to be linked to the watt. Traceability to the candela will still be established with
the same measurement uncertainty via radiometric methods using absolutely-calibrated detectors.

Who will vote and how will the vote take place?

The vote will follow the reading of the proposal on 16th November ~ 13.00 UTC.
The 60 Members States will be invited to vote and the proposal needs a majority to be accepted. We
are, however, expecting all 60 States to agree to this change as work has been truly international in
nature and the potential for the change historic.
Follow the vote live on BIPM’s YouTube channel at 10:50 UTC on 16 November 2018:–gjcrk5ZcWbHVQ

What will happen to the kilogram after the vote?

The International Prototype of the Kilogram (IPK) will remain where it is under the same conditions.
It is an historic artefact that has been under study for 140 years and will retain a bit of metrological
interest even though its mass will no longer define the kilogram.
All other nations holding kilograms will also keep theirs in the same way, as the use of physical
kilogram artefacts will remain an important part of traceability to the new definition of the kilogram
for some years to come – until such experiments to realize the kilogram are readily available by other

When has the International Prototype of the Kilogram (IPK) been used?

Starting in 1885, the kilogram has been used in three calibration campaigns, each lasting several
years. The third of these ended in 1993. In preparation for the upcoming redefinition, the kilogram
was used again in 2014 to supply traceable values to NMIs working on replacing it. This may seem
strange but it helps ensure that the size of the redefined kilogram will be the same as the size of the
present kilogram. Nevertheless, handling and environmental conditions are not conducive to the
mass of an artefact remaining stable. The kilogram is held in a safe which requires the presence of
three people who hold keys for access – the Director of the BIPM, the President of the International
Committee of Weights and Measures (CIPM) and the Director of the French Archives. This procedure
is overseen by the 18 members of the International Committee of Weights and Measures. This last
happened in 2014.

Has the IPK changed?

It is difficult to quantify this as the kilogram, by definition, is a kilogram! The question could be: Has
the mass of the kilogram been seen to change with respect to any constant of nature? The answer is
“not yet”. This is because until quite recently such comparisons were impossible to carry out with the
necessary precision.
But comparisons with other kilograms held in the same conditions show us that over the past 30
years there have been no detectable changes.
However, from 1889 through 1993, the detected differences in mass between supposedly identical
artefacts average out to be about 50 µg per century. The reason for the changes and the reasons the
changes have stopped (or paused) are, and probably will remain, a mystery. These are questions that
artefact definitions invite but do not exist for the redefined kilogram.

Will I get my standard of mass calibrated after the change?

After the redefinition of the kilogram, laboratories will continue sending mass standards to their
National Metrology Institute (NMI) for calibration or to a secondary calibration laboratory, just as
now. However, the traceability path that the NMI will use to link it to the SI kilogram will change and
be a realization of the new definition.
Information for users of the SI:

How can we be sure that laboratories’ realizations of the kilogram are correct?

As with the inter-comparison mechanism that already exists and is widely used for all other units, all
laboratories claiming to realize the kilogram will need to demonstrate traceability to the definition of
the kilogram, by comparing results with their peers. This approach is set out in CIPM Mutual
Recognition Arrangement established in 1999.