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The kilogram is the SI base unit of mass. The modern definition fixes the numerical value of Planck constant to $h=6.62607015\times 10^{-34}\mathrm{J}\mathrm{s}$, with the metre and second defined through the other SI constants. It is no longer defined by the mass of a particular metal artifact.

A realization connects the defining constant to a practical mass measurement. That experiment has uncertainty even though the constant's defining numerical value is exact. The gram and milligram are decimal submultiples of the kilogram.^[ref]

Net mass is the mass attributed to the material of interest after excluding the container or other specified tare mass. For a container-and-contents measurement, the ideal bookkeeping relation iswhere $m_{\mathrm{g}}$ is the gross mass of the complete load and $m_{\mathrm{t}}$ is the mass excluded as tare. State which parts of the load belong to each quantity.^[ref]

For an own weighing example, a container and its contents give a mass estimate of $0.842\mathrm{kg}$, while the corresponding empty container gives $0.217\mathrm{kg}$. Assuming comparable conditions and no change in the container, the contents' estimated net mass isAn instrument's tare operation can instead establish the container reading as its reference before displaying a net indication for the added contents. The recommendation distinguishes this operation from subtracting a stored preset tare value.^[ref]

The physical apparatus is a weighing scale: it determines mass using gravity under its measurement model. The fact that weighing terminology sometimes says weight value does not turn the result in kilograms into a force in newtons.^[ref]

Subtraction also exposes how errors enter. In a simple additive model, let the two estimated masses be $G=m_{\mathrm{g}}+e_g$ and $T=m_{\mathrm{t}}+e_t$. ThenA stable common additive offset cancels when $e_g=e_t$; unequal errors do not. For example, overestimating the tare by $0.005\mathrm{kg}$ while leaving the gross estimate unchanged underestimates net mass by the same amount.

Real measurements additionally involve resolution, drift, measurement uncertainty and the definition of the load. The exact algebraic relation between ideal quantities is not a promise that independently rounded display readings always reproduce the same final digit. Record the relevant tare and measurement conditions along with the net result, rather than treating net mass as a new kind of matter independent of how the excluded mass was defined.

An SI prefix supplies an exact decimal factor when attached to a unit of measurement. For example, milli means $10^{-3}$ and micro means $10^{-6}$; a millimetre and a micrometre are correspondingly scaled metres. The prefix symbol and unit symbol form one unit that can itself be raised to a power.^[ref]

For an original conversion example,The area conversion squares the factor as well as the unit. Applying just one factor of $10^{-3}$ to that area would give $0.004{\mathrm{m}}^2$, a value 1000 times too large. Scientific notation makes the power explicit.

Prefix symbols are case-sensitive and cannot be stacked. For mass, prefixes attach to gram, because the base-unit name kilogram already contains a prefix. SI prefixes denote powers of ten, even in a computing context; they do not silently change into powers of two.^[ref] An exact unit conversion scales any stated measurement uncertainty with the value; it does not make an uncertain measurement exact.

A system of units chooses reference units of measurement for a set of base physical quantities and specifies how other units of measurement are derived. In the International System of Units, length, mass and time are expressed using the metre, kilogram and second; a velocity has derived unit $\mathrm{m}{\mathrm{s}}^{-1}$.

A consistent system allows an equation to connect numerical values without hidden unit-conversion factors. It does not change the underlying physical relation: the same speed may be written as $1\mathrm{m}{\mathrm{s}}^{-1}$ or $3.6\mathrm{km}{\mathrm{h}}^{-1}$. The familiar formulas for electromagnetism can nevertheless contain different constants in SI and Gaussian conventions, because the definitions of the electrical quantities differ as well as their numerical scales.^[ref]

Tare mass is the specified container or packaging mass excluded when reporting net mass. It is also called tare weight in weighing terminology. The intended boundary matters: a jar alone and that jar with its lid have different tare masses. A tare device adjusts a weighing scale's indication with a load present, whereas a zero-setting device establishes zero with the load receptor unloaded.^[ref]

A simple demonstration separates the reading from the physical load. Choose an empty container of mass $m_{\mathrm{t}}=0.120\mathrm{kg}$ and an ideal weighing scale that initially reads zero. After the container is placed on the load receptor, operate the tare device. Its displayed value becomes zero while the container's mass remains $0.120\mathrm{kg}$. In a uniform gravitational field with chosen magnitude $g=9.81\mathrm{m}{\mathrm{s}}^{-2}$, its gravitational weight is stillThe supporting force remains present after the display adjustment.

In an ideal unit-gain measurement model, the tared indication for a complete load of mass $m$ is $I(m)=m-m_{\mathrm{t}}$. Adding $0.300\mathrm{kg}$ of contents gives gross mass $0.420\mathrm{kg}$ and indication $0.300\mathrm{kg}$. Removing the complete load while retaining that reference gives indication $-0.120\mathrm{kg}$. The negative indication expresses the difference from the stored reference; the removed container still has positive mass.

A preset tare device instead uses an introduced preset tare value, which can be recalled or entered without weighing each individual empty container.^[ref] For the same chosen load, using a preset of $0.100\mathrm{kg}$ gives calculated net mass $0.420-0.100=0.320\mathrm{kg}$: the contents are overestimated by 20 g. A persistently low preset produces a systematic error even with otherwise exact readings.

Composite tare mass includes several excluded components, such as all the jars and an outer crate. The gross mass example shows how to account for these components once each. The symbol $m_{\mathrm{t}}$ here denotes a mass; the terminology's use of the word weight does not change the quantity's unit from a kilogram to a unit of force.

A unit of force specifies the reference magnitude used to express a force. The SI derived unit is the newton, defined by $1\mathrm{N}=1\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-2}$, consistent with Newton's second law.

A force of one newton gives a one-kilogram body an acceleration of one metre per second squared when it is the net applied force. A kilogram is a unit of mass, so it cannot replace the newton without also specifying an acceleration or a conventional gravitational reference.^[ref]

A unit of mass is a reference for expressing mass. The SI base unit is the kilogram; the gram is $10^{-3}\mathrm{kg}$. A unit of mass differs from a unit of force: a balance may infer mass from a gravitational force, but a newton measures force and is not another name for a kilogram.

In a weighing experiment the relation $F=mg$ connects the two, with local gravitational acceleration $g$. An instrument calibrated to display mass therefore incorporates a reference or an assumed local $g$. Moving a force-sensing instrument to a different gravitational environment can change that inference even though the specimen's mass is unchanged.^[ref]