= Net mass
{title2=$m_{\mathrm n}=m_{\mathrm g}-m_{\mathrm t}$}
<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 is
$$
m_{\mathrm n}=m_{\mathrm g}-m_{\mathrm t},
$$
where $m_{\mathrm g}$ 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.<Non-automatic weighing instruments, Part 1>[OIML R76-1:2006, T.5.2.1–T.5.2.3, gross/net/tare indications; T.5.3.1–T.5.3.2, preset tare and calculated net value, printed p16]{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 is
$$
0.842-0.217=0.625\,\mathrm{kg}.
$$
An 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.<Non-automatic weighing instruments, Part 1>[T.5.2.2 and T.5.3.1–T.5.3.2, net indication versus calculated net 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.<Non-automatic weighing instruments, Part 1>[T.1.1, weighing instrument and mass/weight-value terminology, printed p5]{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 $T=m_{\mathrm t}+e_t$. Then
$$
G-T=m_{\mathrm n}+e_g-e_t.
$$
A 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.
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