Net mass ($m_{\mathrm{n}}=m_{\mathrm{g}}-m_{\mathrm{t}}$)

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 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.