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A calculated net value is the difference between a measured weighing value, gross or net, and a preset tare value. The key distinction is the origin of the subtracted quantity: it is entered or recalled numerically rather than necessarily being a freshly measured tare for that individual container. OIML R76-1:2006 defines these terms separately from an ordinary net indication after operation of a tare device.^[ref]

Write the arithmetic aswhere $PT$ is the stored preset tare value. Here the weighing values are expressed as mass estimates, for example in kilograms; they are not forces expressed in newtons. The instrument determines mass using gravity under its measurement model.^[ref]

For a chosen packaging example, let a loaded container give $2.480\mathrm{kg}$ and let the stored nominal container tare be $0.180\mathrm{kg}$. The calculated net value is then $2.300\mathrm{kg}$. If this particular empty container actually has mass $0.195\mathrm{kg}$, its contents have ideal net mass $2.285\mathrm{kg}$. The calculated indication overestimates that value by $0.015\mathrm{kg}$. The subtraction has been performed correctly; the discrepancy arises because a nominal tare was substituted for an individual one.

More generally, let the true contents' mass be $m$, the actual container mass be $t$, and the measured gross value be $m+t+e$, where $e$ is measurement error. ThenRepeating weighings can assess variation in $e$, but it does not by itself remove an incorrectly chosen $PT$. A useful check therefore weighs representative empty containers as well as repeating loaded measurements. Report the stored tare and what it represents along with the result. This makes the physical assumptions behind a calculated net value visible rather than treating a precise-looking display as evidence that every container has the nominal tare mass.

A preset tare device subtracts a preset tare value from a measured gross or net indication and displays the calculated result.^[ref] A stored container value can therefore be reused without weighing each empty container individually.

Suppose a container and its contents have measured gross mass $G=2.30\mathrm{kg}$ and the entered preset tare value is $PT=0.30\mathrm{kg}$. Subtraction gives a calculated net mass of $2.00\mathrm{kg}$. If this particular container actually has tare mass $T=0.34\mathrm{kg}$, however, its contents have mass $1.96\mathrm{kg}$. Even an otherwise exact gross measurement would then overestimate the contents by $0.04\mathrm{kg}$.

The error follows without any instrument-specific assumption:A mismatch in the entered container value passes directly into the result. Reusing one value for nominally identical containers is therefore an assumption about their tare masses, rather than a new measurement of each container.

The starting indication also matters. If an ordinary tare device has already removed a $0.30\mathrm{kg}$ container contribution, the displayed $2.00\mathrm{kg}$ is already net for that reference. Subtracting the same $0.30\mathrm{kg}$ again would give $1.70\mathrm{kg}$. A deliberate second subtraction can represent another distinct component, but applying the same container correction twice is a different calculation from removing its tare mass once.

A preset tare value is a numerical tare mass entered or recalled for use in subsequent weighings without individually determining each container's tare. The pinned OIML terminology denotes it by $\mathrm{PT}$ and calls the measured gross or net indication minus this value a calculated net value. It can be supplied through a preset tare device rather than obtained by weighing the current empty container.^[ref]

For an original example, a stored container tare is $\mathrm{PT}=250\mathrm{g}$, but the actual empty container has tare mass $T=263\mathrm{g}$. Adding $500\mathrm{g}$ of material gives gross mass $G=763\mathrm{g}$. Applying the stored value givesalthough the actual net mass is $N=G-T=500\mathrm{g}$. Their difference isAn underestimated preset tare therefore overestimates the contents. Repeating the same discrepancy across ten containers produces a $130\mathrm{g}$ total discrepancy; more displayed digits do not remove it.

A useful physical check independently weighs representative empty containers, including the same lids and attachments used during the loaded measurement, and compares them with the stored value. Record which load components the preset represents. If it is subtracted from an indication that has already had those components removed by a tare device, the same tare mass is deducted twice. The stored number's provenance and variability belong in the measurement model and its measurement uncertainty.

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.