The direction of the calculation matters
The NIST/SEMATECH handbook describes how a calibration curve converts a future instrument reading into a calibrated value. In the linear case, first subtract the estimated offset from the signal, then divide by the estimated slope. The calculation runs backward through the fitted relationship because the measurement supplies the signal and the unknown is the quantity of interest.
An example without laboratory instructions
Consider an invented, unitless teaching relationship: signal = 2 + 3 × quantity. A signal of 17 corresponds to quantity (17 − 2) ÷ 3 = 5. Dividing 17 by 3 alone gives about 5.67 because it ignores the offset. Subtracting 2 alone gives 15 because it ignores the scale. These numbers illustrate algebra only; they are not measurements, concentrations or a preparation recipe.
What to keep beside a reported concentration
A calculation is easier to evaluate when the report identifies the signal, the calibrated quantity and the relationship connecting them. Units belong to those definitions. Do not assume that a value reported by an instrument is already the final concentration, or silently treat an offset as zero because the resulting arithmetic is simpler.
A formula still has conditions
NIST notes that calibration depends on the instrument remaining in statistical control, and that chemical measurements often use a fresh curve for each batch. Its page also treats nonlinear relationships separately. Our small linear example cannot establish that a particular laboratory’s method is valid. The official page is undated; it was reviewed October 14, 2026, rather than presented as a new publication.