The reconstruction of past temperatures is often achieved through measuring the Mg/Ca value of foraminiferal test carbonate. The diversity in foraminiferal Mg/Ca–temperature calibrations suggests that there is also a biological control on this proxy. This study presents a new Mg/Ca–temperature calibration for the benthic foraminifer Bulimina marginata, based on cultures under a range of temperatures (4–14 °C). Measured Mg/Ca values for B. marginata correlate with temperature (Mg/Ca = (1.10 ± 0.10) e(0.045±0.009)T, R2 = 0.28 p < 0.01). The inter-individual variability is, however, also significant (standard deviation is 10–35% of the average). Before applying this or any calibration, the effect of the inter-individual variability on the accuracy of the Mg/Ca–temperature calibration has to be evaluated. The inter-individual variability is quantified and split into three components, namely (1) an analytical error, (2) an environmental effect and (3) a vital effect. The effect of inter-individual variability on the accuracy of Mg/Ca–temperature calibrations depends on the sensitivity of the calibration used and the number of individuals measured (temperature uncertainty = (0.33 · N−0.50)/sensitivity). The less sensitive a calibration, the greater is the impact of inter-individual variability, which can partly be circumvented by measuring more individuals. This study shows the link between inter-individual variability and sensitivity and quantifies their influence on the accuracy of Mg/Ca–temperature calibrations. Differences in the sensitivity of the Mg/Ca–temperature calibration of foraminifera may depend on the environmental conditions in which foraminifera live and their concurring ecological strategies.