AbstractAimVarious factors have been shown to potentially affect the difference between axillary and rectal temperature measurements in newborns. We aimed to explore their roles and, if possible, to construct a formula that explained the difference.MethodsThe study was based on a consecutive sample of 175 infants, with a gestational age of 24–42 weeks, whose rectal and axillary temperatures were measured simultaneously at the neonatal unit at Skaraborg Hospital in Sweden. Data were analysed using multiple regressions.ResultsPremature infants had a significantly smaller mean difference (0.33°C) between rectal and axillary temperatures than full‐term infants (0.43°C). Significant associated factors for premature infants were chronological age (p = 0.025), time of day (p = 0.004) and axillary temperature (p < 0.001). For full‐term infants, the only significant associated factor was axillary temperature (p = 0.015).ConclusionAlthough it is possible to construct a formula that estimates neonate rectal temperature based on axillary temperature with a slightly higher reliability than simply adding a fixed value like 0.4°C, such a formula would be too complex to apply in practice. Adding 0.3°C or 0.4°C to the measured axillary temperature for premature infants or full‐term infants, respectively, yields acceptable approximations in most cases.