This paper presents a methodology and the results of a study to characterize variabilities in temperature dependent thermal conductivity, as defined in EN 13381-8, and dry film thickness (DFT) for one type of intumescent coating under the standard fire condition, and their influences on reliability of intumescent coating protected steel members. The variabilities in temperature dependent thermal conductivity and DFT of the coating were found to be best described by lognormal and normal probability distributions respectively. Monte Carlo simulations were carried out to determine associated probabilities of structural failure based on comparisons of steel limiting temperatures. The EN 13381-8 variable thermal conductivity method was found to give temperature dependent thermal conductivity of the intumescent coating that leads to a probability of failure of 0.1, corresponding to a reliability index of 1.28. This level of reliability may not be sufficient (i.e. not able to achieve the required level of safety), depending on the conditional probabilities of fire occurrence and flashover. To achieve lower probabilities of failure (and thus higher reliability), this paper suggests using a safety factor of greater than 1.0 to multiply the variable thermal conductivity obtained using EN 13381-8. For example, to achieve target conditional probabilities of failure of 0.001 and 0.01, the safety factors are 1.7 and 1.4 respectively.