A series of experiments were performed to measure heat transfer to a cylindrical steel calorimeter engulfed in a 30-minute pool fire. The calorimeter inner surface temperature history was measured at 46 locations. A one-dimensional inverse heat conduction technique was used to determine the net heat flux to the calorimeter as a function of time and location. The uncertainty in heat flux caused by three-dimensional effects is estimated using finite element computer simulations. A Monte Carlo uncertainty simulation is used to estimate the uncertainty in heat flux from propagated uncertainties in dimensions, temperature measurements, and material properties. The estimated uncertainty in the measured heat flux over the 30-minute fire test and the entire calorimeter was found to be ± 18 kW/m2, or 27% of the average heat flux of 66.6 kW/m2. The uncertainties for the early times of the fire test are less than those at later times in the test due to the instability of the inverse conduction calculations caused by the Curie effect of the carbon steel calorimeter material.