Steam Assisted Gravity Drainage (SAGD) is a widely-used thermal oil recovery technique in western Alberta's oil sands reservoirs. Because of reservoir heterogeneity, the wellbore hydraulics and undulation, non-uniform steam chambers will evolve. Numerical simulation allows for the practical prediction of steam chamber size in SAGD. However, the long computation time in 3D scenarios and the impact of uncertainties in input parameters limit its application. In this paper, a numerical simulation based correlation between steam chamber size and temperature falloff data during shut-in was developed in the early period of the SAGD process which is before the moment that the steam chamber starts spreading laterally. The temperature falloff responses and the corresponding steam chamber sizes at different locations in the producer along the lateral were obtained though 3D numerical simulation studies. Based on the simulation results, a correlation among steam chamber, the temperature falloff rate and the height of liquid level in the producer was derived through regression analysis. The same correlation equation with different coefficients was also found at different shut-in times. The applicability of the proposed correlation in estimating steam chamber sizes along horizontal well is also investigated and validated. A synthetic case study shows that the chamber sizes obtained from the correlation and from simulation are in good agreement and also suggests that this correlation is applicable to estimate the chamber size distribution along the horizontal well at the early period of SAGD process. (C) 2015 Elsevier Ltd. All rights reserved.