Numerical calculations of room fire temperature for t-squared fires were carried out by zone model reflecting the room conditions such as room size, wall construction material, opening size and the fire load density. Based on the numerical results, the correlations for flashover occurrence condition are established, in which the fuel surface area and the temperature factor are used as a room size parameter. A simplified formula for determining the critical heat release rate for flashover is also suggested. The wall construction material has little influence on flashover occurrence. For the boundary of flashover in fuel-controlled fires, as the temperature factor becomes larger, the fuel surface area which causes the flashover occurrence becomes smaller. For ventilation controlled fires, critical fuel surface area per unit internal surface area of room corresponds with temperature factor. As the flashover time is smaller, the critical heat release rate for flashover is increased. By comparing the derived formula with the existing experimental data, it was shown that proposed formula corresponds with lower bound of existing experiments. © 2013 International Association for Fire Safety Science.