Extreme flood events often lead to considerable loss of human life, and previous studies on the stability criteria for people in floods do not account for the effect of various slopes. The proposal of an appropriate stability criterion for a range of slopes is therefore important. In this study a mechanics-based incipient velocity formula was extended for a human body in floodwater at toppling instability, with the ground slope being included. Using a laboratory flume, about 250 experimental tests were conducted to obtain water depth and velocity conditions at toppling instability for a small-scale model human body for four slopes. The experimental data obtained for flat ground and slopes of 2 and 4% were used to determine values for two parameters in the derived formula. These parameters were unified to corresponding mean values for various slopes. The calculations using the proposed formula and the unified parameters compare well with the scaled-up experimental data for a slope of 2.5%. Finally, the proposed formula was used to estimate critical velocities under different depths for a typical adult. The differences in stability criteria between the results obtained from the model and prototype experiments are also presented.