Incorporating a measure of stability into a portable sensor has a number of clinically relevant applications. This study investigated the feasibility of developing a real-time assessment tool to monitor balance during quiet standing. Forward and backward perturbations were performed on 16 able-bodied subjects using a pulley system attached to the subjects' waist. A linear relationship was found between the maximum centre of pressure (COP) velocity and the maximum COP position caused by the perturbations. As the maximum COP velocity occurs considerably before the maximum COP position, this model serves as an instability predictor. A sensor that measures COP velocity can use the model to predict a maximum COP position and compare it with an instability threshold. Results show that the proposed model is a viable solution to quantitatively measure stability in real-time.