Competitive bidding is a common practice in business. The main aim of this research is to develop a practical bidding model. This includes studying existing bidding models used by organisations and verifying the developed model. Various factors have been identified and analysed in order to identify the weight and ranking order of each factor in terms of its influence on the bid mark up size. Interviews have been conducted to explore the current practical practice in setting mark up size in Saudi Arabia and to identify factors that may influence bid mark up size. In addition, a questionnaire has been used to identify each factor's level of importance in Saudi Arabia. The findings have been used to establish a ranking order of factors in terms of their influence on bidding decisions based on contractors' size and main client. An important discovery is that the level of importance and rank of factors that influence bid mark up size differ based on contractors' characteristics and main client. The characteristic which has been investigated in this research is the size of the contractor. As a result, a bidding model to determine mark up size based on contractors' size and main clients has been developed. The proposed model has been tested and proved accurate in simulating the contractors' decisions.