The mobility of a vehicle on deformable soil mainly depends on tyre soil interaction and its contact area. The larger the contract area, the lower is the ground pressure and hence greater the mobility on deformable soil. Knowledge of contact area at the tyre soil interface is very much necessary while determining mobility of a vehicle. This paper establishes a generalised relation for determination of contact area between sand tyre and deformable soil based on measurements of contact area of 145/70R20 tyre on a soil bin. It compares the results obtained by different authors on the subject with those using the relation reported in this paper and justifies universality of the relation developed.