Inductive coupling wireless power transfer (ICWPT) is very appropriate to power-up wearable devices. This approach can eliminate the complications related to batteries and power cables, making the wearable device more comfortable for users. Nevertheless, rigid coils such us printed coils are troublesome to users for wearable applications. A flexible textile coil presented in this work is a promising and comfortable solution when integrated into clothing. We propose to approximate the spiral coil shape by a certain number of concentric circular windings, in order to simplify considerably the coil quality factor computation. Consequently, using this approximation, we can deduce easily the optimal coil parameters leading to reach the maximum quality factor. The obtained optimal solution of the coil parameters is expressed analytically in terms of the problem data.