Helical coil tension springs made of shape memory alloy (SMA) materials generally undergo large deflection under loading during which their mean coil diameter changes noticeably. In the design of these helical coil springs, it is necessary to identify the real behaviour which may be affected by the variation in actual coil diameter. Therefore, a simple formulation is proposed in this paper for predicting instantaneous coil diameter. The predictions from present formulation match very closely with experimental measurements. The proposed formulation is relatively easy to adopt for design calculations. The effects of varying coil diameter on the spring characteristics are also discussed. This is very general and can be used for any helical spring which undergoes small or large deflections, although the proposed formulation is derived for SMA helical spring. Copyright © 2020 Inderscience Enterprises Ltd.