A step-down transformer has a turns ratio less than 1.

Ns/Np is known as the turns ratio.
In step down transformer the number of turns in secondary winding (Ns) will be less than the number of turns in primary (Np).
Therefore, the ratio will be less than 1 for step down transformer.
Ex: Np = 10, Ns = 5; Turns ratio = 5/10 = 0.5.