Why are spin values given in terms of $ħ/2$ instead of $1/2$?

Question

In my book it talks about the magnitude of the spin component in the Stern-Gerlach experiment along the $z$-axis being + or $- ħ/2$. I understand this to mean that the deflection of silver atoms going through the stern-gerlach experiment experience a quantized deflection, equally either up or down (if measured in the $z$ axis). But this deflection should logically be based on the atom or the strength of the magnetic field etc of the Stern-Gerlach apparatus right? So what does $ħ/2$ actually mean as a spin component actually mean/come from?