Where does "12" come from in bending stress fomula

[ZHWIL28]

I am wondering what is the meaning of "12" in the equation of bending stress for pressure vessels. S=12M/(I/c)
In the usual stress fomula, only M/(I/c). Is it safety factor?
Thanks to make me clear!

 

[jte]

ZHWIL28-

You didn't include units in the formula. I think what you have is a moment calculated in foot-lb and converted to in-lb (12 inches per foot) in the stress calc.

jt

 

[DSB123]

ZHWIL28,
As stated by jte you must either state Code used or the units of the equation.

 

[ZHWIL28]

Thanks, It is a shame not to take care of units.