# Where does weight mass conversion ratio '0.00259' come from in Nastran?

I think this is very fundamental question.

I am wondering why 0.00259 is used as weight-mass conversion ratio, when inch/pound unit is used in Nastran.

Could anyone explain the reasons?

It's the inverse of the acceleration of gravity g in imperial units. $$1 g = 386.1 in/s^2$$
$$1/g = 0.00259 s^2/in$$
$$weight = mass * g$$