8 PSI is the differential, i.e. the difference between the inside and the outside. So the actual pressure inside the cabin is a bit more than 8 PSI. (8 plus whatever the environmental pressure at the airplane’s altitude is) Sea level pressure is around 15 PSI. At 40,000′ the cabin altitude is between 6,000′ (B787) and 8,500′ (older types).
Hope this helps.