How do you convert a blood pressure reading into pounds per square inch

when they check your 02 level they insert a needle into the wrist and the artery they go into has enough pressure to push the plunger up by itself. so there has to be a p.s.i. reading for that kind of force. so how do you convert the blood pressure reading into a different scale.

•1 mm mercury = 0.0193 lb/in2
so: 120 / 60 mm hg = 2.316 / 1.158 psig