In round numbers atmospheric pressure is 30 inches (of mercury in a
barometer) and is also 15 psi (pounds per square inch). Hence gas
pressure that is 10 inches is about 5 psi. In round numbers, we can
think 2 inches per psi. So gas pressure that has been reduced by
regulator to 2 psi is about 4 inches (of mercury).
To put this into perspective, for my Smith Little Torch, I run my
acetylene at 2 psi and my oxygen at 5 psi.