This question seems to come up a lot, so I figured I would put it in it's own thread.
Manifold Air Pressure (or boost/vacuum) can be measure in a variety of units, these include MPa, bar, atm, psi, and inHg (inches of mercury). If I left anything out, let me know, but I think I covered all of the usual suspects.
0.1 MPa = 1.0 bar = .987 atmospheres = 14.5 psi = 29.53 inHg
These are the real unit conversions from .1 MPa to all the others with minimal rounding.
1 atm = 14.7 psi for further reference.
When the boost gauge reads 14.7 psi, that mean that you have twice the nominal atmospheric pressure available to your intake valves. When the gauge reads 0 you are at atmospheric pressure. When the gauge reads negative, you engine is drawing a vacuum on the intake manifold, just like a naturally aspirated car always does.
I hope this clears up any confusion for our members that are new to the world of turbocharging.
p.s. A Pascal (Pa) is the metric unit for pressure equal to 1 Newton of force distributed across 1 square meter of area (N/m^2). A MegaPascal (MPa) is 1 million Pascals or 1,000,000 Newtons / 1 meter^2.