The maximum hypothetical vacuum at sea level is 29.92 in-Hg, actual pump capabilities are based on and compared to this theoretical value. Depending upon pump design, the vacuum limit ranges from 28 to 29.5 in -Hg, or about 93% or 98% of the maximum abstract value. The vacuum level is determined by the pressure difference between the evacuated volume and the surrounding atmosphere.
Several units of measure can be used. The standard metric unit for a vacuum measurement is the millibar, or mbar. The higher up you are, the less volume you can attain. This is because free air is dense at higher altitudes, which has the effect of reducing the capacity and maximum vacuum levels.