According to the IH Research Foundation:
Intracranial pressure can be measured in millimeters (mm)/water or centimeters (cm)/water. For example, a reading of 200mm/H2O is equal to 20cm/H2O.
For adults:
Normal pressure readings are generally below 200mm/H2O.
Borderline high pressure readings are between 200-250mm/H2O.
Anything above 250mm/H2O is considered a high pressure reading.