hectopascal
An SI unit of pressure equal to 10² pascals. Symbol: hPa
hectopascal: the pressure unit meteorologists and engineers actually use
A hectopascal is a metric unit of pressure equal to 100 pascals, or 1 millibar. It is the standard unit for atmospheric pressure measurement in meteorology, aviation, and industrial process control. The symbol is hPa. One hectopascal represents the pressure exerted by about 1.02 grams of force per square centimeter, which makes it convenient for measuring pressures in the range where most practical engineering happens: roughly 800 to 1100 hPa for atmospheric conditions, or 0 to several hundred hPa for vacuum and low-pressure systems.
The hectopascal replaced the millibar in official SI terminology, though many technicians still use millibar interchangeably since they are numerically identical. A standard atmospheric pressure at sea level is 1013.25 hPa. Aircraft altimeters are calibrated in hectopascals. Barometers, pressure transducers, and weather instruments worldwide display readings in this unit. Industrial vacuum pumps are specified by their ability to reduce pressure from atmospheric (roughly 1000 hPa) down to 1 hPa, 0.1 hPa, or lower depending on the application.
The pascal itself is a small unit, equal to one newton per square meter. Multiplying by 100 to get a hectopascal makes the numbers more manageable in everyday practice. A pressure of 1 hPa is still quite low, close to a laboratory vacuum. Pressures above 2000 hPa are rarely encountered except in compressed gas cylinders or hydraulic lines, where bar or MPa become more practical.
Confusion arises because engineers in some fields prefer kilopascals (kPa) or bars for higher pressures, while meteorologists and aeronautical specialists stick with hectopascals. A pressure of 10 hPa equals 1 kPa, so conversion is straightforward. Analog pressure gauges in older installations may be marked in psi or bar, requiring manual conversion to hectopascals for regulatory documentation or calibration verification.
When calibrating pressure instruments, reference standards and test points are often quoted in hectopascals because the unit aligns with ISO 80000-4 and the SI standard. Pressure transducers sold for general industrial use are commonly specified with output ranges tied to hectopascal setpoints. Understanding this unit is essential for reading technical data sheets, setting alarm thresholds on process controllers, and communicating pressure values across international teams.