pascal
Pressureexact by definition
The pascal is the coherent SI unit of pressure and stress, one newton per square metre. Its factor is 1 by construction and the unit is exact, since it is built from base units with no numerical constant. It is a very small unit: normal atmospheric pressure is about 101 325 Pa.
| 1 Pa | 0.001 kPa |
Named for Blaise Pascal, who in the 1640s demonstrated that atmospheric pressure falls with altitude by having a barometer carried up the Puy de Dôme. The name was adopted by the CGPM in 1971.
About the pascal
A pascal is one newton spread over one square metre, which is about the pressure a sheet of paper exerts lying flat on a table. That smallness is not a design flaw but a consequence of coherence: the SI refuses to insert arbitrary factors, so the unit lands where the base units put it. Practical work therefore lives in prefixed forms. Weather and process pressures use kilopascals or hectopascals, material strengths use megapascals, and elastic moduli use gigapascals.
The same unit measures pressure and stress, because both are force per unit area, and that shared unit is genuinely useful. A steel with a yield strength of 350 MPa is telling you that a bar of 1 mm² cross-section fails at 350 N, since \(1\ \mathrm{MPa} = 1\ \mathrm{N/mm^2}\) exactly. That identity is the most quietly convenient fact in metric structural design, and it is worth memorising: newtons per square millimetre and megapascals are the same number.