The pascal is the SI unit of pressure, written Pa. It is defined as one newton of force spread over one square metre, and it is named after Blaise Pascal, whose experiments with barometers in the 1640s established that air has weight and that its pressure falls with altitude. The General Conference on Weights and Measures adopted the name in 1971.
It is a remarkably small unit. Atmospheric pressure at sea level is 101,325 pascals, so the pressure everyone lives under is a six-figure number, and almost every practical use of the unit therefore carries a prefix. That awkwardness is not a design fault but a consequence of coherence: the newton and the square metre were fixed first, and the pascal is whatever falls out of dividing one by the other.
Acoustics is the field where bare pascals are natural. The reference pressure for the decibel scale is twenty micropascals, taken as the quietest sound a healthy young ear can detect, and ordinary conversation is around 0.02 pascals of sound pressure. The threshold of pain sits near 63 pascals, so the entire range of human hearing spans about six orders of magnitude in this unit.
Everyday objects give a sense of the scale. A sheet of office paper lying flat presses on the desk with about 0.8 pascals. A gentle breeze exerts a few pascals on a wall, and the pressure difference that drives ventilation through a building is typically between ten and fifty. Anything a person can feel as force is already in the thousands.
Prefixed forms carry the real work. Weather uses hectopascals, which are numerically identical to the older millibars, so a forecast reading of 1013 needed no relearning. Engineering uses kilopascals for tyre and fluid pressures, and materials science uses megapascals and gigapascals for strength and stiffness, where one megapascal is one newton per square millimetre.
The pascal also appears wherever a stress rather than a pressure is meant, since the two have the same dimensions. Young's modulus, yield strength and shear stress are all quoted in pascals or their multiples, which is why a single unit spans the pressure in a tyre and the stiffness of steel.
One pascal equals 1 newton per square metre, 0.01 millibars, about 0.0000099 atmospheres, or about 0.000145 pounds per square inch.
| Unit | Symbol | 1 Pascal equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanopascal | nPa | 1000000000 nPa | 0.000000001 Pa | Pascals to Nanopascals |
| Micropascal | µPa | 1000000 µPa | 0.000001 Pa | Pascals to Micropascals |
| Millipascal | mPa | 1000 mPa | 0.001 Pa | Pascals to Millipascals |
| Centipascal | cPa | 100 cPa | 0.01 Pa | Pascals to Centipascals |
| Decipascal | dPa | 10 dPa | 0.1 Pa | Pascals to Decipascals |
| Newton per square meter | N/m² | 1 N/m² | 1 Pa | Pascals to Newtons per square meter |
| Decapascal | daPa | 0.1 daPa | 10 Pa | Pascals to Decapascals |
| Hectopascal | hPa | 0.01 hPa | 100 Pa | Pascals to Hectopascals |
| Kilopascal | kPa | 0.001 kPa | 1000 Pa | Pascals to Kilopascals |
| Bar | bar | 0.00001 bar | 100000 Pa | Pascals to Bars |
| Megapascal | MPa | 0.000001 MPa | 1000000 Pa | Pascals to Megapascals |
| Gigapascal | GPa | 0.000000001 GPa | 1000000000 Pa | Pascals to Gigapascals |
| Terapascal | TPa | 1 × 10-12 TPa | 1000000000000 Pa | Pascals to Terapascals |
| Atmosphere | atm | 0.00000986923266716 atm | 101325 Pa | Pascals to Atmospheres |
| Pound per square inch | psi | 0.00014503773773 psi | 6894.75729317 Pa | Pascals to Pounds per square inch |
| Kilopound per square inch | ksi | 0.000000145037743897 ksi | 6894757 Pa | Pascals to Kilopounds per square inch |
| Megapound per square inch | Mpsi | 1.45037743897 × 10-10 Mpsi | 6894757000 Pa | Pascals to Megapounds per square inch |