| Millipascals (mPa) | Pascals (Pa) |
|---|---|
| 1 Millipascal | 0.001 Pa |
| 2 Millipascals | 0.002 Pa |
| 3 Millipascals | 0.003 Pa |
| 4 Millipascals | 0.004 Pa |
| 5 Millipascals | 0.005 Pa |
| 10 Millipascals | 0.01 Pa |
| 20 Millipascals | 0.02 Pa |
| 25 Millipascals | 0.025 Pa |
| 50 Millipascals | 0.05 Pa |
| 100 Millipascals | 0.1 Pa |
| Reference | Millipascals (mPa) | Pascals (Pa) |
|---|---|---|
| Atmospheric pressure at sea level | 101325000 mPa | 101325 Pa |
| Healthy blood pressure (120 mmHg) | 16000000 mPa | 16000 Pa |
| A car tyre | 220000000 mPa | 220000 Pa |
| A racing bicycle tyre | 600000000 mPa | 600000 Pa |
The millipascal is a unit of pressure equal to one thousandth of a pascal. Its symbol is mPa. The pascal is already a very small unit — it is the pressure of a sheet of paper lying on a table — so a thousandth of one is smaller than almost any pressure a person encounters. It survives because two fields genuinely work at that scale: acoustics and viscosity.
Sound is a pressure wave, and the pressures involved are tiny. Ordinary conversation at a metre carries a sound pressure of about 20 millipascals. A whisper is nearer 2, and the threshold of hearing, the quietest sound a healthy young ear can detect, is 0.02 millipascals, which is 20 micropascals. A loud rock concert reaches a few pascals. The entire useful range of human hearing therefore lives between a hundredth of a millipascal and a few thousand of them.
That is why sound is reported in decibels rather than in pressure units. A range spanning a factor of a million is unwieldy in linear numbers, so acoustics takes the logarithm and anchors it at the threshold of hearing. But the decibel is not a unit of pressure at all: behind every decibel figure is a pressure in pascals or millipascals, and instrument calibration is done in those real units.
The second use is stranger, because it is not a pressure at all. Dynamic viscosity is measured in pascal seconds, and almost every liquid people care about lands in the millipascal second range. Water at room temperature is 1 mPa·s exactly enough for practical purposes. That happens to equal one centipoise in the older CGS system, so the switch to SI left every viscosity table numerically unchanged, which is why the millipascal second took hold where the millipascal alone did not.
With that scale in hand, the numbers become legible. Petrol is about 0.6 mPa·s, olive oil about 80, honey several thousand, and glycerol around 1,400. Blood plasma is about 1.3, and whole blood nearer 4, which is one reason blood flow is harder to model than water flow.
For pressure itself, outside acoustics, the millipascal appears in vacuum work and in the gentlest of laboratory measurements — the pressure differences that drive slow gas flow, or the residual pressure in a chamber that has been pumped down hard. In those settings the alternative units are the micropascal below and the pascal above.
One millipascal equals 0.001 pascals, one thousand micropascals, 0.00001 millibars, or about 0.000000145 pounds per square inch.
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.