| Millipascals (mPa) | Centipascals (cPa) |
|---|---|
| 1 Millipascal | 0.1 cPa |
| 2 Millipascals | 0.2 cPa |
| 3 Millipascals | 0.3 cPa |
| 4 Millipascals | 0.4 cPa |
| 5 Millipascals | 0.5 cPa |
| 10 Millipascals | 1 cPa |
| 20 Millipascals | 2 cPa |
| 25 Millipascals | 2.5 cPa |
| 50 Millipascals | 5 cPa |
| 100 Millipascals | 10 cPa |
| Reference | Millipascals (mPa) | Centipascals (cPa) |
|---|---|---|
| Atmospheric pressure at sea level | 101325000 mPa | 10132500 cPa |
| Healthy blood pressure (120 mmHg) | 16000000 mPa | 1600000 cPa |
| A car tyre | 220000000 mPa | 22000000 cPa |
| A racing bicycle tyre | 600000000 mPa | 60000000 cPa |
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 centipascal is a unit of pressure equal to one hundredth of a pascal. Its symbol is cPa. It is a legitimate metric combination that almost nobody writes, and understanding why it is unused says more about how pressure is actually recorded than a list of its applications ever could.
The centi prefix has one great success behind it, the centimetre, and that success came from the human scale of the metre. A metre is too long for the width of a hand and a millimetre too short, so the centimetre filled a real gap. Pressure has no comparable gap. The pascal is already so small that dividing it further gives numbers no instrument in ordinary use can resolve.
A centipascal is one hundredth of the pressure of a sheet of paper on a table. It is the pressure difference across a millimetre of air, or roughly what a grain of sand exerts spread over a fingernail. Nothing in engineering, meteorology or building practice needs that resolution: airtightness testing works at fifty pascals, ventilation at a few hundred, and weather at tens of thousands.
Where pressures smaller than a pascal do matter — acoustics and vacuum work — the convention jumps straight to the millipascal and the micropascal, because those fit the thousand-step ladder that the rest of the measurement system uses. A sound pressure of 20 millipascals is written that way rather than as 2 centipascals, even though the two are the same, because everything around it on the page is in thousand-step units.
The unit is nevertheless well defined, and that is the metric system working as designed. Every prefix combines with every unit without exception, so a reader who has never seen cPa can still decode it on sight from the prefix alone. A system with gaps would require a table; a system without them requires only the rule.
Occasional appearances do occur. Older instrument manuals sometimes use it, some national standards mention it in passing, and a few specialist fields adopt it locally when their numbers happen to fall between one and a hundred in that unit. A converter has to handle it for exactly those cases.
One centipascal equals 0.01 pascals, 10 millipascals, 0.1 decipascals, or about 0.00000145 pounds per square inch.