| Millipascals (mPa) | Nanopascals (nPa) |
|---|---|
| 1 Millipascal | 1000000 nPa |
| 2 Millipascals | 2000000 nPa |
| 3 Millipascals | 3000000 nPa |
| 4 Millipascals | 4000000 nPa |
| 5 Millipascals | 5000000 nPa |
| 10 Millipascals | 10000000 nPa |
| 20 Millipascals | 20000000 nPa |
| 25 Millipascals | 25000000 nPa |
| 50 Millipascals | 50000000 nPa |
| 100 Millipascals | 100000000 nPa |
| Reference | Millipascals (mPa) | Nanopascals (nPa) |
|---|---|---|
| Atmospheric pressure at sea level | 101325000 mPa | 1.01325 × 1014 nPa |
| Healthy blood pressure (120 mmHg) | 16000000 mPa | 1.6 × 1013 nPa |
| A car tyre | 220000000 mPa | 2.2 × 1014 nPa |
| A racing bicycle tyre | 600000000 mPa | 6 × 1014 nPa |
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 nanopascal is a unit of pressure equal to one billionth of a pascal. Its symbol is nPa. It marks the far end of the pressure scale, the region where the idea of pressure as a push on a surface stops being useful and becomes a statement about how few particles are present.
The clearest home for the unit is extreme high vacuum. Ordinary laboratory vacuum reaches millipascals; ultra-high vacuum, used for surface physics and for the beam pipes of particle accelerators, reaches micropascals. Below that lies extreme high vacuum at nanopascals and lower, and reaching it takes a sealed chamber, hours of baking at two hundred degrees to drive gas out of the metal itself, and pumps that trap molecules rather than push them.
At those pressures a chamber is not empty. A nanopascal still contains something like a quarter of a million molecules per cubic centimetre, which sounds like a great many until you compare it with the twenty-five billion billion in the same volume of room air. What matters is not the count but the mean free path: a molecule now travels thousands of kilometres before striking another, so it hits the walls long before it meets a neighbour.
That is exactly the point. Surface science needs a sample to stay clean for the length of an experiment, and at ordinary pressures a fresh surface is covered by a layer of adsorbed gas in about a nanosecond. At a nanopascal the same surface stays clean for days. The vacuum is not there to remove air but to buy time.
Space provides the natural comparison. Low Earth orbit is around a micropascal, still dense enough that the atmosphere drags on satellites and eventually pulls them down. Interplanetary space is nanopascals. Interstellar space is far lower still, roughly a femtopascal, which no terrestrial pump has ever matched — the best laboratory vacuums are still denser than the space between the stars.
Radiation pressure lands in similar territory. Sunlight falling on a perfectly absorbing surface at Earth's distance exerts about 4.5 micropascals, and at the distance of the outer planets it falls to nanopascals. Solar sails work with these numbers, which is why they must be enormous and light to gather a usable force.
One nanopascal equals 0.000000001 pascals, 0.001 micropascals, 1,000 picopascals, or about 0.000000000000145 pounds per square inch.