Conversion from Megapascals to Millipascals

Information about the Megapascal (MPa)

The megapascal is a unit of pressure equal to one million pascals, written MPa. It is ten bar, and it is exactly one newton per square millimetre. That last identity is the reason the unit dominates engineering: a stress calculated from a force in newtons and an area in square millimetres comes out in megapascals with no conversion at all.

Materials strength is stated in it almost universally. A common structural steel yields at 355 megapascals, high-strength alloy steels at over a thousand, aluminium alloys between 200 and 500, and structural concrete at 25 to 50 in compression. Because those numbers are what a designer compares against a calculated stress, the megapascal is the unit in which most of the built world is specified.

Structural drawings often write N/mm² instead, and the two are the same. Some national codes prefer one form and some the other, but a plate marked 355 N/mm² and one marked 355 MPa carry identical steel. Knowing that they are the same unit removes what looks like a discrepancy between drawings from different countries.

Hydraulics works at these pressures too. Construction machinery runs at twenty to thirty-five megapascals, industrial presses higher, and water-jet cutting reaches four hundred, at which pressure a stream of water a fraction of a millimetre across will cut steel. Pressure vessels, pipelines and hydrogen storage are all designed and tested against figures in this range.

Above the megapascal, the gigapascal takes over for stiffness rather than strength. Young's modulus for steel is about 200 gigapascals, for aluminium 70 and for concrete 30, and those numbers describe how much a material deflects rather than when it breaks. Keeping strength in megapascals and stiffness in gigapascals is a convention that makes the two easy to tell apart at a glance.

For conversion, one megapascal is ten bar, 145 pounds per square inch, and just under ten atmospheres. The last comparison is worth holding: a pressure of one megapascal is about ten times what the air outside exerts, which puts even modest industrial pressures in a useful human perspective.

One megapascal equals 1,000,000 pascals, 10 bar, 1 newton per square millimetre, or about 145 pounds per square inch.

Information about the Millipascal (mPa)

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.