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.

Megapascal conversion table