| Pascals (Pa) | Megapounds per square inch (Mpsi) |
|---|---|
| 1 Pascal | 1.45037743897 × 10-10 Mpsi |
| 2 Pascals | 2.90075487795 × 10-10 Mpsi |
| 3 Pascals | 4.35113231692 × 10-10 Mpsi |
| 4 Pascals | 5.80150975589 × 10-10 Mpsi |
| 5 Pascals | 7.25188719486 × 10-10 Mpsi |
| 10 Pascals | 0.00000000145037743897 Mpsi |
| 20 Pascals | 0.00000000290075487795 Mpsi |
| 25 Pascals | 0.00000000362594359743 Mpsi |
| 50 Pascals | 0.00000000725188719486 Mpsi |
| 100 Pascals | 0.0000000145037743897 Mpsi |
| Reference | Pascals (Pa) | Megapounds per square inch (Mpsi) |
|---|---|---|
| Atmospheric pressure at sea level | 101325 Pa | 0.0000146959 Mpsi |
| Healthy blood pressure (120 mmHg) | 16000 Pa | 0.0000023206 Mpsi |
| A car tyre | 220000 Pa | 0.0000319083 Mpsi |
| A racing bicycle tyre | 600000 Pa | 0.0000870226 Mpsi |
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.
The megapound per square inch is a unit of pressure equal to one million pounds per square inch, or 6.895 gigapascals. Its symbol is Mpsi. Nothing operates at this pressure; the unit exists solely to state elastic modulus, which is the quantity that describes how stiff a material is rather than how strong.
Two numbers in it are memorised by every American mechanical engineer. Steel has a Young's modulus of 29 million pounds per square inch and aluminium of 10 million, and that ratio of roughly three to one governs how the two behave in every structure. A steel part and an aluminium part of identical shape and identical strength will not deflect identically, and the difference is exactly that factor.
The rest of the common metals fill in around them. Titanium sits at about 16.5 million pounds per square inch, copper at 17, magnesium at 6.5, and cast iron between 12 and 20 depending on grade. Because modulus barely changes with heat treatment or alloying within a metal family, these figures are far more stable than strength figures and can be relied on across grades.
Composites are where the unit does its most interesting work. Standard carbon fibre reaches about 33 million pounds per square inch along the fibre, high-modulus grades 55 to 85, and glass fibre only about 10, the same as aluminium. Since a composite laminate has a different modulus in every direction, designers work with a set of values rather than one, and the unit appears many times on a single datasheet.
Its metric equivalent is the gigapascal, and the conversion is 6.895 gigapascals to the megapound per square inch. Steel at 29 million pounds per square inch is 200 gigapascals, and aluminium at 10 million is 69, which are the two most quoted stiffness figures in metric engineering as well. The two traditions are describing exactly the same materials.
Distinguishing modulus from strength matters more here than anywhere. A steel with a modulus of 29 million pounds per square inch might yield at 50 thousand, a difference of nearly six hundred times, and the two properties are independent: heat treatment can double the strength of a steel while leaving its stiffness untouched.
One megapound per square inch equals 1,000,000 pounds per square inch, about 6.895 gigapascals, about 68,950 bar, or 1000 kilopounds per square inch.