| Hectopascals (hPa) | Millipascals (mPa) |
|---|---|
| 1 Hectopascal | 100000 mPa |
| 2 Hectopascals | 200000 mPa |
| 3 Hectopascals | 300000 mPa |
| 4 Hectopascals | 400000 mPa |
| 5 Hectopascals | 500000 mPa |
| 10 Hectopascals | 1000000 mPa |
| 20 Hectopascals | 2000000 mPa |
| 25 Hectopascals | 2500000 mPa |
| 50 Hectopascals | 5000000 mPa |
| 100 Hectopascals | 10000000 mPa |
| Reference | Hectopascals (hPa) | Millipascals (mPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1013.25 hPa | 101325000 mPa |
| Healthy blood pressure (120 mmHg) | 160 hPa | 16000000 mPa |
| A car tyre | 2200 hPa | 220000000 mPa |
| A racing bicycle tyre | 6000 hPa | 600000000 mPa |
The hectopascal is a unit of pressure equal to one hundred pascals. Its symbol is hPa. It is exactly one millibar, and that identity is the whole reason it exists: when meteorology moved to SI units in the 1980s, the hectopascal let every barometer, chart and forecast keep its numbers unchanged while changing the name on them.
Weather is its home and almost its only use. Standard atmospheric pressure at sea level is 1013.25 hectopascals, a settled high-pressure system reads 1020 to 1035, and a deep Atlantic depression can fall below 950. Isobars on a synoptic chart are drawn at four-hectopascal intervals, so the spacing of the lines is a direct picture of how hard the wind will blow.
Storm intensity is reported the same way. The central pressure of a tropical cyclone is the single most quoted measure of its strength, and the lowest sea-level pressure ever recorded was 870 hectopascals in Typhoon Tip in 1979. A drop of a few tens of hectopascals over a day is enough to turn ordinary weather into a severe event.
Altitude changes the reading steadily. Near sea level pressure falls by roughly one hectopascal for every eight metres of height gained, which is why a barometric altimeter works at all and why a barometer at home responds to being carried upstairs. That gradient weakens with altitude, so the relationship holds only in the lowest few kilometres.
Aviation depends on the unit for safety. Pilots set their altimeters to a local pressure figure given in hectopascals so that everyone in the same airspace measures height from the same datum, and above a defined transition altitude all aircraft switch to the standard setting of 1013 hectopascals so that vertical separation is preserved regardless of the weather. North American practice uses inches of mercury for the same purpose, and the two must never be confused.
For conversion, one hectopascal is one millibar exactly, 0.1 kilopascals, and about 0.0295 inches of mercury. The last of those is the figure to watch, since a reading of 30 inches of mercury and one of 1016 hectopascals describe the same afternoon.
One hectopascal equals 100 pascals, 1 millibar, 0.1 kilopascals, or about 0.0295 inches of mercury.
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.