The Pascal: From Torricelli's Barometer to the SI Standard
Source: NIST. Published: 2025-05-01.
Torricelli's Barometer: Discovering Atmospheric Pressure
In 1643, the Italian physicist and mathematician Evangelista Torricelli, a student of Galileo, performed one of the most elegant experiments in the history of science. He filled a long glass tube with mercury, sealed one end, inverted it, and placed the open end in a dish of mercury. The mercury column inside the tube dropped until it was about 760 millimetres high, leaving a vacuum — now known as a Torricelli vacuum — above it. Torricelli concluded that the atmosphere was pressing down on the mercury in the dish, and that this pressure was sufficient to support a column of mercury 760 mm tall. He had discovered atmospheric pressure and invented the barometer simultaneously. The experiment established for the first time that air has weight, overturning the ancient belief that vacuums were impossible and setting the stage for two centuries of pressure measurement.
Blaise Pascal Proves Atmospheric Weight
Torricelli's finding was confirmed and extended by the French mathematician and philosopher Blaise Pascal, whose name the SI unit of pressure would eventually bear. Pascal reasoned that if atmospheric pressure was holding up the mercury column, then the pressure should be lower at higher altitudes, where there is less atmosphere above. In 1648, Pascal arranged for his brother-in-law to carry a barometer to the summit of Puy de Dôme, a volcanic peak in the Auvergne region of France. The mercury column was indeed shorter at the summit than at the base — by about 85 mm — precisely confirming Torricelli's hypothesis. Pascal also demonstrated that mercury could be replaced by other fluids: a tube of water would need to be about 10 metres tall to support the same atmospheric pressure, consistent with the density difference between mercury and water. His systematic investigation of pressure earned him recognition as a founding figure of hydrodynamics.
Pressure in the Industrial Age: PSI and Steam
The Industrial Revolution created a pressing practical need for pressure measurement. Steam engines required boilers that operated at pressures well above atmospheric, and measuring those pressures accurately was a matter of safety and efficiency. The unit that emerged in the English-speaking world was pounds per square inch, or PSI — the force in pounds exerted on one square inch of area. PSI remains widely used today in the United States for tyre inflation, hydraulic systems, and industrial equipment. The bar — equal to 100 000 pascals and approximately equal to one standard atmosphere — was widely adopted in continental Europe and in meteorology. Atmospheres (atm), millimetres of mercury (mmHg), and inches of mercury (inHg) were used in scientific, medical, and meteorological contexts respectively. The proliferation of pressure units reflected the independent development of pressure measurement in different industries and countries, each establishing conventions before international standardisation existed.
The Pascal Becomes the SI Unit of Pressure
The pascal (Pa) was adopted as the SI unit of pressure in 1971 by the 14th General Conference on Weights and Measures, named in honour of Blaise Pascal. It is defined as one newton per square metre — that is, a force of one newton distributed uniformly over an area of one square metre. One pascal is a very small pressure: the standard atmospheric pressure at sea level is approximately 101 325 Pa, or about 101.3 kilopascals (kPa). Because the pascal is so small relative to pressures encountered in everyday life, the kilopascal and megapascal are the more commonly used multiples. Meteorologists use hectopascals (hPa), where one hPa equals one millibar — a convenient historical coincidence. NIST maintains pressure standards through a hierarchy of calibrated instruments, from mercury manometers at the top of the traceability chain to working-level pressure gauges used in calibration laboratories.
Pressure in Everyday Life
Pressure appears in a surprising range of everyday contexts. Tyre pressure, typically around 30-35 PSI (207-241 kPa) for a passenger car, affects fuel economy, handling, and tyre wear — and is checked with a gauge that measures gauge pressure, meaning pressure above atmospheric rather than absolute pressure. Blood pressure is still measured in millimetres of mercury (mmHg) by convention, even in countries that have otherwise adopted SI units: a reading of 120/80 mmHg means a systolic pressure of 120 mmHg above zero and a diastolic of 80 mmHg. Weather forecasts express atmospheric pressure in hectopascals or millibars, with lower values indicating stormy weather and higher values clear skies. Industrial and scuba diving applications require pressure gauges calibrated in bar, and compressed gas cylinders are rated in megapascals. Understanding pressure units and being able to convert between them is genuinely useful across a wide range of practical situations.
Using the Pressure Converter
The Unit Master Pressure Converter supports eight common pressure units: pascal (Pa), kilopascal (kPa), megapascal (MPa), bar, PSI, atmosphere (atm), millimetres of mercury (mmHg), and inches of mercury (inHg). All conversions use the pascal as the base unit, with exact or high-precision conversion factors derived from SI definitions and accepted measurement practice. For example, one standard atmosphere is defined as exactly 101 325 Pa — a value established by the International Organization for Standardization and adopted by NIST. One bar is defined as exactly 100 000 Pa. One mmHg (at 0 °C, standard gravity) equals approximately 133.322 Pa, a value based on the density of mercury and standard gravitational acceleration. Whether you are inflating a tyre, reading a weather report, or converting a laboratory pressure reading, the converter provides an instant cross-reference across all eight units.
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