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What is Pressure? The Physics Explained

Pressure is defined as force applied perpendicularly per unit area. In the International System of Units (SI), the unit of pressure is the Pascal (Pa), where one Pascal equals one Newton per square meter.

The Pascal is named after Blaise Pascal (1623–1662), the French mathematician and physicist who made foundational contributions to fluid mechanics. His principle — that pressure applied to a confined fluid is transmitted equally in all directions — underpins hydraulic engineering to this day.

Because the Pascal is quite small (standard atmospheric pressure is about 101,325 Pa), pressure is frequently expressed in kilopascals (kPa), megapascals (MPa), or hectopascals (hPa). The bar, millibar, PSI, atm, mmHg, and Torr are all non-SI units that remain in widespread use for historical and practical reasons.

The Major Pressure Units

Pascal (Pa) and Its Multiples

The Pascal is the SI base unit of pressure. Because it represents a very small force (roughly the pressure exerted by a dollar bill resting flat on a table), most practical applications use multiples:

• 1 hPa (hectopascal) = 100 Pa — used in meteorology; equivalent to 1 millibar
• 1 kPa (kilopascal) = 1,000 Pa — used in engineering, tire pressure in some countries
• 1 MPa (megapascal) = 1,000,000 Pa — used in materials science and structural engineering

Bar and Millibar (mbar)

The bar is a metric unit approximately equal to atmospheric pressure at sea level. It equals exactly 100,000 Pa (100 kPa). Despite not being an SI unit, the bar is widely used in engineering, chemistry, and weather forecasting. The millibar (mbar), equal to 0.001 bar or 100 Pa, is used synonymously with the hectopascal in meteorology: 1 mbar = 1 hPa.

Pounds per Square Inch (PSI)

PSI (pounds-force per square inch) is the dominant pressure unit in the United States and is still used in the United Kingdom for tire inflation. One PSI equals approximately 6,895 Pascals. PSI appears on tire gauges, pressure washers, oxygen cylinders, and in most American engineering contexts.

Atmosphere (atm)

The standard atmosphere (atm) is defined as exactly 101,325 Pa — the approximate average pressure of the Earth's atmosphere at sea level. It's commonly used in chemistry and physics when describing gas conditions.

Millimeters of Mercury (mmHg) and Torr

The mmHg is a manometric unit defined as the pressure exerted by a column of mercury 1 mm high under standard gravity. The Torr is defined as exactly 1/760 of a standard atmosphere, making 1 Torr ≈ 1 mmHg (they differ by less than 0.000015%). Both units are used in medicine (blood pressure), vacuum science, and laboratory work.

The History of Pressure Measurement: Torricelli's Barometer

In 1643, Italian physicist Evangelista Torricelli performed one of the most elegant experiments in the history of science. He filled a glass tube (closed at one end) with mercury, inverted it into a bowl of mercury, and observed that the mercury column in the tube settled at about 760 mm — it didn't fall all the way down. The space above the mercury column was a near-perfect vacuum.

Atmospheric Pressure and Altitude

Atmospheric pressure decreases as altitude increases, because there is less air above you pressing down. This has significant practical implications for aviation, mountaineering, and even cooking (see our cooking measurements article for altitude baking tips).

Blood Pressure: Reading mmHg in Medical Context

Blood pressure is reported as two numbers in mmHg — for example, 120/80 mmHg. These represent:

• Systolic pressure (120): The peak pressure in the arteries when the heart contracts (beats). This is the higher number.
• Diastolic pressure (80): The minimum pressure between heartbeats when the heart is relaxed. This is the lower number.

Tire Pressure: PSI, Bar, and kPa Around the World

Tire pressure is one of the most practical applications of pressure measurement for everyday people. The recommended unit varies by country:

• United States: PSI (typically 30–35 PSI for passenger cars)
• United Kingdom: PSI is still most common, though bar appears on newer vehicles
• Continental Europe: bar (typically 2.0–2.5 bar)
• Japan and much of Asia: kPa (typically 200–250 kPa)

Scuba Diving and Underwater Pressure

Water is approximately 800 times denser than air. This means pressure increases dramatically with depth — far more rapidly than altitude changes air pressure. Understanding underwater pressure is fundamental to safe scuba diving.

This relationship (Boyle's Law) explains why a diver who holds their breath while ascending can suffer a pulmonary embolism — the air in the lungs expands as pressure decreases. Scuba cylinders typically hold air at 200–300 bar (2,900–4,350 PSI).

Absolute vs. Gauge Pressure

Absolute Pressure (psia / bara)

Absolute pressure is measured relative to perfect vacuum (0 Pa). It includes atmospheric pressure. Scientific calculations (Boyle's Law, ideal gas law) always use absolute pressure. Denoted with an 'a' suffix: psia, bara, kPa(a).

Gauge Pressure (psig / barg)

Gauge pressure is measured relative to local atmospheric pressure. A tire gauge reading 30 psig means the tire is inflated to 30 PSI above atmospheric pressure. Most everyday pressure instruments read gauge pressure. Denoted with a 'g' suffix: psig, barg, kPa(g).

Vacuum Measurement

Negative gauge pressure indicates a partial vacuum — pressure below atmospheric. In vacuum engineering, pressure is often expressed in Torr or mbar for rough vacuum, and in Pa for high and ultra-high vacuum:

• Rough vacuum: 100,000 Pa down to ~100 Pa (1 to 0.001 bar)
• Medium vacuum: 100 Pa down to 0.1 Pa
• High vacuum: 0.1 Pa down to 10⁻⁷ Pa
• Ultra-high vacuum: Below 10⁻⁷ Pa (used in particle accelerators)

Weather Forecasting and Barometric Pressure

Meteorologists measure atmospheric pressure in hPa (hectopascals), numerically identical to the millibar. Standard sea-level pressure is 1013.25 hPa. Pressure changes are highly predictive of weather:

• High pressure (above ~1020 hPa): Associated with descending air, stable and clear conditions
• Low pressure (below ~1000 hPa): Associated with rising air, clouds, precipitation, and storms
• Strong hurricanes can have central pressures below 900 hPa — the lowest recorded was Typhoon Tip at 870 hPa (1979)
• A rapid pressure drop of 3+ hPa in 3 hours often signals an approaching storm system

Industrial Applications: Hydraulics and Steam

Industry relies on high-pressure systems for power transmission and process engineering:

• Hydraulic systems in construction equipment: 150–350 bar (2,200–5,000 PSI)
• Hydraulic presses in manufacturing: up to 700 bar (10,000+ PSI)
• Steam boilers in power plants: 30–300 bar (440–4,350 PSI)
• Pneumatic tools (compressed air): 6–10 bar (87–145 PSI)
• Oxygen cylinders for medical use: 138–200 bar (2,000–2,900 PSI)
• Diesel engine fuel injection: up to 2,500 bar (36,000 PSI) in modern common-rail systems

Comprehensive Pressure Unit Conversion Table

The table below shows conversion factors between the most common pressure units, anchored to 1 Pascal as the SI reference.

Quick Conversion Formulas

Safety Considerations

• Always confirm whether readings are absolute or gauge pressure
• Use appropriate pressure relief devices for all pressurized systems
• Regularly calibrate pressure measuring instruments
• Follow manufacturer specifications for pressure ratings
• Never exceed the maximum working pressure of any equipment
• When in doubt, convert to Pa and verify against specifications

Conclusion

Pressure is measured in an unusually diverse range of units, each with historical roots in a particular industry or scientific tradition. The Pascal unifies them all through SI base units, but mmHg still dominates medicine, PSI dominates American engineering, bar is preferred in Europe and for diving, and hPa rules meteorology. Understanding which unit applies in which context — and how to convert reliably between them — is essential knowledge for engineers, scientists, divers, pilots, healthcare workers, and curious minds alike.