Understanding Pressure Units: From Pascal to PSI

What Is Pressure?

Pressure is defined as force applied per unit area. When you push on something, the pressure depends not just on how hard you push, but on how small the contact area is. A thumbtack hurts because the same force is concentrated on a tiny point.

The SI unit of pressure is the pascal (Pa), defined as one newton per square metre (1 Pa = 1 N/m²). But because a pascal is an extremely small amount of pressure, you'll encounter many other units in practice — atmospheres, bars, PSI, and millimetres of mercury — each rooted in a different historical or practical context.

Pressure can be measured in two ways:

• Absolute pressure — measured relative to a perfect vacuum (zero pressure)
• Gauge pressure — measured relative to atmospheric pressure. A tire gauge reading of 32 PSI means 32 PSI above atmospheric pressure

The SI Unit: Pascal

The pascal is named after Blaise Pascal (1623–1662), the French mathematician, physicist, and philosopher who made groundbreaking contributions to fluid mechanics and hydrostatics. In 1648, Pascal confirmed Torricelli's theory that atmospheric pressure decreases with altitude by sending his brother-in-law up the Puy de Dôme mountain with a mercury barometer.

One pascal is defined as one newton of force applied over one square metre. To put that in perspective:

• 1 Pa is roughly the pressure exerted by a dollar bill resting flat on a table
• Standard atmospheric pressure is 101,325 Pa — over a hundred thousand pascals
• A car tire is inflated to about 220,000 Pa

Because one pascal is so small, larger multiples are used in practice:

Atmosphere (atm)

The standard atmosphere (atm) is a reference pressure defined as exactly 101,325 Pa. It approximates the average atmospheric pressure at sea level at 15 °C.

The concept dates back to Evangelista Torricelli, who in 1643 invented the mercury barometer and demonstrated that the atmosphere has weight. He showed that air pressure could support a column of mercury about 760 mm tall — which is why 1 atm = 760 mmHg.

How Atmospheric Pressure Varies

Atmospheric pressure is not constant. It changes with:

• Altitude: Pressure drops roughly 12 Pa per metre of elevation gain near sea level. At the summit of Mount Everest (8,849 m), pressure is only about 0.33 atm — one-third of sea level
• Weather: High-pressure systems (clear skies) typically read 1.02–1.04 atm; low-pressure systems (storms) can drop below 0.97 atm
• Temperature: Warmer air is less dense, producing slightly lower surface pressure

Bar and Millibar

The bar was introduced in 1909 by the British meteorologist Napier Shaw. It's defined as exactly 100,000 Pa (100 kPa), making it very close to — but not exactly equal to — one atmosphere.

The millibar (mbar) — one-thousandth of a bar — is the traditional unit for weather forecasting. Standard sea-level pressure is 1013.25 mbar. In modern meteorology, the equivalent unit hectopascal (hPa) has largely replaced the millibar, but they are numerically identical: 1 mbar = 1 hPa.

Where Bar Is Used

• Weather maps and forecasts — isobars on weather charts are labelled in mbar or hPa
• Scuba diving — depth is often expressed in bar (every 10 metres of water adds about 1 bar)
• European tire pressure — often specified in bar (e.g., 2.2 bar ≈ 32 PSI)
• Industrial compressed air — compressor ratings in continental Europe use bar

PSI: Pounds Per Square Inch

PSI (pounds per square inch) is the most common pressure unit in the United States and is widely used in the UK for certain applications. It's part of the imperial/US customary system, defined as one pound-force applied to an area of one square inch.

Common PSI Values

You may also encounter PSIA (PSI absolute) and PSIG (PSI gauge). A tire gauge reads in PSIG — the pressure above atmospheric. In engineering specifications, the distinction matters: a system rated for 150 PSIG has a very different burst threshold from one rated at 150 PSIA.

Mercury Columns: mmHg and inHg

Before electronic sensors, pressure was measured by observing how high a column of liquid could be supported against gravity. Mercury was the liquid of choice because of its high density — 13.6 times denser than water — which kept the column to a practical height.

Torricelli's Barometer

In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish, and observed that the mercury dropped to about 760 mm, with a vacuum above it. The weight of the mercury column was balanced by the atmospheric pressure pushing on the dish. This was the first barometer.

Where Mercury Units Are Still Used

• Blood pressure — always reported in mmHg worldwide (e.g., 120/80 mmHg). Even digital monitors that don't use mercury display results in mmHg by convention
• Aviation weather — altimeter settings in the US and Canada use inHg (e.g., 29.92 inHg). Pilots set their altimeters to the local barometric pressure reported in inHg
• Vacuum systems — laboratory and industrial vacuum levels are often specified in torr or mmHg
• HVAC and refrigeration — manifold gauges may show inHg for vacuum readings

Pressure in Daily Life

Pressure is one of those physics concepts that's invisible but affects us constantly. Here are the most common places you encounter it:

Tire Pressure

Proper tire inflation is critical for safety, fuel efficiency, and tire lifespan. Under-inflated tires increase rolling resistance, waste fuel, and wear unevenly. Over-inflated tires reduce grip and make the ride harsh.

Blood Pressure

A blood pressure reading of 120/80 mmHg means the systolic pressure (during heartbeat) is 120 mmHg and the diastolic pressure (between beats) is 80 mmHg. In other units, that's about 16.0/10.7 kPa or 2.32/1.55 PSI — but no doctor will ever use those units.

Weather Forecasting

Barometric pressure is one of the most important indicators in meteorology:

• High pressure (above 1020 hPa) — generally clear, stable weather
• Low pressure (below 1000 hPa) — clouds, rain, and storms
• Rapidly falling pressure — approaching storm front
• Hurricane central pressure — can drop below 920 hPa. Hurricane Wilma (2005) recorded 882 hPa, the lowest in Atlantic history

Altitude and Cabin Pressure

Commercial aircraft cabins are pressurized to the equivalent of about 1,800–2,400 m (6,000–8,000 ft) above sea level, or roughly 0.75–0.80 atm. This is why your ears pop during ascent and descent — the pressure difference across your eardrum changes faster than your body can equalize.

Diving

Underwater pressure increases by about 1 atm for every 10.06 metres of depth in seawater. At 30 metres, a diver experiences 4 atm of absolute pressure (1 atm air + 3 atm water). Understanding pressure is essential in diving to avoid decompression sickness ("the bends"), nitrogen narcosis, and oxygen toxicity.

Pressure Cookers

A pressure cooker traps steam to raise the internal pressure to about 1 atm above atmospheric (2 atm absolute, or roughly 15 PSI gauge). This raises the boiling point of water from 100 °C to about 121 °C, cooking food significantly faster — typically 50–70% less time than conventional boiling.

Conversion Quick Reference

The following table shows how to convert between the most common pressure units. All values are based on exact or internationally agreed definitions.

Common Pressure Values at a Glance