Pressure Units Guide: PSI, Bar, Pascal & ATM Conversion (2026)

What Is Pressure?

Pressure is force applied per unit area. Push harder on a surface, or push the same force through a smaller area, and pressure goes up. The formal definition: pressure = force ÷ area.

The SI unit is the Pascal (Pa), defined as 1 Newton per square meter. It was named after Blaise Pascal, the French mathematician and physicist who described atmospheric pressure in the 1640s — source: SI Brochure, BIPM. One Pascal is quite small. A dollar bill resting flat on a table exerts roughly 5 Pa. That's why practical applications use kilopascals (kPa), megapascals (MPa), bar, or PSI instead.

Pressure shows up everywhere: weather forecasting, tire inflation, scuba diving, hydraulics, medical diagnostics, and cooking. Each field tends to use the unit most intuitive for its scale.

Pressure Unit Conversion Table

All values show how many Pascals equal one unit of the listed measure.

A few quick conversions worth memorizing: 1 bar ≈ 14.5 PSI. 1 atm ≈ 14.7 PSI. 1 kPa ≈ 0.145 PSI. For everything else, use the pressure converter.

Real-World Pressure Reference Table

Numbers are more useful when anchored to things you know. Here’s pressure in context:

The range is staggering. From the near-perfect vacuum of space to the crushing pressure at the bottom of Challenger Deep in the Mariana Trench — approximately 1,086 bar (15,750 psi or 1,071 atm), according to NOAA — pressure varies by more than 10 orders of magnitude across nature and engineering.

Medical Pressure: Blood Pressure and mmHg

Blood pressure is measured in millimeters of mercury (mmHg), a unit that dates to the mercury sphygmomanometer invented in the late 19th century. Despite the availability of SI units, medicine has kept mmHg because its scale is intuitive for clinical ranges and the historical data is all in mmHg.

A blood pressure reading has two numbers. The higher number (systolic) is the pressure when the heart contracts and pushes blood out. The lower number (diastolic) is the pressure when the heart relaxes between beats.

Source: American Heart Association (2023). Normal blood pressure is below 120/80 mmHg. Hypertension is defined as ≥130/80 mmHg. In SI units, 120 mmHg ≈ 16 kPa and 80 mmHg ≈ 10.7 kPa — but clinicians virtually never use kPa for blood pressure.

One mmHg equals 133.322 Pascals. The unit is sometimes called a Torr (named after Evangelista Torricelli, who invented the barometer). For most purposes, 1 mmHg = 1 Torr.

Tire Pressure: Why PSI Matters

Under-inflated tires are a genuine safety issue. According to NHTSA (2024), under-inflation contributes to approximately 11,000 tire-related crashes per year in the US. The failure mode is heat: an under-inflated tire flexes more with each rotation, generating heat that can lead to tread separation or blowout.

Most passenger car tires should be inflated to 32–35 PSI. But the correct value for your specific vehicle is on a sticker inside the driver-side door jamb — not the number stamped on the tire sidewall. That sidewall number is the maximum safe pressure, not the recommended operating pressure.

Tire pressure varies with temperature. For every 10°F (5.6°C) drop in temperature, tires lose roughly 1 PSI. Check pressure in the morning before driving when tires are cold, since heat from driving temporarily inflates readings.

Road bike tires run 80–110 PSI — three times car tire pressure — because the narrow contact patch needs high pressure to roll efficiently without pinch flats. Mountain bike tires run much lower, 25–35 PSI, to maximize grip and absorb impacts.

Weather Pressure: How Barometers Work

Atmospheric pressure is the weight of the air column above you. At sea level, that column exerts about 101,325 Pa (1013.25 mbar). As you go up in altitude, there is less air above you, so pressure drops. At 5,500 m (18,000 ft), pressure is roughly half sea level.

Weather forecasters track pressure in millibars (mbar) or hectopascals (hPa) — these are the same unit, 1 mbar = 1 hPa = 100 Pa. Standard pressure is 1013.25 hPa.

• High pressure (>1020 hPa): sinking air, clear skies, stable weather
• Low pressure (<1000 hPa): rising air, cloud formation, precipitation, storms
• Hurricane-force systems: central pressure can drop below 900 hPa; the 2005 Hurricane Wilma set the Atlantic record at 882 hPa

A mercury barometer works by balancing air pressure against a column of liquid mercury in a sealed tube. At sea level, that column is 760 mm tall — the origin of “760 mmHg” as standard pressure. Aneroid barometers use a sealed, flexible metal capsule instead of mercury. As pressure changes, the capsule expands or contracts, moving a needle across a dial. These are safer to transport and are used in aircraft altimeters and home weather stations.

Aviation uses inHg (inches of mercury) in the United States. Standard sea-level pressure is 29.92 inHg. Pilots set altimeters to local barometric pressure during flight to get accurate altitude readings.

Industrial and Scientific Pressure Uses

Bar and MPa dominate industrial applications. Hydraulic systems in construction equipment typically operate at 200–350 bar (2,900–5,000 PSI). High-pressure water jet cutting can reach 4,000–6,000 bar. Natural gas distribution pipelines operate at pressures ranging from a few mbar for residential lines to 70+ bar for transmission lines.

In chemistry, pressure affects reaction rates, boiling points, and gas behavior. The ideal gas law ties pressure directly to temperature and volume. Autoclave sterilization uses 121°C steam at 1 atm above ambient pressure (about 2 atm total) — the elevated pressure raises the boiling point of water, enabling temperatures above 100°C.

Scuba tanks are typically filled to 200–300 bar (2,900–4,350 PSI). A standard aluminum 80 tank holds enough air to supply a diver for roughly 45–60 minutes at 10 m depth.

Frequently Asked Questions

What is PSI in pressure?

PSI stands for pounds per square inch. It measures how much force (in pounds) is applied over one square inch of area. It’s the standard pressure unit in the United States for everyday applications like tire inflation, hydraulics, and gas line pressure. One PSI equals 6,894.76 Pascals.

How do I convert bar to PSI?

Multiply bar by 14.5038. So 2 bar = 29.0 PSI, and 3 bar = 43.5 PSI. To go the other direction, divide PSI by 14.5038 to get bar. For example, 35 PSI ÷ 14.5038 ≈ 2.41 bar.

What is standard atmospheric pressure?

Standard atmospheric pressure at sea level is exactly 101,325 Pascals (Pa), which equals 1 atmosphere (atm), 1013.25 millibars (mbar), 1.01325 bar, 14.696 PSI, and 760 mmHg. This value is defined by NIST and ISO 2533 and represents average air pressure at Earth’s surface.

What is normal blood pressure in kPa?

Normal blood pressure is below 120/80 mmHg, which equals roughly 16/10.7 kPa. The systolic reading of 120 mmHg converts to about 16 kPa. Hypertension is defined as 130/80 mmHg or higher (approximately 17.3/10.7 kPa) by the American Heart Association (2023). Clinically, blood pressure is almost always reported in mmHg.

What PSI should my car tires be?

Most passenger vehicles recommend 32–35 PSI (220–240 kPa). The correct value is printed on a sticker inside your driver-side door jamb — not on the tire sidewall, which shows the maximum pressure. According to NHTSA (2024), under-inflated tires contribute to approximately 11,000 tire-related crashes per year in the US. Check pressure when tires are cold for the most accurate reading.

How does a barometer measure atmospheric pressure?

A barometer measures atmospheric pressure by balancing the weight of the atmosphere against a column of fluid. A mercury barometer uses a sealed tube of mercury in a dish — air pressure pushes down on the dish and holds the mercury column up. At sea level, this column stands about 760 mm tall (760 mmHg). Aneroid barometers use a sealed metal capsule that expands and contracts with pressure changes, which is why they’re used in weather stations and aircraft altimeters.