Pressure Units: Converting Pascals, Bar, PSI, Atmospheres, Torr, and More

Pressure measures force per unit area, with the SI unit being the pascal — one newton per square meter. Evangelista Torricelli created the first mercury barometer in 1643, establishing the millimeter of mercury (mmHg) as the original pressure standard. Today, pressure measurement spans six orders of magnitude: from the near-vacuum of outer space (10⁻¹⁴ Pa) to the 100,000 atm at Earth's core. Each industry has its preferred unit — meteorology uses millibars and inches of mercury, engineering uses PSI and kPa, chemistry uses atmospheres and torr, and tire gauges use PSI worldwide. Understanding the conversion factors between these units enables cross-disciplinary reading of weather reports, scuba dive tables, industrial specifications, and medical measurements.

Key Takeaways

• 1 atm = 101,325 Pa = 1.01325 bar = 14.6959 PSI = 760 torr = 760 mmHg = 29.9213 inHg.
• 1 Pa = 1 N/m². 1 kPa = 1,000 Pa.
• 1 bar = 100,000 Pa (exactly). 1 mbar = 0.001 bar.
• PSI = pounds-force per square inch — US customary unit.

1. The pascal and SI pressure measurement
2. Conversion reference table
3. Atmospheres, bar, and meteorology
4. PSI and US customary pressure
5. Torr, mmHg, and vacuum measurement
6. Frequently Asked Questions

The pascal and SI pressure measurement

Definition. The pascal (Pa), named after French mathematician Blaise Pascal, is the SI derived unit of pressure. One pascal equals one newton of force distributed over one square meter: 1 Pa = 1 N/m². A one-newton force is approximately the weight of a 102 g apple; spread over a full square meter, that force produces a pressure of just 1 Pa — a very small unit, which is why kPa (1,000 Pa) and MPa (1,000,000 Pa) are more common in practice.

Blaise Pascal's contribution. In the 1640s, Pascal demonstrated that atmospheric pressure decreases with altitude by having a barometer carried up the Puy-de-Dôme mountain in France. The 1,000 m ascent showed a measurable 84 mmHg drop, proving that air has weight and that pressure decreases predictably with elevation. This principle underlies all modern altimetry and weather forecasting.

Practical examples of pascals.

• Atmospheric pressure at sea level: ~101,325 Pa (101.325 kPa)
• Standard car tire pressure: ~220,000 Pa (220 kPa / 32 PSI)
• Household water pressure: ~350,000 Pa (350 kPa / 50 PSI)
• Scuba tank (full): ~20,000,000 Pa (20 MPa / 3,000 PSI)

Conversion reference table

Fundamental conversion relationships:

• 1 atm = 101,325 Pa
• 1 bar = 100,000 Pa (exactly)
• 1 PSI = 6,894.76 Pa
• 1 torr = 133.322 Pa
• 1 mmHg = 133.322 Pa (same value as torr)
• 1 inHg = 3,386.39 Pa

Pressure Unit Conversion Reference

Worked example: converting 35 PSI to kPa and bar.

35 PSI × 6,894.76 = 241,317 Pa ÷ 1,000 = 241.3 kPa. 241,317 ÷ 100,000 = 2.413 bar. Typical car tire pressure: 35 PSI = 2.4 bar = 241 kPa.

Worked example: converting 1,020 mbar to atm and inHg.

1,020 mbar ÷ 1,000 = 1.020 bar. 1.020 × 0.9869 = 1.007 atm. 1.007 × 29.9213 = 30.13 inHg. A high-pressure weather system.

Atmospheres, bar, and meteorology

Standard atmosphere. The standard atmosphere (atm) was originally defined as the average sea-level atmospheric pressure. It was later fixed at exactly 101,325 Pa by international agreement in 1954. One atmosphere is the pressure that supports a 760 mm column of mercury at 0 °C at standard gravity.

The bar and millibar in weather. The bar (from Greek baros, meaning weight) was introduced in 1909 by British meteorologist William Napier Shaw. One bar = 100,000 Pa — close enough to 1 atm (101,325 Pa) that the 1.3% difference was acceptable for weather work. Millibars (mbar) became the standard weather pressure unit worldwide until the 1990s, when most countries switched to hectopascals (hPa), which are numerically identical: 1 hPa = 1 mbar.

Worked example: reading a weather chart.

A weather map shows a high-pressure center at 1,035 mbar. In hPa: 1,035 hPa. In atm: 1,035 × 0.9869 ÷ 1,000 = 1.021 atm. In inHg: 1.021 × 29.9213 = 30.55 inHg. A strong high-pressure system bringing clear weather.

Worked example: aviation altimeter setting.

An airport reports altimeter setting 29.92 inHg (standard pressure). In hPa: 29.92 × 33.8639 = 1,013.2 hPa. In mbar: 1,013.2 mbar. Pilots set this value into their altimeter so it reads 0 ft at sea level in standard conditions.

PSI and US customary pressure

Pounds per square inch. PSI (pounds-force per square inch) is the standard pressure unit in the United States for almost all practical applications: tire pressure, HVAC refrigerant pressures, hydraulic systems, and scuba tanks. One PSI = 6,894.76 Pa. The conversion to bar is approximately 1 bar = 14.5 PSI.

Worked example: converting a scuba tank pressure.

A scuba tank filled to 3,000 PSI. In bar: 3,000 ÷ 14.504 = 206.8 bar. In Pa: 3,000 × 6,894.76 = 20,684,280 Pa = 20.68 MPa. A standard aluminum 80 cubic-foot tank holds 3,000 PSI at room temperature.

PSI and gauge pressure. PSIG (PSI gauge) measures pressure relative to atmospheric pressure. A tire gauge reading 32 PSIG means 32 PSI above the surrounding 14.7 PSI of atmosphere — or 46.7 PSIA (PSI absolute). This distinction matters for engineering calculations involving gas laws.

Worked example: absolute vs gauge pressure.

A hydraulic system reads 2,500 PSIG. Absolute pressure: 2,500 + 14.7 = 2,514.7 PSIA. In Pa: 2,514.7 × 6,894.76 = 17,338,000 Pa = 17.34 MPa. In bar: 17.34 × 10 = 173.4 bar.

Torr, mmHg, and vacuum measurement

Torricelli's legacy. Evangelista Torricelli, a student of Galileo, invented the mercury barometer in 1643. He filled a 1 m glass tube with mercury, inverted it into a dish, and observed that the mercury column stood at about 760 mm above the dish surface — the weight of the atmosphere supporting the column. This gave the world the millimeter of mercury (mmHg) as a pressure unit.

Torr as a unit. One torr is defined as 1/760 of a standard atmosphere — making it exactly equal to 1 mmHg for most practical purposes (the difference is 1.9 × 10⁻⁷ Pa, negligible outside metrology). The torr is the standard unit for vacuum measurement in physics and industrial vacuum systems.

Worked example: converting vacuum pump rating.

A vacuum pump is rated at 0.1 torr. In Pa: 0.1 × 133.322 = 13.33 Pa. In atm: 0.1 ÷ 760 = 1.32 × 10⁻⁴ atm. This is a medium vacuum — approximately 99.987% of the gas has been removed from the chamber.

Worked example: blood pressure reading.

A blood pressure reading of 120/80 mmHg. Systolic: 120 mmHg = 120 × 133.322 = 15,998.6 Pa = 16.0 kPa. Diastolic: 80 mmHg = 80 × 133.322 = 10,665.8 Pa = 10.7 kPa. This is the gauge pressure relative to atmospheric — the pressure inside the artery above ambient.