What is atmospheric pressure?
Atmospheric pressure is the force exerted on a surface by the weight of air above it per unit area. At sea level, the column of air stretching from the ground to the edge of the atmosphere weighs approximately 10.3 tonnes per square metre — a force of 101,325 Newtons per square metre, or 101,325 Pa (Pascals).
This is the International Standard Atmosphere (ISA) at sea level, defined as exactly 101,325 Pa. It goes by many names depending on the unit system: 1 atm, 1013.25 hPa, 1013.25 mbar, 760 mmHg, 14.696 PSI, 29.92 inHg.
To quickly convert between any of these units, use the Pressure Converter — enter a value in any unit and see all others simultaneously.
Why atmospheric pressure isn’t constant
We often speak of “1 atmosphere” as if pressure were fixed, but in reality atmospheric pressure varies continuously due to:
Altitude
Pressure decreases with altitude because there is less air overhead. The relationship is approximately exponential (the barometric formula):
P(h) ≈ P₀ × e^(−h/H)
where P₀ is sea-level pressure, h is altitude in metres, and H is the scale height (~8,500 m for air at 15°C).
In practice:
- Sea level: 1013 hPa
- 500 m (Delhi): ~955 hPa
- 1,000 m: ~899 hPa
- 2,000 m (many hill stations): ~795 hPa
- 3,500 m (Leh, Ladakh): ~655 hPa
- 5,380 m (Everest base camp): ~500 hPa
- 8,849 m (Everest summit): ~337 hPa
This is why aircraft cabins are pressurised to roughly 750 hPa (equivalent to ~2,400 m altitude), and why altitude sickness affects climbers above ~2,500 m.
Weather systems
Atmospheric pressure at a fixed altitude varies day-to-day with weather patterns:
High pressure systems (anticyclones): Pressure above ~1020 hPa at sea level. Associated with settled, dry, sunny weather. Air descends slowly, suppressing cloud formation.
Low pressure systems (cyclones / depressions): Pressure below ~1000 hPa at sea level. Associated with unsettled, cloudy, rainy weather. Air rises and cools, forming clouds and precipitation.
Extreme weather:
- Strongest recorded hurricane/cyclone: central pressure ~870 hPa
- Strongest high pressure: ~1083.8 hPa (Agata, Siberia, December 2022)
- Average sea-level pressure: 1013.25 hPa
The units of atmospheric pressure
Different fields standardised on different units, all in common use today:
Pascal (Pa) and derivatives
The Pascal is the SI unit of pressure: 1 Pa = 1 N/m². It is the base unit for all pressure conversions.
- hPa (hectopascal) = 100 Pa. The unit of choice in meteorology. Standard atmosphere = 1013.25 hPa.
- kPa (kilopascal) = 1,000 Pa. Used in weather and physics. Standard atmosphere = 101.325 kPa.
- MPa (megapascal) = 1,000,000 Pa. Used in engineering (material strength, deep-earth geology).
Bar and millibar
- bar = 100,000 Pa = 100 kPa. Close to 1 atm (which is 101.325 kPa). Used in diving, meteorology (older literature), and European engineering.
- mbar (millibar) = 100 Pa = 1 hPa. Numerically identical to hPa. Weather maps often label isobars in mbar.
mmHg and Torr
- mmHg = the pressure exerted by a 1 mm column of mercury at 0°C. Defined as exactly 133.322 Pa.
- Torr ≈ 1 mmHg (exactly 101325/760 Pa ≈ 133.322 Pa). Originally defined differently but now effectively the same for practical purposes.
Used in: blood pressure measurements, vacuum technology, gas chemistry, ophthalmology (intraocular pressure in mmHg).
inHg (inches of mercury)
inHg = the pressure exerted by a 1-inch mercury column = 25.4 mmHg = 3,386.4 Pa. Standard atmosphere = 29.92 inHg. Used in US aviation (altimeter settings are in inHg) and US weather forecasts.
atm (standard atmosphere)
1 atm = exactly 101,325 Pa. A defined constant — not a measured physical quantity. Used as a reference in chemistry, scuba diving (depth-pressure relationships), and aviation.
PSI (pounds per square inch)
1 PSI = 6,894.76 Pa ≈ 6.895 kPa. Standard atmosphere ≈ 14.696 PSI. Used in US engineering, automotive tyre pressure, and industrial equipment specifications.
How a barometer measures atmospheric pressure
A mercury barometer (invented by Evangelista Torricelli in 1643) is a tube sealed at one end, filled with mercury, and inverted into a dish of mercury. The mercury column rises to a height proportional to atmospheric pressure — about 760 mm at standard sea-level conditions. The vacuum above the mercury column (Torricelli’s vacuum) is essentially a perfect vacuum at negligible pressure.
An aneroid barometer uses a sealed flexible capsule that expands and contracts with pressure changes, mechanically linked to a needle. It doesn’t need liquid mercury, making it portable and safe.
Modern digital barometers use MEMS (Micro-Electro-Mechanical Systems) pressure sensors on a silicon chip. These are in every smartphone and smartwatch, accurate to ±1 hPa or better.
Applications by unit
| Application | Unit | Typical range |
|---|---|---|
| Weather forecasts | hPa / mbar | 950–1050 hPa |
| Aviation altimeter | inHg | 28–31 inHg |
| Blood pressure | mmHg | 60–200 mmHg |
| Tyre pressure | bar or PSI | 2–3 bar / 30–45 PSI |
| Scuba diving | bar or atm | 1–10 bar |
| Industrial equipment | bar or MPa | varies widely |
| Laboratory vacuum | Pa or Torr | 0.001–760 Torr |
Converting atmospheric pressure between units
The Pressure Converter handles all conversions in real time. Enter the value in any unit — Pa, kPa, MPa, bar, mbar, PSI, atm, mmHg, Torr, inHg, or inH₂O — and see the equivalent in all other units simultaneously, to 8 significant figures.
Reference table for 1 standard atmosphere:
| Unit | Value |
|---|---|
| Pa | 101,325 |
| kPa | 101.325 |
| hPa / mbar | 1013.25 |
| bar | 1.01325 |
| PSI | 14.696 |
| atm | 1.000 |
| mmHg | 760.00 |
| Torr | 760.00 |
| inHg | 29.921 |
Summary
Atmospheric pressure is the weight of the air column above a surface per unit area — 101,325 Pa at sea level under standard conditions. It varies with altitude (decreasing exponentially) and weather (ranging roughly 950–1050 hPa at sea level). Different fields use different units: hPa for weather, inHg for US aviation, mmHg for medicine, bar for diving. The Pressure Converter converts between all of them instantly.