Pressure describes force spread over an area. Simple concept, many units. If you have ever looked at a tire sidewall, a weather report, and a blood pressure reading on the same day, you have already encountered three different pressure units without trying.
This guide explains the most common ones, how to convert between them, and which unit to use when.
What Is Pressure?
Pressure is force per unit area:
Pressure = Force ÷ Area
The SI unit is the pascal (Pa), equal to one newton per square meter. In practice, a single pascal is tiny — atmospheric pressure is about 101,325 Pa — so larger multiples and alternative units are used depending on the field.
The Seven Most Common Pressure Units
Pascal (Pa) and Kilopascal (kPa)
The pascal is the base SI unit. Because one pascal is small, kilopascals (1 kPa = 1,000 Pa) and megapascals (1 MPa = 1,000,000 Pa) are more practical.
- Used in: engineering specifications, scientific research, tire pressure (in kPa) in some countries
- Key relationship: 1 atm = 101.325 kPa
Bar
One bar equals exactly 100,000 Pa. It is close to one atmosphere (1 bar ≈ 0.987 atm), which makes it an intuitive metric unit for everyday pressure.
- Used in: European tire pressure, industrial equipment, compressed gas, scuba diving
- Key relationship: 1 bar = 14.5038 psi
Millibar (mbar) and Hectopascal (hPa)
One millibar equals 100 Pa. One hectopascal also equals 100 Pa. They are identical — meteorologists gradually adopted hPa as the SI-aligned name, but mbar remains common in weather reports and aviation.
- Used in: weather forecasting, barometric pressure, aviation altimetry
- Key relationship: standard sea-level pressure = 1013.25 mbar
PSI (Pounds per Square Inch)
PSI measures how many pounds of force act on one square inch. It is the dominant pressure unit in the United States and United Kingdom for consumer and industrial applications.
- Used in: tire pressure (US/UK), water systems, HVAC, compressed air
- Key relationship: 1 atm = 14.696 psi
Atmosphere (atm)
One standard atmosphere is defined as exactly 101,325 Pa. It approximates average sea-level air pressure and serves as a convenient reference point.
- Used in: chemistry (gas laws), diving depth calculations, altitude discussions
- Key relationship: 1 atm = 1.01325 bar = 14.696 psi = 760 mmHg
mmHg (Torr)
Millimeters of mercury measure the height of a mercury column supported by the pressure. Named after Evangelista Torricelli, who invented the mercury barometer in 1643.
- Used in: blood pressure, vacuum technology, older barometric instruments
- Key relationship: 1 atm = 760 mmHg; normal blood pressure ≈ 120/80 mmHg
Inches of Mercury (inHg)
The imperial equivalent of mmHg. Measures mercury column height in inches instead of millimeters.
- Used in: US aviation (altimeter settings), US weather reports
- Key relationship: 1 atm = 29.921 inHg
How to Convert Between Pressure Units
All pressure units can be converted through pascals as a common base. Multiply the input by its "to-pascal" factor, then divide by the target unit's factor.
Most-Used Conversion Formulas
Bar ↔ PSI:
- psi = bar × 14.5038
- bar = psi × 0.0689476
Atmosphere ↔ PSI:
- psi = atm × 14.696
- atm = psi ÷ 14.696
Bar ↔ Atmosphere:
- atm = bar × 0.986923
- bar = atm × 1.01325
kPa ↔ PSI:
- psi = kPa × 0.145038
- kPa = psi × 6.89476
mmHg ↔ atm:
- atm = mmHg ÷ 760
- mmHg = atm × 760
For any conversion, the pressure unit converter handles all 8 units instantly.
Worked Examples
Example 1: Tire Pressure (psi to bar)
A US tire gauge reads 35 psi. What is that in bar?
35 × 0.0689476 = 2.413 bar
European gauges would show approximately 2.4 bar.
Example 2: Weather Report (mbar to inHg)
A forecast says barometric pressure is 1023 mbar. Convert to inHg:
1023 mbar = 102,300 Pa 102,300 ÷ 3,386.389 = 30.21 inHg
This is slightly above standard atmospheric pressure (29.921 inHg), indicating fair weather.
Example 3: Diving Depth (meters to atm)
Every 10 meters of seawater adds approximately 1 atmosphere of pressure. At 30 meters depth:
Absolute pressure = 1 atm (surface) + 3 atm (water) = 4 atm = 58.78 psi = 4.053 bar
Example 4: Blood Pressure (mmHg to kPa)
A reading of 120/80 mmHg in kilopascals:
- Systolic: 120 × 0.133322 = 16.0 kPa
- Diastolic: 80 × 0.133322 = 10.7 kPa
Some countries (notably France and parts of Europe) report blood pressure in kPa.
Which Unit Should You Use?
Different fields have settled on different units. Using the right one avoids confusion:
| Application | Common Unit | Why |
|---|---|---|
| Car/truck tires (US, UK) | psi | Industry standard in those countries |
| Car/truck tires (Europe, Asia) | bar or kPa | Metric convention |
| Blood pressure | mmHg | Medical tradition, worldwide |
| Weather reports (most countries) | mbar or hPa | Meteorological standard |
| Weather reports (US) | inHg | Legacy convention |
| Aviation altimeters | mbar, hPa, or inHg | Depends on region |
| Scuba diving | bar or atm | Simple depth-to-pressure math |
| Industrial equipment | bar or psi | Depends on region |
| Chemistry / gas laws | atm or kPa | Convenient for standard conditions |
| Vacuum technology | mmHg (torr) or Pa | Low-pressure precision |
Quick Reference Table: 1 atm in Every Unit
| Unit | Value at 1 atm |
|---|---|
| Pascal (Pa) | 101,325 |
| Kilopascal (kPa) | 101.325 |
| Bar | 1.01325 |
| Millibar (mbar) | 1,013.25 |
| PSI | 14.696 |
| mmHg (Torr) | 760 |
| Inches of Mercury (inHg) | 29.921 |
Common Mistakes
-
Confusing gauge pressure with absolute pressure. Tire gauges read pressure above atmospheric. A tire at 32 psi (gauge) has an absolute pressure of about 46.7 psi. Most everyday tools use gauge pressure.
-
Treating mbar and bar as the same order of magnitude. 1 bar = 1,000 mbar. Mixing them up shifts your value by three orders of magnitude.
-
Using mmHg for non-medical contexts. Outside medicine and vacuum tech, mmHg can confuse readers. Convert to bar, psi, or kPa for general use.
-
Forgetting that water depth pressure is gauge pressure. At 10 m underwater, the gauge pressure is ~1 atm, but absolute pressure is ~2 atm (water + air above).
Frequently Asked Questions
Why are there so many pressure units?
Different fields developed independently. Medicine inherited mmHg from mercury instruments. Engineering in the US adopted psi from the imperial system. Meteorology uses mbar/hPa for convenient atmospheric-range numbers. The SI system defines the pascal, but it is too small for everyday use on its own.
Is kPa the same as mbar?
No, but they are related. 1 kPa = 10 mbar. Both are metric, but kPa is SI-aligned while mbar is traditionally used in meteorology.
What does "gauge pressure" mean?
Gauge pressure is the difference between absolute pressure and atmospheric pressure. A tire at 32 psi gauge reads zero when flat and exposed to air. Absolute pressure would be about 46.7 psi for the same tire.
Does altitude affect pressure?
Yes. Atmospheric pressure drops roughly 12 mbar (about 0.12 kPa) per 100 meters of elevation gain near sea level. At 5,500 meters (18,000 feet), pressure is about half of sea-level pressure.
Can I use this converter for vacuum pressures?
Yes. Enter the absolute pressure value and convert normally. For very low pressures (high vacuum), values in pascal or torr will be most practical.
Use the pressure unit converter to convert between any of these units instantly. For specific bar-to-psi conversions, see the bar to psi converter. For the reverse, use the psi to bar converter.