Pressure is force spread over an area, and nearly every field has settled on its own unit for it. American tire gauges read psi, European ones bar; doctors record blood pressure in millimeters of mercury, meteorologists use hectopascals or inches of mercury, HVAC technicians measure duct pressure in inches of water, and engineers use kilopascals and megapascals. This converter covers 18 of those units with exact factors from NIST SP 811, and also shows what a gauge reading means in absolute terms.
How to use the pressure converter
- Enter the reading in Value.
- Choose the unit you have in From and the unit you want in To.
- The tape shows the result, both factors and the absolute pressure if your value is a gauge reading taken at sea level.
- The table lists the same pressure in every unit.
Pressure conversion formula
Every unit is defined as a number of pascals (1 Pa = 1 N/m²):
Key definitions: 1 psi = 6,894.757293168 Pa (one pound-force on one square inch), 1 bar = 100,000 Pa, 1 atm = 101,325 Pa, and 1 torr = 101,325/760 Pa. The mercury and water-column units are conventional values based on a standard density and standard gravity.
Worked example: tire pressure
The door sticker says 32 psi, but your compressor shows kPa and bar:
32 psi × 6.894757 = 220.63 kPa
220,632 Pa ÷ 100,000 = 2.21 bar
Absolute pressure at sea level: 220.63 + 101.325 = 321.96 kPa (46.70 psia).
The gauge value is what you set; the absolute value is what the gas laws need if you are calculating how pressure changes with temperature.
Common pressure conversions
| From | To | Multiply by |
|---|---|---|
| psi | kPa | 6.8947573 |
| kPa | psi | 0.1450377 |
| bar | psi | 14.503774 |
| psi | bar | 0.0689476 |
| atm | psi | 14.695949 |
| atm | kPa | 101.325 (exact) |
| atm | mmHg | 760 (by the torr definition) |
| inHg | hPa (mbar) | 33.86389 |
| mmHg | kPa | 0.1333224 |
| MPa | psi | 145.03774 |
| ksi | MPa | 6.8947573 |
Reference pressures
| Situation | Typical value |
|---|---|
| Standard sea-level air | 101.325 kPa · 14.696 psi · 29.92 inHg · 1,013.25 hPa |
| Car tire (gauge) | 30–35 psi · 2.1–2.4 bar |
| Road bike tire (gauge) | 80–100 psi · 5.5–6.9 bar |
| Normal blood pressure | below 120/80 mmHg (16.0/10.7 kPa) |
| Household water supply | 40–80 psi · 280–550 kPa |
| Scuba tank, full | about 3,000 psi · 207 bar |
Gauge versus absolute pressure
Most everyday instruments read gauge pressure, the difference from the air around them. An “empty” tire still contains air at atmospheric pressure, yet the gauge says zero. Absolute pressure counts from a perfect vacuum and is what physics equations such as the ideal gas law require. Engineers mark the difference with suffixes: psig for gauge and psia for absolute. Vacuum gauges sometimes read negative values or “inches of vacuum,” which are gauge readings below atmospheric pressure.
Choosing a unit
- Pa, kPa, MPa: the SI family. Building codes, material strengths and scientific work.
- psi and ksi: US tires, hydraulics, compressors and structural steel stress.
- bar and mbar: European tire gauges, diving, weather (1 mbar = 1 hPa).
- atm: chemistry and high-pressure reactions.
- mmHg and torr: blood pressure, vacuum systems, physiology.
- inHg: US weather reports and aircraft altimeter settings.
- inH₂O and cmH₂O: low pressures in ductwork, gas appliances and ventilators.
To calculate pressure from a force and an area, use the pressure calculator. The force converter handles the force half of the units if you need to build a custom one, such as newtons per square centimeter.
Frequently asked questions
How do I convert psi to kPa?
Multiply by 6.894757. A tire inflated to 32 psi holds 32 × 6.894757 = 220.6 kPa. To go from kPa to psi, multiply by 0.1450377.
How many psi is one bar?
One bar is 100,000 Pa, which is about 14.504 psi. It is close to, but slightly less than, one standard atmosphere (1.01325 bar, or 14.696 psi).
What is the difference between psi and psig or psia?
psig is gauge pressure, measured relative to the surrounding air, so an open tire gauge reads zero. psia is absolute pressure, measured from a perfect vacuum. At sea level, psia ≈ psig + 14.696.
Are torr and mmHg the same?
They differ by less than one part in seven million. A torr is defined as exactly 1/760 of a standard atmosphere; the conventional millimeter of mercury is 133.322387415 Pa. For any practical purpose they are interchangeable.
Why do weather reports use millibars or hectopascals?
One millibar equals one hectopascal, 100 Pa, and normal sea-level air pressure is about 1,013 of them, a convenient range for weather maps. US broadcasts often give the same reading in inches of mercury, around 29.92 inHg.