Pressure Unit Converter
Convert between Pascal, psi, atm, bar, mmHg, and torr pressure units.
What Is the Pressure Unit Converter?
Pressure is the force applied perpendicular to a surface divided by the area of that surface. The SI unit, pascal (Pa), is extremely small, one pascal is the weight of an apple spread over a square metre. In practice, multiples like kPa, MPa, and non-SI units like bar and psi are far more common in engineering.
- ›SI group, Pa, kPa, MPa, hPa. Used in physics, structural engineering, and formally in all scientific publications. Hectopascal (hPa) is identical to millibar and is the standard in meteorology.
- ›Engineering group, bar, mbar, atm, technical atm. The bar (100 kPa) is widely used in European industrial equipment; atm is the reference standard for chemistry gas laws.
- ›Imperial group, psi (lb-force per square inch). Standard in US engineering for tyre pressure, hydraulic systems, boilers, and plumbing.
- ›Medical/weather group, mmHg, cmHg, inHg. Mercury-manometer-based units that persist in medicine (blood pressure), vacuum systems, and aviation altimetry.
Formula
SI Base Unit: Pascal (Pa)
1 Pa = 1 N/m² = 1 kg/(m·s²)
Conversion Factors to Pascal (NIST SP 811 / ISO 80000-4)
1 kPa = 1,000 Pa
1 MPa = 1,000,000 Pa
1 bar = 100,000 Pa
1 atm = 101,325 Pa (exact, by BIPM definition)
1 psi = 6,894.757 Pa
1 mmHg = 133.322 Pa
1 inHg (32°F) = 3,386.389 Pa
| Unit | Symbol | Pa equivalent | Field |
|---|---|---|---|
| Pascal | Pa | 1 | Physics, SI base |
| Kilopascal | kPa | 1,000 | Engineering, medicine |
| Megapascal | MPa | 1,000,000 | Structural, hydraulics |
| Hectopascal | hPa | 100 | Meteorology (= mbar) |
| Bar | bar | 100,000 | Industrial, HVAC |
| Millibar | mbar | 100 | Weather reports |
| Atmosphere | atm | 101,325 | Chemistry, gas laws |
| PSI | psi | 6,895 | US engineering, tyres |
| mmHg / Torr | mmHg | 133.3 | Medical, vacuum |
| Inches Hg | inHg | 3,386 | Aviation altimetry |
How to Use
- 1Enter a numeric pressure value in the "Value" field.
- 2Select your source unit from the "From unit" dropdown.
- 3All 12 equivalent values appear instantly, grouped by category (SI, Engineering, Imperial, Medical/Weather).
- 4Click any result row to set that unit as the new source unit, the converter reconfigures with your new value.
- 5Use the copy button on any row to copy that converted value to clipboard.
- 6Press Clear to reset and remove the saved state.
Example Calculation
Converting blood pressure: a systolic reading of 120 mmHg to other units:
120 mmHg × 133.322 Pa/mmHg = 15,999 Pa
= 16.0 kPa = 0.160 bar = 0.158 atm = 2.32 psi
= 12.0 cmHg = 0.472 inHg
Converting tyre pressure from US to metric: 35 psi to kPa:
35 psi × 6,894.757 = 241,317 Pa = 241.3 kPa ≈ 2.41 bar
Standard Reference Pressures
- ›1 atm = 101,325 Pa = 14.696 psi = 760 mmHg = 1013.25 mbar
- ›Sea-level average ≈ 101.3 kPa (varies with weather)
- ›Bicycle tyre (road) = 690–830 kPa (100–120 psi)
- ›Car tyre = 220–260 kPa (32–38 psi)
Understanding Pressure Unit Converter
Pressure Unit History and Usage
The diversity of pressure units reflects the fact that pressure was independently measured in different contexts long before international standards existed. Mercury manometers gave us mmHg; early industrial steam gauges gave us bar and psi; atmospheric science gave us mbar and inHg; and the SI system gave us the official pascal. Each community settled on units that made their numbers convenient to work with.
Quick Conversion Reference
| From → To | Multiply by |
|---|---|
| psi → kPa | 6.89476 |
| kPa → psi | 0.14504 |
| bar → psi | 14.5038 |
| atm → kPa | 101.325 |
| mmHg → Pa | 133.322 |
| inHg → hPa | 33.864 |
Frequently Asked Questions
What is the difference between Pa, kPa, and MPa?
The three SI pressure units span different engineering scales:
- ›Pa (pascal), the base unit. 1 Pa ≈ the weight of a 10 g mass on 1 m². Used for sound pressure levels, very low differential pressures, and formal scientific notation.
- ›kPa (kilopascal), the practical everyday unit. Tyre pressure (220–260 kPa), atmospheric pressure (101.3 kPa), blood pressure (10–16 kPa), HVAC duct pressure (0.1–2.5 kPa).
- ›MPa (megapascal), for high-pressure systems. Hydraulic machinery (7–35 MPa), concrete compressive strength (20–80 MPa), steel yield strength (250–1,500 MPa).
- ›GPa (gigapascal), for extreme pressures. Diamond hardness (≈ 90–100 GPa), Earth's core (≈ 360 GPa). Not included in this converter due to rarity in everyday use.
Why is blood pressure still measured in mmHg?
The persistence of mmHg in medicine is a classic example of institutional inertia:
- ›Historical origin, early blood pressure measurement used a U-tube with mercury. The height of mercury supported by blood pressure became the natural measurement unit.
- ›Clinical familiarity, "120/80 mmHg" is recognized by every clinician globally. Switching to kPa (16/10.7) would require updating guidelines, textbooks, and electronic health records worldwide.
- ›SI recommendation, many countries (UK, Australia) officially recommend kPa for blood pressure, but adoption has been slow even there due to clinical resistance.
- ›Quick conversion, 1 mmHg = 133.322 Pa ≈ 0.1333 kPa. To convert: multiply mmHg by 0.1333 to get kPa.
What is the difference between bar and atm?
Bar and atm are close but not identical:
- ›1 bar = 100,000 Pa exactly. Defined by BIPM as a convenient engineering unit. 1 mbar = 1 hPa.
- ›1 atm = 101,325 Pa exactly. Defined as standard atmospheric pressure at sea level and 0°C. Slightly larger than 1 bar.
- ›Difference, 1 atm = 1.01325 bar. A 1.3% discrepancy. In gas chromatography and thermodynamics tables, mixing up the two can introduce small but real errors.
- ›Technical atmosphere (at), a third unit: 1 at = 98,066.5 Pa = 1 kgf/cm². Used in older German and Russian technical literature.
What does hPa mean in weather forecasts?
hPa is the standard meteorological pressure unit:
- ›Definition, 1 hPa = 100 Pa = 1 mbar. The prefix "hecto" means 100. Meteorologists adopted it when updating from millibar to SI without changing numeric values.
- ›Standard sea level, 1013.25 hPa (= 1 atm = 29.92 inHg). Pressure above this means high pressure; below means low pressure.
- ›Weather significance, pressure gradient drives wind. A 10 hPa difference over 100 km creates moderate winds. Hurricane centers can drop to 870–950 hPa.
- ›Aviation, altimeter settings are in hPa in most of the world (inHg in the US/Canada). 1 hPa ≈ 8 m of altitude change near sea level.
What is PSI and where is it used?
PSI is the dominant pressure unit in US engineering and daily life:
- ›Definition, 1 psi = 1 lbf/in² = 6,894.757 Pa. It measures force in pounds-force per square inch of area.
- ›Tyre pressure, car tyres 32–38 psi, truck tyres 75–110 psi, bicycle road tyres 80–130 psi, motorcycle tyres 30–42 psi.
- ›Plumbing, municipal water supply: 40–80 psi. Residential recommended: 40–60 psi. PRV (pressure reducing valve) typically set to 50–60 psi.
- ›Hydraulics, construction equipment: 2,000–5,000 psi. Industrial hydraulic presses: up to 10,000 psi. Scuba tanks: 2,900–3,000 psi.
Why does aviation use inHg for altimeter settings?
inHg persists in aviation for historical and safety reasons:
- ›Standard atmosphere, 29.92 inHg = 1013.25 hPa. Used as the default setting above 18,000 ft (Flight Level) in North America.
- ›QNH setting, pilots adjust the altimeter sub-scale to local sea-level pressure so altitude reads correctly on the ground. Keeps all aircraft on the same reference.
- ›US/Canada vs rest of world, the US and Canada use inHg; most other countries and ICAO officially use hPa. Conversion: multiply inHg by 33.864 to get hPa.
- ›Precision, altimeter settings are given to 2 decimal places in inHg (e.g., 29.88 inHg). Each 0.01 inHg represents about 0.34 hPa, or roughly 3 m of altitude near sea level.
What units does this pressure converter include?
The 12 units cover all major pressure measurement contexts:
- ›SI group (4), Pa, kPa, MPa, hPa. Covers everyday to structural engineering pressures.
- ›Engineering group (4), bar, mbar, atm, technical atm (at). Covers industrial equipment and chemical processes.
- ›Imperial group (1), psi. Covers US engineering, plumbing, and HVAC.
- ›Medical/Weather group (3), mmHg, cmHg, inHg. Covers clinical measurement, vacuum systems, and aviation.
- ›Accuracy, all conversion factors from NIST SP 811 and ISO 80000-4. Calculations run live in your browser with full floating-point precision.
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