Pressure Unit Converter
Convert between any pair of pressure units — Pascal, kilopascal, bar, atmosphere, psi, mmHg, Torr, and more — using conversion factors verified against NIST and BIPM references. Type a value and the result updates instantly.
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What is pressure, and why are there so many units?
Pressure is force distributed over an area. In SI units, one Pascal (Pa) is one newton of force pressed onto one square meter — a tiny amount, roughly the weight of a sheet of paper resting on your hand. Because real-world pressures span more than fifteen orders of magnitude — from the near-vacuum of a freeze-dryer to the multi-gigapascal forging of metals — engineers have inherited a long list of unit families to keep numbers in a comfortable range.
The reason there are so many pressure units is historical, not scientific. Tire shops grew up using psi, weather services adopted millibar (now relabelled hPa), hospitals never moved off mmHg for blood pressure, plumbing standards in metric countries default to bar, and high-pressure hydraulics speak in MPa. Each industry locked in a unit before the SI system was finalised, and switching costs are high enough that the old units persist.
The pressure units, explained
Pascal (Pa) and its multiples — the SI baseline
The Pascal is the coherent SI derived unit: 1 Pa = 1 N/m². It is too small for most engineering work, so the kilopascal (kPa, 10³ Pa) and megapascal (MPa, 10⁶ Pa) dominate practical use. Materials engineers quote tensile strength in MPa; HVAC and structural pressures land in kPa; the hectopascal (hPa, 10² Pa) is the modern label for atmospheric pressure on weather charts.
Bar and millibar — convenient near-atmospheric scale
One bar is exactly 100,000 Pa, chosen so that 1 bar ≈ 1 atm within 1.3%. It is widely used in pneumatics, hydraulics, and meteorology because round-number values fall in everyday ranges. The millibar (1 mbar = 100 Pa = 1 hPa) was the standard in weather forecasting before the WMO recommended hPa; the two units are numerically identical.
Standard atmosphere (atm) — the historical reference
The standard atmosphere is fixed at 101,325 Pa exactly — defined to match average sea-level air pressure at 15 °C. It was demoted from official SI use in 1982 but survives in chemistry tables, gas laws, and dive-table equations.
Pounds per square inch (psi) — the imperial workhorse
1 psi is defined as 1 lbf / 1 in² = 6,894.757293168 Pa. It is the de-facto unit for car and bicycle tires, US plumbing, compressed-air tools, scuba cylinders, and any equipment specified to imperial standards. Variants — psia (absolute), psig (gauge), psid (differential) — describe the reference, not the unit.
Millimeter of mercury (mmHg) and Torr — clinical and vacuum work
Both units descend from Torricelli's mercury barometer. The conventional mmHg is fixed at 133.322387415 Pa, while the Torr is defined as exactly 1/760 of a standard atmosphere (133.3223684… Pa). The two differ only in the eighth decimal place, but mmHg is preferred in medicine (blood pressure cuffs read mmHg) and Torr in vacuum technology.
Inch of mercury (inHg) and kgf/cm² — niche but persistent
Aviation altimeters and US weather reports still use inHg (1 inHg ≈ 3,386.389 Pa). The kilogram-force per square centimeter (kgf/cm² ≈ 98,066.5 Pa) survives in older European and Russian engineering documents, where it sits within 4% of the bar.
Real-world applications and which unit to expect
- Automotive tires: Most cars list cold pressure in psi (typically 30–35 psi) or bar (2.0–2.4 bar). 1 bar ≈ 14.5 psi, which is why "30 psi" and "2.07 bar" appear together on tire placards.
- Weather forecasting: Surface charts use hPa; a typical sea-level reading is 1013 hPa (= 1.013 bar = 1 atm). A hurricane core can drop below 920 hPa.
- Medicine — blood pressure: Sphygmomanometers measure in mmHg. A reading of 120/80 means systolic pressure of 120 mmHg over a diastolic 80 mmHg, equivalent to ~16/10.7 kPa.
- Industrial hydraulics: Hydraulic presses, injection molders, and heavy machinery routinely operate at 10–70 MPa (≈100–700 bar).
- Vacuum systems: Mechanical pumps reach the millitorr range (1 mTorr ≈ 0.133 Pa), turbomolecular pumps reach 10⁻⁷ Torr or below — Torr is the lingua franca.
- Scuba diving: Cylinder pressures are quoted in bar (300 bar = 4,350 psi service pressure) or psi (3,000 psi). Depth pressure is +1 atm per 10 meters of seawater.
- Materials testing: Tensile strength, yield strength, and Young's modulus are reported in MPa or GPa. Steel rebar yields around 400–500 MPa.
How much is 1 unit of each in Pascal?
| Unit | Value in Pascal (Pa) |
|---|---|
| 1 Pa (Pascal) | 1 Pa |
| 1 hPa (Hectopascal) | 100 Pa |
| 1 kPa (Kilopascal) | 1000 Pa |
| 1 MPa (Megapascal) | 1000000 Pa |
| 1 bar (Bar) | 100000 Pa |
| 1 mbar (Millibar) | 100 Pa |
| 1 atm (Standard atmosphere) | 101325 Pa |
| 1 psi (Pound per square inch) | 6894.757293168 Pa |
| 1 mmHg (Millimeter of mercury) | 133.322387415 Pa |
| 1 Torr (Torr) | 133.32236842105263 Pa |
| 1 inHg (Inch of mercury) | 3386.389 Pa |
| 1 kgf/cm² (Kilogram-force per square centimeter) | 98066.5 Pa |
Frequently asked questions about pressure units
How do I convert between any two pressure units?
Multiply by the source unit's Pascal-factor and divide by the target's. Example: convert 50 psi to kPa → 50 × 6894.757 ÷ 1000 = 344.74 kPa. The converter above does this in real time.
Are bar and atmosphere the same thing?
Almost — but not exactly. 1 bar = 100,000 Pa; 1 atm = 101,325 Pa. The atmosphere is about 1.325% larger than a bar. Engineers often round them as equal in casual conversation, but precision work uses the exact values.
Why does mmHg differ slightly from Torr?
Both originated from Torricelli's mercury column experiments. mmHg was redefined in 1985 against a fixed mercury density and gravity (1 mmHg = 133.322387415 Pa, exactly), while Torr was redefined as 1/760 of a standard atmosphere (133.3223684... Pa). The difference appears only in the eighth decimal place.
Should I use absolute or gauge pressure?
Gauge pressure (psig, barg) is what most pressure gauges show: pressure relative to local atmospheric. Absolute pressure (psia, bara) measures relative to a perfect vacuum, and is what physics equations like the ideal gas law require. To convert: P_absolute = P_gauge + P_atmospheric (≈ 14.7 psi or 1 atm at sea level).
What is a typical tire pressure in different units?
A common passenger-car cold tire pressure is 32 psi, which equals 220.6 kPa, 2.21 bar, 2.18 atm, or 1655 mmHg. Always follow the placard inside your driver's door — manufacturers specify in either psi or bar.
How precise are these conversion factors?
All factors used by this converter are sourced from NIST SP 811 and the BIPM SI Brochure. Definitions like 1 psi = 6894.757293168 Pa are exact by definition, not rounded. The display rounds to ten significant digits, which is more than sufficient for any engineering purpose.
Why does atmospheric pressure change with weather and altitude?
Atmospheric pressure is the weight of the air column above you. At higher elevations the column is shorter, so pressure drops about 1 hPa per 8 meters near sea level. Storm systems push or pull air horizontally, causing the small day-to-day variations weather services track.
Is psi the same as psia, psig, and psid?
No — they share the unit "pound per square inch" but use different references. psia = absolute (vs vacuum). psig = gauge (vs ambient atmosphere). psid = differential (between two points). Always check which one a spec sheet calls for.
Which unit should I use for scientific publication?
ISO 80000-4 and most journals require SI units, so use Pa, kPa, or MPa. Torr is still accepted in vacuum-physics papers, and mmHg in clinical medicine, but adding the SI value in parentheses is now expected.
Can I link to a specific conversion?
Yes. The URL updates as you change units and values, so you can copy the address bar after any conversion. Example: ?from=psi&to=bar&x=32.
References
Dedicated converter pages
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