Enter one pressure and see it in 19 units at once — SI, imperial, atmospheric, fluid column and industrial. Handles the gauge versus absolute distinction explicitly, which is the difference that quietly ruins more pressure calculations than any conversion error.
Pressure Converter
LiveGauge versus absolute — the distinction that breaks calculations
Almost every pressure gauge you will ever read shows gauge pressure: the amount above the surrounding atmosphere. A tyre gauge reading zero on a flat tyre does not mean the tyre contains no air — it means the inside matches the outside.
| Use | Which reference | Why |
|---|---|---|
| Tyres, hydraulics, most gauges | Gauge | What matters is the difference across the wall, which is what causes stress and drives flow |
| Gas laws, thermodynamics | Absolute | PV = nRT requires absolute pressure. Using gauge here gives answers that are simply wrong, and the error grows as pressures fall |
| Vacuum work | Absolute | Gauge pressure goes negative in vacuum, which makes ratios meaningless |
| Weather and altitude | Absolute | Barometric pressure is inherently absolute — there is no reference to subtract |
| Pressure vessel design | Both, stated | Design pressure is usually gauge; codes require the reference to be explicit precisely because it is ambiguous otherwise |
Where each unit lives
| Unit | Exact value | Used for |
|---|---|---|
| Pascal (Pa) | 1 N/m² — the SI unit | Scientific work. Too small for most practical pressures, hence the prefixes |
| Kilopascal (kPa) | 1,000 Pa | Tyre pressures and general engineering in metric countries |
| Bar | Exactly 100,000 Pa | European industry, diving, hydraulics. Close to one atmosphere, which is why it caught on |
| Millibar / hPa | 100 Pa — identical values | Meteorology worldwide. Sea-level standard is 1013.25 of either |
| psi | 6,894.757 Pa | US and UK engineering, tyres, plumbing, compressed air |
| Atmosphere (atm) | Exactly 101,325 Pa | Chemistry and physics as a reference point rather than a working unit |
| Torr / mmHg | 1/760 atm — nearly identical | Vacuum systems and medicine. Blood pressure is quoted in mmHg universally |
| inHg | 3,386.389 Pa | US aviation altimeters and weather reports |
| inH₂O | 249.089 Pa | HVAC duct pressure, filter differentials, gas appliance regulation — where pressures are too small for psi to be readable |
| kgf/cm² | 98,066.5 Pa | Legacy industrial gauges, still common in parts of Asia. Roughly equal to one atmosphere, and often confused with bar |
Pressures worth having a feel for
| Situation | Pressure | In other units |
|---|---|---|
| Deep vacuum (laboratory) | 10⁻⁶ Torr | About 0.13 mPa |
| Cabin altitude at cruise | ~75 kPa absolute | 10.9 psia · 0.74 atm — equivalent to 2,400 m |
| Standard atmosphere | 101.325 kPa | 14.696 psi · 1.013 bar · 760 mmHg · 29.92 inHg |
| Blood pressure 120/80 | 120 mmHg gauge | 16.0 kPa · 2.32 psi above atmospheric |
| Car tyre | 32 psi gauge | 220.6 kPa · 2.21 bar · 46.7 psia |
| Espresso extraction | 9 bar gauge | 130.5 psi · 900 kPa |
| Mains water supply | 3–6 bar | 44–87 psi |
| Scuba cylinder, full | 200–300 bar | 2,900–4,350 psi |
| Mariana Trench floor | ~1,100 bar | 16,000 psi · roughly 1,086 atmospheres |
Why fluid columns are temperature-dependent
Units like mmHg and inH₂O measure pressure as the height of a liquid column that would balance it. That is convenient and slightly imprecise, because liquid density changes with temperature.
The conventional values are defined at those reference temperatures. Water at 20 °C is about 0.2% less dense than at 4 °C, so a water column measured warm reads slightly differently from the definition. For HVAC and most practical work this is negligible. For calibration and metrology it is not, which is why laboratory standards specify the fluid temperature alongside the reading.
Torr and mmHg are defined slightly differently — Torr as exactly 1/760 of an atmosphere, mmHg from the mercury column. They differ by about one part in seven million, which matters only in precision metrology.
Where a pressure error has real consequences
| Setting | The confusion | What follows |
|---|---|---|
| Gas law calculations | Gauge fed into PV = nRT | The equation needs absolute. A gauge reading of zero would mean no gas at all |
| Compressor ratios | Gauge divided by gauge | 100 psig to 200 psig is not a 2:1 ratio — it is 114.7 to 214.7, or 1.87:1 |
| Vacuum work | Negative gauge readings | A gauge can read below zero; absolute cannot. Vacuum is quoted in absolute or as inches of mercury below atmosphere |
| Altitude | Atmospheric assumed at 14.7 | At 2,000 m it is about 11.5 psi. A gauge calibrated at sea level reads differently in Denver or Riyadh |
| Medical | Blood pressure in mmHg | Always gauge, relative to atmosphere. 120/80 means 120 mmHg above ambient, not absolute |
The suffix exists for a reason: psig, psia, barg, bara. Where a specification omits it, assume gauge for anything mechanical and absolute for anything scientific — then confirm, because that assumption is exactly the one that fails.
Why a millimetre of mercury is not a fixed pressure
Pressure units defined by a fluid column depend on the density of that fluid, and density changes with temperature:
| Unit | Defined at | Note |
|---|---|---|
| mmHg | 0 °C, standard gravity | 133.322 Pa. Mercury expands about 0.018% per degree, so a warm column reads slightly low |
| inHg | 0 °C | 3,386.39 Pa. Aviation altimeters use this and correct for temperature |
| Torr | 1/760 of an atmosphere | Almost identical to mmHg but defined independently, so the two differ in the seventh digit |
| mH₂O | 4 °C, where water is densest | 9,806.65 Pa. Used in plumbing and hydrology |
| Pascal | One newton per square metre | No fluid, no temperature dependence. Which is why SI chose it |
Standard gravity is part of the definition too, at 9.80665 m/s². Real gravity varies by about 0.5% between the equator and the poles, so a mercury manometer is technically location-dependent as well as temperature-dependent. Modern instruments are electronic and report pascals internally for exactly this reason.
Common mistakes
Frequently asked questions
What is the difference between psi, psig and psia?
psig is gauge — pressure above the surrounding atmosphere, which is what almost every gauge displays. psia is absolute, measured from a perfect vacuum. Plain psi is ambiguous and usually means gauge by convention. The gap between them is one atmosphere, 14.696 psi, so a tyre at 32 psig holds 46.7 psia. That difference is constant, and it is large enough to invalidate any gas calculation that ignores it.
How many psi is 1 bar?
14.5038 psi. One bar is defined as exactly 100,000 pascals, which makes it very close to but not identical with one standard atmosphere at 101,325 Pa. That 1.3% gap is why the two are not interchangeable in precise work, despite being used loosely as though they were.
Why is blood pressure measured in mmHg?
Historical continuity. The original sphygmomanometer used a mercury column, and readings were literally the height in millimetres. Modern devices are electronic and still report mmHg because a century of clinical thresholds, research and training is expressed in it. A reading of 120 mmHg is 16.0 kPa — accurate, and meaningless to anyone in medicine.
Are millibar and hectopascal the same?
Numerically identical — both are 100 pascals, so 1013 mbar and 1013 hPa describe exactly the same pressure. Meteorology moved to hPa for SI consistency, which conveniently required no renumbering of any chart or record. Some services still print mbar out of habit.
Which pressure should I use for gas law calculations?
Absolute, always. PV = nRT and every derived relationship assume pressure measured from a true zero. Using gauge introduces an error of one atmosphere, which is proportionally enormous at low pressures and still significant at moderate ones. Switch the reference toggle above to absolute before reading off a value for this purpose.
Why does HVAC use inches of water?
Because duct pressures are tiny. A typical residential system runs around 0.5 inH₂O, which is 0.018 psi — a number with too many leading zeros to read at a glance. Water columns give a readable scale for small pressures, and the same logic applies to gas appliance regulation and filter differentials.
Do fluid column units change with temperature?
Slightly, yes. They are defined by liquid density, and density falls as temperature rises — water at 20 °C is about 0.2% less dense than at the 4 °C reference. For HVAC and general engineering this is negligible. For calibration and metrology it is not, which is why standards specify the fluid temperature alongside the value.
Is my input stored?
No. The conversion runs in your browser with no server request, and works offline once the page has loaded.
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