Force spread over an area
Pressure is force per unit area. The SI unit, the pascal (Pa), is one newton on one square meter, which is tiny, so practical readings use multiples: the kilopascal (1,000 Pa), the hectopascal (100 Pa) and the bar (100,000 Pa). The other units come from different traditions:
- psi, pounds-force per square inch: tire and air-compressor gauges in the United States and on many dual-scale gauges worldwide. One psi is about 6,894.76 Pa.
- bar and kPa: tire pressures in much of Europe and Asia are printed in bar, kPa or both. One bar is exactly 100 kPa.
- hPa (millibars): weather maps and forecasts. A hectopascal and a millibar are the same size, so meteorologists switched names without changing numbers.
- atm, the standard atmosphere: exactly 101,325 Pa by definition, a reference for average sea-level air pressure in physics and chemistry.
- kgf/cm²: kilograms-force per square centimeter, 0.980665 bar, still seen on older gauges and some equipment.
- mmHg and inHg: the height of a mercury column the pressure would support. Today they are conventional units with fixed pascal values. Blood pressure is reported in mmHg, and US weather reports and US aviation altimeter settings give barometric pressure in inches of mercury.
The converter turns any value into pascals and then into the target unit; the working line shows the direct factor, and Show in every unit lists them all at once.
Gauge pressure and absolute pressure
A tire gauge reads zero when the tire is flat, even though the air inside is still at atmospheric pressure. It measures gauge pressure: the amount above the surrounding air. Absolute pressure counts from a perfect vacuum, so it is gauge pressure plus the local atmospheric pressure (about 1 atm, or 14.7 psi, at sea level). Tire, compressor and boiler gauges show gauge pressure; barometers and scientific data use absolute. Labels such as “psig” and “psia” or “bar(g)” and “bar(a)” say which one is meant. The converter changes the unit only; it does not add or remove the atmosphere.
Pressure formulas
The psi factor comes from the exact pound and inch and standard gravity, so it is exact but has a long decimal; the psi factors shown here are rounded. The bar, kPa, hPa and atm values are exact round numbers.
Worked examples
- One psi is 0.0689475729 bar: 32 × 0.0689475729 = 2.2063 bar.
- One bar is 100 kPa, so 2.2063 bar = 220.63 kPa.
- Multiply by 14.503773773: 2.5 × 14.503773773 = 36.259 psi.
- Most tire gauges read to 0.5 or 1 psi, so set 36 psi.
- One inch of mercury is 33.86389 hPa.
- 980 ÷ 33.86389 = 28.939 inHg, well below the standard 29.92 inHg.
- Standard atmospheric pressure is 14.69595 psi.
- Add it to the gauge reading: 32 + 14.69595 = 46.696 psia.
- At altitude the atmosphere is lower, so the absolute figure would be lower for the same gauge reading.
Pressure tables
| Pounds per square inch (psi) | Bar (bar) | Kilopascals (kPa) |
|---|---|---|
| 20 | 1.379 | 137.9 |
| 25 | 1.724 | 172.4 |
| 28 | 1.931 | 193.1 |
| 30 | 2.068 | 206.8 |
| 32 | 2.206 | 220.6 |
| 35 | 2.413 | 241.3 |
| 36 | 2.482 | 248.2 |
| 40 | 2.758 | 275.8 |
| 44 | 3.034 | 303.4 |
| 50 | 3.447 | 344.7 |
| 65 | 4.482 | 448.2 |
| 80 | 5.516 | 551.6 |
| 100 | 6.895 | 689.5 |
| Hectopascals (millibars) (hPa) | Inches of mercury (inHg) | Millimeters of mercury (mmHg) | Standard atmospheres (atm) |
|---|---|---|---|
| 950 | 28.053 | 712.56 | 0.93758 |
| 970 | 28.644 | 727.56 | 0.95732 |
| 990 | 29.235 | 742.56 | 0.97705 |
| 1,000 | 29.53 | 750.06 | 0.98692 |
| 1,013.25 | 29.921 | 760 | 1 |
| 1,020 | 30.121 | 765.06 | 1.0067 |
| 1,040 | 30.711 | 780.06 | 1.0264 |
Pressure mistakes to avoid
- Comparing a gauge reading with an absolute one. A tire at 32 psi gauge is at about 46.7 psi absolute, so mixing the two kinds puts a 14.7 psi error into the comparison. Check the label or the source before comparing numbers.
- Treating bar and atm as identical. They differ by 1.3% (1 atm = 1.01325 bar). For tires that is negligible; for scientific work it is not.
- Reading the wrong ring of a dual gauge. Many gauges print psi alongside bar or kPa. A target of 2.2 bar read off the psi ring would leave the tire almost empty, while 220 kPa and 2.2 bar are the same pressure.
- Expecting 760 mmHg to equal exactly 1 atm. The conventional millimeter of mercury is defined from a mercury density and standard gravity, so 1 atm comes out as 759.9998917 mmHg in the converter. The difference is about one part in seven million.
Converting a blood pressure reading from mmHg to kPa changes only the unit. For what a reading means for health, ask a doctor or another qualified professional.
Questions people ask
What is normal air pressure in hPa?
The standard atmosphere is 1,013.25 hPa, a reference value for sea level. Actual sea-level pressure moves above and below it with the weather, and pressure falls as you climb.
Is kgf/cm² the same as bar?
Nearly. One kgf/cm² is 0.980665 bar, or 14.22 psi, so treating them as equal adds an error of about 2%.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B, National Institute of Standards and Technology.
- The International System of Units (SI Brochure), 9th edition, Bureau International des Poids et Mesures.