Pressure Converter
Pressure is force per unit area, and the number of units in active use is unusually large because different industries standardised separately. The conversions themselves are simple multiplications; the thing that causes actual errors is gauge versus absolute pressure, which no unit conversion will catch.
How to use it
- Enter a value and pick its unit.
- Pick the target unit.
- Read the converted value.
Gauge and absolute pressure
A gauge reads the difference between the pressure inside a system and the atmosphere around it. Absolute pressure is measured from a true vacuum. The gap between them is one atmosphere, about 14.7 PSI or 101.325 kPa, and it is not small.
Every tyre gauge, air compressor, and hydraulic gauge reads gauge pressure. A tyre at 32 PSI gauge is at about 46.7 PSI absolute. Notation distinguishes them where it matters: PSIG for gauge, PSIA for absolute, and bar(g) against bar(a).
The distinction bites in three places. Thermodynamic calculations — gas laws, compressor work, steam properties — require absolute pressure, and using gauge values produces answers that are wrong by a factor rather than a percentage at low pressures. Vacuum work is expressed as absolute by necessity, since gauge pressure below atmosphere is negative. And weather and aviation pressures are absolute, which is why a barometric reading and a tyre gauge cannot be compared directly.
This converter handles unit conversion only. Whether a figure is gauge or absolute is information that travels with the number and has to be tracked separately.
The units and where they come from
Each unit survives because an industry standardised on it, and knowing which tells you what a figure probably means.
- The pascal is the SI unit, one newton per square metre. It is inconveniently small — atmospheric pressure is about 101,325 Pa — so kilopascals and megapascals are used in practice. Structural and materials work uses MPa, where 1 MPa is about 145 PSI.
- The bar is exactly 100,000 Pa, defined for convenience because it is close to atmospheric pressure. Common in European industry and in automotive work outside North America.
- PSI, pounds per square inch, dominates North American industry: tyres, compressed air, hydraulics, plumbing. One PSI is about 6,895 Pa.
- The standard atmosphere is exactly 101,325 Pa, defined as sea-level pressure. Used as a reference in chemistry and physics rather than for measurement.
- Millimetres of mercury, about 133.3 Pa each, persist in medicine and vacuum work because early instruments were mercury columns. Blood pressure is still reported this way. The torr is nearly identical.
- Inches of water column, roughly 249 Pa, are used for very low pressures in HVAC duct work and gas appliance regulation, where a PSI figure would be an inconveniently small decimal.
Useful approximations
A few relationships are close enough to exact to be worth remembering, and they make it easy to spot a conversion that has gone wrong by an order of magnitude.
1 bar is about 14.5 PSI, and 1 atmosphere is about 14.7 PSI — so bar and atmospheres are interchangeable to within 1.3 percent for rough work. 100 kPa is about 14.5 PSI. 1 MPa is about 145 PSI. Car tyre pressures of 30 to 35 PSI are about 2.1 to 2.4 bar.
For water, roughly 1 PSI per 2.31 feet of head, or about 9.8 kPa per metre. This is why a building water system needs a booster above a certain height and why a header tank position determines available pressure.
Pressure changes with temperature
For a fixed volume of gas, pressure rises with absolute temperature. This is why tyre pressures read differently in different conditions and why checking them cold is specified rather than advisory.
The rule of thumb is about 1 PSI per 10 degrees Fahrenheit, or roughly 1 PSI per 5.5 degrees Celsius. A tyre set to 32 PSI on a 70 °F afternoon reads about 28 PSI on a 30 °F morning, which is enough to trigger a low-pressure warning and enough to matter for handling and wear.
Driving heats tyres by 5 to 10 °F, so a hot reading is several PSI above the cold value. Topping up to specification while hot leaves the tyre underinflated once it cools. The manufacturer figure on the door jamb is always a cold pressure, meaning after at least three hours parked or under a mile of driving.
At a glance
| 1 atmosphere | Exactly 101,325 Pa, about 14.696 PSI |
|---|---|
| 1 bar | Exactly 100,000 Pa, about 14.504 PSI |
| 1 PSI | About 6,894.76 Pa |
| Gauge vs absolute | Not handled, track it with the value |
Frequently asked questions
What is the difference between gauge and absolute pressure?
Gauge is measured relative to atmosphere, absolute from a vacuum. They differ by about 14.7 PSI. Tyre and compressor gauges read gauge; thermodynamic calculations need absolute.
Is 1 bar the same as 1 atmosphere?
Nearly. 1 bar is exactly 100,000 Pa and 1 atmosphere is 101,325 Pa, a difference of about 1.3 percent.
Why do my tyres read low in winter?
Gas pressure falls with temperature, roughly 1 PSI per 10 °F. A tyre set to 32 PSI at 70 °F reads about 28 PSI at 30 °F.
Why is blood pressure in mmHg?
Because early instruments used a mercury column and the convention stuck. One mmHg is about 133.3 Pa.
Read more
Unit conversion done properly — Most conversions are exact multiplications. The ones that are not cause the expensive mistakes.