Pressure is force spread over area
Pressure is simply force divided by the area it acts on. Push with the same force on a drawing pin and on a flat palm, and the pin hurts far more because the same force is concentrated on a tiny area. That single idea — force per unit area — is what every pressure unit measures. The differences between units come down to which force and which area each one was built around, and which industry adopted it. The Pressure Converter lets you move between all of them at once; this guide explains where they come from.
The pascal — the SI base unit
The pascal (Pa) is the SI unit: one newton of force spread over one square metre. It is a small unit — a single pascal is roughly the pressure of a sheet of paper resting on a table — so in practice you see multiples. A kilopascal (kPa) is 1,000 Pa, a megapascal (MPa) is a million, and a hectopascal (hPa) is 100. Because the pascal is the base, every other unit can be defined as some number of pascals, which is exactly how conversion works: translate everything to pascals, then back out to the unit you want. The Pressure Converter does this internally for all eleven units it supports.
Bar and millibar
The bar was defined to be close to average atmospheric pressure: 1 bar = 100,000 Pa. It is popular in European industry, diving, and weather. Its thousandth, the millibar (mbar), equals 100 Pa — and here’s a useful coincidence: a millibar is numerically identical to a hectopascal. That’s why meteorologists could switch from mbar to hPa without changing any familiar numbers; sea-level pressure is about 1013 in both.
PSI — pounds per square inch
In the United States and in much of mechanical engineering, pressure is measured in PSI — pounds-force per square inch. One PSI is about 6,895 Pa, or roughly 0.069 bar. The most everyday encounter is tyre pressure: a car tyre at 2.2 bar reads about 32 PSI, which is why gauges often print both scales. When you see a spec in PSI and a pump calibrated in bar, the Pressure Converter settles it instantly.
Atmospheres and mercury columns
The standard atmosphere (atm) is defined as exactly 101,325 Pa — a stand-in for typical sea-level air pressure. It’s common in chemistry and physics. Closely related are the mercury-column units: millimetres of mercury (mmHg) and Torr, both about 133.322 Pa. They come from the original barometer, where atmospheric pressure was read off the height of a mercury column — 760 mmHg at one atmosphere. These units survive in two places especially: blood pressure, reported as something like 120/80 mmHg, and vacuum technology, where Torr is standard. The imperial cousin, inches of mercury (inHg), appears in aviation altimeters and US weather reports.
Putting the relationships together
It helps to anchor everything to one atmosphere. Standard atmospheric pressure equals 1 atm, which is 101,325 Pa, 1.01325 bar, 1013.25 hPa (or mbar), 760 mmHg (or Torr), 29.92 inHg, and 14.696 PSI. Memorising even a couple of these — “about 1 bar, about 14.7 PSI, about 760 mmHg” — gives you a sanity check on any conversion. For anything precise, though, reach for the Pressure Converter, which keeps eight significant figures and shows every unit in one table.
A note on gauge vs absolute pressure
One subtlety the units themselves don’t capture: gauge versus absolute pressure. A tyre gauge reading “32 PSI” is measuring pressure above the surrounding atmosphere — that’s gauge pressure. Absolute pressure adds the roughly 14.7 PSI of atmosphere on top. Sometimes you’ll see this written as “PSIG” (gauge) versus “PSIA” (absolute). The conversion factors are the same either way — a bar is a bar — but make sure you’re comparing like with like before you convert, especially in engineering contexts.
Converting reliably
The safe way to convert pressure is always the same: take the source value into pascals using its defined factor, then divide by the target unit’s factor. Doing this by hand invites slips, particularly with the mercury and imperial units whose factors aren’t round numbers. A converter removes that risk and, by showing every unit at once, also catches the “wrong unit entirely” mistakes that cause real problems. If you work across measurement systems often, the Pressure Converter pairs naturally with the Length Converter and Energy Converter — and our area converter guide covers the “per unit area” half of the pressure definition in more depth.
Quick reference: everyday pressure values
It helps to carry a few real-world anchors so conversions pass the sniff test. A car tyre runs around 32 PSI, which is about 2.2 bar or 220 kPa. Atmospheric pressure at sea level is about 1 atm, 1.013 bar, 1013 hPa, or 14.7 PSI. Healthy blood pressure is reported around 120/80 mmHg. A household water supply is often 40–60 PSI (about 2.8–4.1 bar). A scuba tank might be filled to 200 bar (roughly 2,900 PSI). When you punch a value into the Pressure Converter and the answer is wildly off one of these anchors, you’ve probably picked the wrong source unit — a quick mental check that catches real mistakes.
These anchors also explain why showing every unit at once is so useful. A weather app reports hPa, a dive computer reads bar, a US tyre gauge shows PSI, and a blood-pressure cuff uses mmHg — yet they’re all the same physical quantity. Rather than memorising a dozen pairwise factors, you can let the converter translate once and read whichever scale you need. For broader measurement work the Length Converter and Energy Converter follow the same “convert through a base unit” approach.
The takeaway
Every pressure unit measures the same thing — force per area — but different fields settled on different scales: pascals in science, bar in European industry and diving, PSI in US engineering and tyres, hPa/mbar in weather, and mmHg/Torr in medicine and vacuum work. Anchor them to one atmosphere, watch out for gauge versus absolute, and let the Pressure Converter handle the exact factors so you can move between units without error.