Pressure Converter

Pressure is force distributed over an area — mathematically, P = F/A, with units of newtons per square meter in SI. The SI unit, the pascal (Pa), is defined as exactly one newton per square meter and is the fundamental unit underlying all pressure measurements in science and engineering. While the pascal is physically clean and intuitive, practical pressures in daily life and engineering often involve very large numbers — atmospheric pressure at sea level is about 101,325 Pa — so derived units like bar (100,000 Pa), atmosphere (101,325 Pa), and kilopascal (kPa, 1,000 Pa) are widely used to keep the numbers manageable.

Other units persist in specific industries and countries: pounds per square inch (psi) dominates US pressure measurements for tires, plumbing, and industrial fluid systems; inches of mercury (inHg) and millimeters of mercury (mmHg, also called Torr) dominate medical and meteorological measurements because they historically came from mercury barometers; millibar (mbar) is standard for weather forecasts in some countries. The calculator converts between all major units using the underlying pascal relationship, so you can translate, for example, a tire-pressure specification from US psi to European bar without manual unit math. Pressure differences and gauge pressures (see next section) are the same unit as absolute pressure but measured relative to a different reference point.

Tire gauges and most industrial instruments measure gauge pressure — the pressure above ambient atmospheric pressure, with 0 gauge pressure meaning "equal to surrounding atmosphere." Absolute pressure is measured relative to a perfect vacuum and includes the atmospheric contribution. A tire at 32 psi gauge has an absolute pressure of about 46.7 psi (32 + 14.7 psi atmospheric at sea level), which is why letting air out of a tire to "zero" still leaves it at atmospheric pressure rather than actual vacuum.

Scientific calculations involving gas laws (PV = nRT, adiabatic compression, phase diagrams) typically require absolute pressure because the underlying physics treats vacuum as the reference zero. Engineering calculations for pressure vessels, pumps, and fluid systems often use gauge pressure because the material stress depends on the pressure difference between inside and outside the vessel. Always clarify which reference is being used — confusing gauge and absolute pressure by adding or forgetting the 14.7 psi atmospheric offset is one of the most common errors in undergraduate chemistry and engineering. Gauge pressure units are typically annotated with "g" (e.g., psig, barg) and absolute pressure with "a" (psia, bara), though this convention is inconsistently applied in practice.

The calculator uses pascal as the internal base unit, with conversion factors for all supported units: 1 atm = 101,325 Pa, 1 bar = 100,000 Pa, 1 psi = 6,894.76 Pa, 1 mmHg (Torr) = 133.322 Pa, 1 inHg = 3,386.39 Pa, 1 kPa = 1,000 Pa. Converting from unit A to unit B happens in two steps internally: first convert A to pascals, then pascals to B. This ensures all pairwise conversions are consistent and avoids the accumulated rounding that can occur with direct unit-to-unit tables.

Small changes in input produce proportional changes in output, so precision in source measurements matters when conversions feed downstream safety-critical calculations (pressure vessel design, medical oxygen delivery, HVAC system sizing, scuba diving decompression). For simple unit conversions in everyday contexts like checking that a tire's recommended European-spec bar rating matches your US-spec psi gauge, the calculator produces the exact equivalent without rounding artifacts. Always verify you're converting the right type of pressure (gauge vs absolute) before trusting results, because the conversion factors are the same but the reference points differ.