Pressure Converter

Understanding Pressure

Pressure is force spread across a surface. When you inflate a tire, you're pushing air molecules against the walls - more molecules or smaller space means higher pressure. Weather changes because air masses of different pressures move around the globe. Blood pressure shows how hard your heart pumps blood through your arteries.

The Main Pressure Units

Pascal (Pa) is the SI unit of pressure, named after Blaise Pascal who studied fluid mechanics in the 1600s. One pascal equals one newton of force per square meter. Since a pascal is quite small, kilopascals (kPa) or megapascals (MPa) work better in practice. A car tire sits at roughly 200,000 Pa, which is easier to express as 200 kPa.

In the United States and UK, PSI (Pounds per Square Inch) handles tire pressure, compressed gas, and hydraulics. One PSI means one pound of force pushing on every square inch. Most car tires run 30-35 PSI, while industrial hydraulic systems can hit thousands.

Europe and most metric countries use bar. One bar is close to atmospheric pressure at sea level (0.987 atm), so 2 bar means roughly twice atmospheric pressure. Tire pressure gauges in metric countries show bar or kPa.

Earth's average air pressure at sea level defines the atmosphere (atm). Divers use atmospheres to track depth pressure - every 10 meters underwater adds another atmosphere.

Mercury barometers gave us mmHg and Torr. A column of mercury 760mm tall at sea level balances atmospheric pressure. Blood pressure readings use mmHg (120/80 means the systolic pressure equals 120mm of mercury). Torr and mmHg are nearly the same - Torr is named after Evangelista Torricelli, who invented the barometer.

Gauge vs Absolute Pressure

This catches people off guard. Gauge pressure measures relative to atmospheric pressure - your tire gauge reads 30 PSI above atmosphere, not 30 PSI total. Absolute pressure includes atmospheric pressure - that same tire actually contains about 44.7 PSI absolute (30 PSI gauge + 14.7 PSI atmosphere). Most pressure gauges read relative to atmosphere: tire gauges, blood pressure cuffs, air compressors. Engineering and lab work usually call for absolute pressure. Negative gauge pressure means a partial vacuum—pressure below atmosphere.

When in doubt, ask which type of pressure the situation requires.

Why Different Units Exist

These units came out of different industries - mercury barometers gave us mmHg, British engineers brought PSI, and the metric system added pascals. Each field picked units that kept their numbers manageable - PSI for medium pressures, bar for ease of use, kPa for precision, atmospheres for reference. You'll hit this when a European car manual specifies tire pressure in bar while your American gauge shows PSI.

Practical Applications

Tire pressure and weather are the two places most people encounter pressure readings. Under-inflated tires waste fuel and wear unevenly at the edges, while over-inflation reduces grip and wears the center - temperature shifts pressure too, so check tires when cold. Barometric pressure drives weather forecasting: high pressure usually means clear skies, low pressure brings storms, and a rapidly falling reading warns of severe weather approaching. At altitude, pressure drops about 12% per 1,000 meters - Mount Everest's summit sits at roughly one-third sea-level pressure.

Industrial systems run on precise pressure control, from shop air compressors at 90-120 PSI to hydraulic presses exceeding 10,000 PSI. Pneumatic tools, steam boilers, pipelines, and water jet cutters all run within tight pressure limits. Engineers use whatever unit their industry settled on.

Blood pressure monitors and ventilators both rely on accurate pressure readings. Medicine settled on mmHg decades ago and stuck with it.

Safety: High-pressure systems pack serious energy - a burst line or failed vessel can injure. Stick to manufacturer ratings and check equipment regularly.

Common Questions About Pressure Conversion

• Can I convert negative pressure values? Negative gauge pressures represent partial vacuums - pressure below atmospheric. The converter handles these. Absolute pressure can't go negative though - if you get negative absolute pressure, check your numbers.
• Why do my tire pressure readings differ from the converter? Tire gauges show gauge pressure (above atmospheric), not absolute pressure. Pressure also varies with temperature - a tire inflated to 30 PSI on a cold morning might read 33 PSI after highway driving. Always check tires when cold for consistent readings.
• What's the difference between bar and atm? Almost nothing. One bar equals 0.987 atm. Close enough for quick math; use exact factors when precision matters.
• Why is blood pressure always in mmHg? It's a holdover from mercury column devices. Doctors everywhere adopted mmHg, and while some countries officially switched to kPa, clinics still use mmHg.
• How does altitude affect pressure readings? Atmospheric pressure drops about 12% per 1,000 meters of altitude gain, which is why boiling points change at elevation and why aircraft cabins need pressurization. Tire gauges still read correctly at altitude because they measure gauge pressure (the difference from local atmosphere), but weather barometers need altitude correction to produce sea-level equivalent readings that can be compared across locations.