Pressure units evolved from different measurement methods and industries. Atmospheric pressure led to 'atmospheres' and 'bars.' Mercury barometers gave us 'mmHg', 'cmHg', and 'torr.' PSI comes from mechanical systems. Pascals are the scientific standard. Weather uses millibars and hectopascals, while industrial applications often use megapascals and metric units like kgf/cm².

Kilogram-force per square centimeter (kgf/cm²) is a metric unit commonly used in automotive and industrial applications, especially in Europe and Asia. 1 kgf/cm² ≈ 0.98 bar ≈ 14.22 psi ≈ 98.07 kPa. It's intuitive because it represents the pressure of a 1kg weight on a 1cm² area.

Millibar (mbar) and hectopascal (hPa) are exactly equivalent units: 1 mbar = 1 hPa. Meteorologists often use both terms interchangeably for weather reporting. Standard atmospheric pressure is 1013.25 mbar/hPa. These units are preferred over kilopascals in weather because the numbers are more convenient.

Absolute pressure measures total pressure including atmospheric pressure (14.7 PSI at sea level). Gauge pressure measures pressure relative to atmospheric pressure. A tire showing 32 PSI is actually 46.7 PSI absolute (32 + 14.7). Most pressure gauges show gauge pressure.

Use kgf/cm² for automotive applications (tire pressure), industrial equipment, and when working with European/Asian specifications. Use gf/cm² for smaller pressures and laboratory work. Use kgf/m² for very low pressures like wind loads. These units are intuitive and widely used in mechanical engineering outside North America.

Megapascals (MPa) are ideal for high-pressure applications like hydraulic systems, material testing, and structural engineering. 1 MPa = 1,000,000 Pa = 145.04 PSI = 10.2 kgf/cm². Industrial hydraulics often operate at 10-350 MPa, while material strength is commonly expressed in MPa for international standards.

Atmospheric pressure decreases with altitude. At sea level, it's about 1 atmosphere (14.7 PSI, 101.3 kPa, 1013 mbar, 1.03 kgf/cm²). At 10,000 feet, it drops to about 10.1 PSI (697 mbar, 0.71 kgf/cm²). This affects everything from cooking times to tire pressure readings and aircraft performance.

Check your vehicle's door jamb or manual for recommended pressure (typically 30-35 PSI, 2.1-2.4 bar, or 2.1-2.5 kgf/cm²). This is 'cold' pressure—measured when tires haven't been driven recently. Tire pressure increases ~1 PSI per 10°F temperature rise, and decreases in cold weather.

Blood pressure is measured in mmHg (millimeters of mercury). Normal pressure is around 120/80 mmHg (16.0/10.7 kPa or 0.163/0.109 kgf/cm²). The numbers represent systolic (heart contracting) and diastolic (heart relaxing) pressures. Some medical equipment also uses cmHg (centimeters of mercury) for higher pressure readings.

Vacuum refers to pressure below atmospheric pressure. A 'perfect vacuum' is 0 absolute pressure. Vacuum is often measured in negative gauge pressure (like -10 PSI or -0.7 kgf/cm²) or as a percentage of atmospheric pressure. High vacuum systems use specialized units like torr or microbar.

Water pressure increases by 1 atmosphere every 33 feet (10 meters) of depth. At 33 feet, pressure doubles to 2 atmospheres (29.4 PSI or 2.06 kgf/cm²). This affects air consumption, nitrogen absorption, and decompression requirements. Rapid pressure changes can cause serious medical conditions.

Barometers measure atmospheric pressure in millibars (mb), hectopascals (hPa), or inches of mercury (inHg). Standard atmospheric pressure is 1013.25 mb/hPa, 29.92 inHg, or 1.033 kgf/cm². Rising pressure often indicates improving weather, while falling pressure suggests storms. Pressure changes help predict weather patterns.

PSI (pounds per square inch) and PSF (pounds per square foot) differ by area. 1 PSI = 144 PSF (since 1 square foot = 144 square inches). Similarly, 1 kgf/cm² = 10,000 kgf/m². PSF and kgf/m² are often used in construction for distributed loads like wind pressure or snow loads on roofs.