What Is Pressure — and How Is It Measured?

Pressure is force applied per unit area. The SI unit is the pascal (Pa), named after the 17th-century French mathematician and physicist Blaise Pascal: 1 Pa = 1 newton per square metre. Standard atmospheric pressure at sea level is 101,325 Pa — also expressed as 1 atmosphere (atm) or approximately 1 bar (100,000 Pa), and in imperial units as 14.696 psi (pounds per square inch). Pressure measurement appears across meteorology, medicine, aviation, deep-sea diving, and everyday engineering.

Pressure in Context

A car tyre is typically inflated to 32–35 psi (220–240 kPa) — well above atmospheric pressure. Average human blood pressure is 120/80 mmHg (about 16/10.7 kPa). A road bicycle tyre runs at 100–130 psi, roughly seven times atmospheric pressure. At the bottom of the Mariana Trench, the deepest point in the ocean, pressure reaches about 1,086 atm or 15,750 psi — enough to crush an unprotected submarine and compact the density of water measurably. A fully inflated NBA basketball sits at just 8 psi.

Gauge vs Absolute Pressure — and Weather

Tyre pressure gauges measure relative to atmospheric pressure — "gauge pressure." An apparently empty tyre reads 0 psi on a gauge, but still contains 14.7 psi of absolute pressure from the atmosphere. Absolute pressure = gauge pressure + atmospheric pressure. Blood pressure is measured in mmHg (millimetres of mercury), a legacy unit from early mercury manometers still standard in medicine worldwide. In weather forecasting, pressure is measured in hectopascals (hPa). High-pressure systems (anticyclones) push air downward, suppressing cloud formation and producing clear skies. Low-pressure systems (cyclones) allow air to rise, cool, and condense — causing clouds and rain. A barometric pressure drop of 4–6 hPa over several hours typically signals an approaching storm.