PSI, Bar & Atmospheres
Last reviewed: May 2026
Convert between all common pressure units: PSI, bar, atmospheres, kilopascals, millimeters of mercury, and inches of mercury. Pressure measurement spans everyday applications (tire inflation, weather forecasts) to industrial and scientific use (hydraulics, vacuum systems, blood pressure).1
| Unit | 1 atm Equals | Common Use |
|---|---|---|
| PSI | 14.696 | Tires, compressed air (US) |
| Bar | 1.01325 | Tires, industrial (metric) |
| kPa | 101.325 | SI standard, engineering |
| mmHg (torr) | 760 | Blood pressure, vacuum |
| inHg | 29.921 | Weather, aviation |
| Pa | 101,325 | SI base unit |
Pressure is force per unit area, expressed in dozens of different units across industries. PSI (pounds per square inch): U.S. standard for tire pressure, plumbing, and HVAC. Bar: Metric unit, approximately equal to atmospheric pressure. 1 bar = 14.504 PSI. Pascal (Pa): SI unit. 1 Pa = 1 N/m². Very small — weather uses hectopascals (hPa) or kilopascals (kPa). Atmosphere (atm): Reference unit — 1 atm = 14.696 PSI = 101.325 kPa = 1.01325 bar. mmHg (Torr): Used in medicine (blood pressure) and vacuum systems. 1 atm = 760 mmHg. inHg: Used in U.S. weather reporting and aviation altimeters. Standard sea level = 29.92 inHg.
Tire pressure: Passenger cars: 30-35 PSI (207-241 kPa). Bicycle tires: 40-130 PSI depending on tire type. Under-inflation by just 5 PSI reduces fuel economy by 2% and increases tire wear by 10%. Water pressure: Residential: 40-60 PSI (276-414 kPa). Below 40 PSI causes weak flow; above 80 PSI can damage fixtures and pipes. Blood pressure: 120/80 mmHg. Measured relative to atmospheric pressure — a reading of 120 mmHg means 120 mmHg above ambient. Atmospheric pressure: Varies with altitude and weather. Sea level: ~14.7 PSI. Denver (5,280 ft): ~12.2 PSI — 17% less. Everest summit: ~4.9 PSI — not enough to sustain consciousness without supplemental oxygen.
Scuba diving: Pressure increases by 1 atm per 33 feet of seawater depth. At 66 feet, a diver experiences 3 atm — tank regulators must deliver air at matching pressure. Rapid ascent causes decompression sickness ("the bends") as dissolved nitrogen forms bubbles. Weather forecasting: Falling barometric pressure indicates approaching storms; rising pressure suggests clearing skies. A rapid drop of 0.06 inHg/hour signals severe weather. Autoclaves: Sterilize medical equipment at 121°C (250°F) under 15 PSI above atmospheric — the elevated pressure raises water's boiling point, enabling higher sterilization temperatures. Hydraulics: Pascal's law (pressure applied anywhere in a confined fluid transmits equally everywhere) enables hydraulic lifts, brakes, and construction equipment to multiply force by ratios of 10:1 to 100:1.
Pressure measures force distributed over an area — the same force concentrated on a smaller area creates higher pressure. This explains why a 120-pound person in stiletto heels (0.1 sq inch heel tip) exerts roughly 1,200 psi on the floor — more than a 6,000-pound elephant standing on flat feet (~75 psi per foot). The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Because a pascal is tiny (atmospheric pressure is 101,325 Pa), practical measurements typically use kilopascals (kPa), megapascals (MPa), or one of several other units depending on the field. Converting between pressure units is essential because different industries and countries standardize on different units, often for historical reasons rather than technical ones.
Meteorology uses millibars (mbar) or hectopascals (hPa) — they're identical (1 mbar = 1 hPa). Standard atmospheric pressure is 1013.25 mbar. A hurricane's central pressure might drop to 920-950 mbar. Tire pressure is measured in psi (pounds per square inch) in the US: typical car tires run 30-35 psi, bicycle tires 80-130 psi, and truck tires 80-120 psi. European tire gauges read in bar: 2.0-2.4 bar for cars (1 bar = 14.504 psi). HVAC and ventilation systems measure in inches of water column (inWC or "WC): duct pressure runs 0.5-2.0 inWC, and a manometer reading of 3.5 inWC means the fan generates 3.5 inches of water column pressure (0.126 psi). Medical blood pressure uses mmHg (millimeters of mercury): 120/80 mmHg means systolic pressure pushes a mercury column up 120 mm (2.32 psi). The torr is nearly identical to mmHg (1 torr = 1.000000142 mmHg) and is used in vacuum technology. Scuba diving uses atmospheres (atm) or bar: at 10 meters depth, pressure is 2 atm (1 atmospheric + 1 from water weight), doubling the pressure on your body and the air in your lungs.
Most pressure measuring instruments — tire gauges, blood pressure cuffs, boiler gauges — read gauge pressure (psig), which measures pressure above atmospheric. A tire at 35 psig actually contains air at 35 + 14.7 = 49.7 psia (pounds per square inch absolute). This distinction matters critically in engineering calculations involving gas laws (PV = nRT requires absolute pressure) and vacuum systems. A "perfect vacuum" reads -14.7 psig (zero absolute) at sea level. Industrial vacuum systems describe their depth in inches of mercury (inHg) below atmospheric: a vacuum pump pulling 25 inHg creates an absolute pressure of about 5 inHg, or roughly 2.5 psia. Altitude affects the reference: atmospheric pressure drops roughly 1 inHg per 1,000 feet of elevation, so a tire gauge at 5,000 feet reads against a lower atmospheric baseline — though the difference is negligible for consumer applications.
Pressure affects daily activities in ways people rarely consider. Atmospheric pressure changes of 10-20 mbar associated with weather fronts trigger headaches and joint pain in sensitive individuals — barometric pressure sensitivity is medically recognized though poorly understood. Water pressure in residential plumbing runs 40-80 psi: below 40 psi, showerheads drizzle and irrigation systems underperform; above 80 psi, pipes and fittings face accelerated wear and toilets can become noisy. Pressure cookers operate at roughly 15 psi above atmospheric (2 atm total), raising water's boiling point to 250°F (121°C) and reducing cooking time by 60-70%. Espresso machines force water through coffee grounds at 9 bar (130 psi) — this high pressure is what creates the crema and intense flavor that distinguishes espresso from drip coffee. Car engine combustion chambers reach peak pressures of 700-1,000 psi during the power stroke, while diesel engines exceed 2,000 psi — one reason diesels require heavier, more expensive engine blocks than gasoline engines.
Pressure measures force distributed over an area — the same force concentrated on a smaller area creates higher pressure. This explains why a 120-pound person in stiletto heels (0.1 sq inch heel tip) exerts roughly 1,200 psi on the floor — more than a 6,000-pound elephant standing on flat feet (~75 psi per foot). The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Because a pascal is tiny (atmospheric pressure is 101,325 Pa), practical measurements typically use kilopascals (kPa), megapascals (MPa), or one of several other units depending on the field. Converting between pressure units is essential because different industries and countries standardize on different units, often for historical reasons rather than technical ones.
Meteorology uses millibars (mbar) or hectopascals (hPa) — they're identical (1 mbar = 1 hPa). Standard atmospheric pressure is 1013.25 mbar. A hurricane's central pressure might drop to 920-950 mbar. Tire pressure is measured in psi (pounds per square inch) in the US: typical car tires run 30-35 psi, bicycle tires 80-130 psi, and truck tires 80-120 psi. European tire gauges read in bar: 2.0-2.4 bar for cars (1 bar = 14.504 psi). HVAC and ventilation systems measure in inches of water column (inWC or "WC): duct pressure runs 0.5-2.0 inWC, and a manometer reading of 3.5 inWC means the fan generates 3.5 inches of water column pressure (0.126 psi). Medical blood pressure uses mmHg (millimeters of mercury): 120/80 mmHg means systolic pressure pushes a mercury column up 120 mm (2.32 psi). The torr is nearly identical to mmHg (1 torr = 1.000000142 mmHg) and is used in vacuum technology. Scuba diving uses atmospheres (atm) or bar: at 10 meters depth, pressure is 2 atm (1 atmospheric + 1 from water weight), doubling the pressure on your body and the air in your lungs.
Most pressure measuring instruments — tire gauges, blood pressure cuffs, boiler gauges — read gauge pressure (psig), which measures pressure above atmospheric. A tire at 35 psig actually contains air at 35 + 14.7 = 49.7 psia (pounds per square inch absolute). This distinction matters critically in engineering calculations involving gas laws (PV = nRT requires absolute pressure) and vacuum systems. A "perfect vacuum" reads -14.7 psig (zero absolute) at sea level. Industrial vacuum systems describe their depth in inches of mercury (inHg) below atmospheric: a vacuum pump pulling 25 inHg creates an absolute pressure of about 5 inHg, or roughly 2.5 psia. Altitude affects the reference: atmospheric pressure drops roughly 1 inHg per 1,000 feet of elevation, so a tire gauge at 5,000 feet reads against a lower atmospheric baseline — though the difference is negligible for consumer applications.
Pressure affects daily activities in ways people rarely consider. Atmospheric pressure changes of 10-20 mbar associated with weather fronts trigger headaches and joint pain in sensitive individuals — barometric pressure sensitivity is medically recognized though poorly understood. Water pressure in residential plumbing runs 40-80 psi: below 40 psi, showerheads drizzle and irrigation systems underperform; above 80 psi, pipes and fittings face accelerated wear and toilets can become noisy. Pressure cookers operate at roughly 15 psi above atmospheric (2 atm total), raising water's boiling point to 250°F (121°C) and reducing cooking time by 60-70%. Espresso machines force water through coffee grounds at 9 bar (130 psi) — this high pressure is what creates the crema and intense flavor that distinguishes espresso from drip coffee. Car engine combustion chambers reach peak pressures of 700-1,000 psi during the power stroke, while diesel engines exceed 2,000 psi — one reason diesels require heavier, more expensive engine blocks than gasoline engines.
Pressure measures force distributed over an area — the same force concentrated on a smaller area creates higher pressure. This explains why a 120-pound person in stiletto heels (0.1 sq inch heel tip) exerts roughly 1,200 psi on the floor — more than a 6,000-pound elephant standing on flat feet (~75 psi per foot). The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Because a pascal is tiny (atmospheric pressure is 101,325 Pa), practical measurements typically use kilopascals (kPa), megapascals (MPa), or one of several other units depending on the field. Converting between pressure units is essential because different industries and countries standardize on different units, often for historical reasons rather than technical ones.
Meteorology uses millibars (mbar) or hectopascals (hPa) — they're identical (1 mbar = 1 hPa). Standard atmospheric pressure is 1013.25 mbar. A hurricane's central pressure might drop to 920-950 mbar. Tire pressure is measured in psi (pounds per square inch) in the US: typical car tires run 30-35 psi, bicycle tires 80-130 psi, and truck tires 80-120 psi. European tire gauges read in bar: 2.0-2.4 bar for cars (1 bar = 14.504 psi). HVAC and ventilation systems measure in inches of water column (inWC or "WC): duct pressure runs 0.5-2.0 inWC, and a manometer reading of 3.5 inWC means the fan generates 3.5 inches of water column pressure (0.126 psi). Medical blood pressure uses mmHg (millimeters of mercury): 120/80 mmHg means systolic pressure pushes a mercury column up 120 mm (2.32 psi). The torr is nearly identical to mmHg (1 torr = 1.000000142 mmHg) and is used in vacuum technology. Scuba diving uses atmospheres (atm) or bar: at 10 meters depth, pressure is 2 atm (1 atmospheric + 1 from water weight), doubling the pressure on your body and the air in your lungs.
Most pressure measuring instruments — tire gauges, blood pressure cuffs, boiler gauges — read gauge pressure (psig), which measures pressure above atmospheric. A tire at 35 psig actually contains air at 35 + 14.7 = 49.7 psia (pounds per square inch absolute). This distinction matters critically in engineering calculations involving gas laws (PV = nRT requires absolute pressure) and vacuum systems. A "perfect vacuum" reads -14.7 psig (zero absolute) at sea level. Industrial vacuum systems describe their depth in inches of mercury (inHg) below atmospheric: a vacuum pump pulling 25 inHg creates an absolute pressure of about 5 inHg, or roughly 2.5 psia. Altitude affects the reference: atmospheric pressure drops roughly 1 inHg per 1,000 feet of elevation, so a tire gauge at 5,000 feet reads against a lower atmospheric baseline — though the difference is negligible for consumer applications.
Pressure affects daily activities in ways people rarely consider. Atmospheric pressure changes of 10-20 mbar associated with weather fronts trigger headaches and joint pain in sensitive individuals — barometric pressure sensitivity is medically recognized though poorly understood. Water pressure in residential plumbing runs 40-80 psi: below 40 psi, showerheads drizzle and irrigation systems underperform; above 80 psi, pipes and fittings face accelerated wear and toilets can become noisy. Pressure cookers operate at roughly 15 psi above atmospheric (2 atm total), raising water's boiling point to 250°F (121°C) and reducing cooking time by 60-70%. Espresso machines force water through coffee grounds at 9 bar (130 psi) — this high pressure is what creates the crema and intense flavor that distinguishes espresso from drip coffee. Car engine combustion chambers reach peak pressures of 700-1,000 psi during the power stroke, while diesel engines exceed 2,000 psi — one reason diesels require heavier, more expensive engine blocks than gasoline engines.
Pressure measures force distributed over an area — the same force concentrated on a smaller area creates higher pressure. This explains why a 120-pound person in stiletto heels (0.1 sq inch heel tip) exerts roughly 1,200 psi on the floor — more than a 6,000-pound elephant standing on flat feet (~75 psi per foot). The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Because a pascal is tiny (atmospheric pressure is 101,325 Pa), practical measurements typically use kilopascals (kPa), megapascals (MPa), or one of several other units depending on the field. Converting between pressure units is essential because different industries and countries standardize on different units, often for historical reasons rather than technical ones.
Meteorology uses millibars (mbar) or hectopascals (hPa) — they're identical (1 mbar = 1 hPa). Standard atmospheric pressure is 1013.25 mbar. A hurricane's central pressure might drop to 920-950 mbar. Tire pressure is measured in psi (pounds per square inch) in the US: typical car tires run 30-35 psi, bicycle tires 80-130 psi, and truck tires 80-120 psi. European tire gauges read in bar: 2.0-2.4 bar for cars (1 bar = 14.504 psi). HVAC and ventilation systems measure in inches of water column (inWC or "WC): duct pressure runs 0.5-2.0 inWC, and a manometer reading of 3.5 inWC means the fan generates 3.5 inches of water column pressure (0.126 psi). Medical blood pressure uses mmHg (millimeters of mercury): 120/80 mmHg means systolic pressure pushes a mercury column up 120 mm (2.32 psi). The torr is nearly identical to mmHg (1 torr = 1.000000142 mmHg) and is used in vacuum technology. Scuba diving uses atmospheres (atm) or bar: at 10 meters depth, pressure is 2 atm (1 atmospheric + 1 from water weight), doubling the pressure on your body and the air in your lungs.
Most pressure measuring instruments — tire gauges, blood pressure cuffs, boiler gauges — read gauge pressure (psig), which measures pressure above atmospheric. A tire at 35 psig actually contains air at 35 + 14.7 = 49.7 psia (pounds per square inch absolute). This distinction matters critically in engineering calculations involving gas laws (PV = nRT requires absolute pressure) and vacuum systems. A "perfect vacuum" reads -14.7 psig (zero absolute) at sea level. Industrial vacuum systems describe their depth in inches of mercury (inHg) below atmospheric: a vacuum pump pulling 25 inHg creates an absolute pressure of about 5 inHg, or roughly 2.5 psia. Altitude affects the reference: atmospheric pressure drops roughly 1 inHg per 1,000 feet of elevation, so a tire gauge at 5,000 feet reads against a lower atmospheric baseline — though the difference is negligible for consumer applications.
Pressure affects daily activities in ways people rarely consider. Atmospheric pressure changes of 10-20 mbar associated with weather fronts trigger headaches and joint pain in sensitive individuals — barometric pressure sensitivity is medically recognized though poorly understood. Water pressure in residential plumbing runs 40-80 psi: below 40 psi, showerheads drizzle and irrigation systems underperform; above 80 psi, pipes and fittings face accelerated wear and toilets can become noisy. Pressure cookers operate at roughly 15 psi above atmospheric (2 atm total), raising water's boiling point to 250°F (121°C) and reducing cooking time by 60-70%. Espresso machines force water through coffee grounds at 9 bar (130 psi) — this high pressure is what creates the crema and intense flavor that distinguishes espresso from drip coffee. Car engine combustion chambers reach peak pressures of 700-1,000 psi during the power stroke, while diesel engines exceed 2,000 psi — one reason diesels require heavier, more expensive engine blocks than gasoline engines.
Pressure measures force distributed over an area — the same force concentrated on a smaller area creates higher pressure. This explains why a 120-pound person in stiletto heels (0.1 sq inch heel tip) exerts roughly 1,200 psi on the floor — more than a 6,000-pound elephant standing on flat feet (~75 psi per foot). The SI unit of pressure is the pascal (Pa), equal to one newton per square meter. Because a pascal is tiny (atmospheric pressure is 101,325 Pa), practical measurements typically use kilopascals (kPa), megapascals (MPa), or one of several other units depending on the field. Converting between pressure units is essential because different industries and countries standardize on different units, often for historical reasons rather than technical ones.
Meteorology uses millibars (mbar) or hectopascals (hPa) — they're identical (1 mbar = 1 hPa). Standard atmospheric pressure is 1013.25 mbar. A hurricane's central pressure might drop to 920-950 mbar. Tire pressure is measured in psi (pounds per square inch) in the US: typical car tires run 30-35 psi, bicycle tires 80-130 psi, and truck tires 80-120 psi. European tire gauges read in bar: 2.0-2.4 bar for cars (1 bar = 14.504 psi). HVAC and ventilation systems measure in inches of water column (inWC or "WC): duct pressure runs 0.5-2.0 inWC, and a manometer reading of 3.5 inWC means the fan generates 3.5 inches of water column pressure (0.126 psi). Medical blood pressure uses mmHg (millimeters of mercury): 120/80 mmHg means systolic pressure pushes a mercury column up 120 mm (2.32 psi). The torr is nearly identical to mmHg (1 torr = 1.000000142 mmHg) and is used in vacuum technology. Scuba diving uses atmospheres (atm) or bar: at 10 meters depth, pressure is 2 atm (1 atmospheric + 1 from water weight), doubling the pressure on your body and the air in your lungs.
Most pressure measuring instruments — tire gauges, blood pressure cuffs, boiler gauges — read gauge pressure (psig), which measures pressure above atmospheric. A tire at 35 psig actually contains air at 35 + 14.7 = 49.7 psia (pounds per square inch absolute). This distinction matters critically in engineering calculations involving gas laws (PV = nRT requires absolute pressure) and vacuum systems. A "perfect vacuum" reads -14.7 psig (zero absolute) at sea level. Industrial vacuum systems describe their depth in inches of mercury (inHg) below atmospheric: a vacuum pump pulling 25 inHg creates an absolute pressure of about 5 inHg, or roughly 2.5 psia. Altitude affects the reference: atmospheric pressure drops roughly 1 inHg per 1,000 feet of elevation, so a tire gauge at 5,000 feet reads against a lower atmospheric baseline — though the difference is negligible for consumer applications.
Pressure affects daily activities in ways people rarely consider. Atmospheric pressure changes of 10-20 mbar associated with weather fronts trigger headaches and joint pain in sensitive individuals — barometric pressure sensitivity is medically recognized though poorly understood. Water pressure in residential plumbing runs 40-80 psi: below 40 psi, showerheads drizzle and irrigation systems underperform; above 80 psi, pipes and fittings face accelerated wear and toilets can become noisy. Pressure cookers operate at roughly 15 psi above atmospheric (2 atm total), raising water's boiling point to 250°F (121°C) and reducing cooking time by 60-70%. Espresso machines force water through coffee grounds at 9 bar (130 psi) — this high pressure is what creates the crema and intense flavor that distinguishes espresso from drip coffee. Car engine combustion chambers reach peak pressures of 700-1,000 psi during the power stroke, while diesel engines exceed 2,000 psi — one reason diesels require heavier, more expensive engine blocks than gasoline engines.
→ Tire pressure: check cold. Heat from driving increases pressure 2–5 PSI.
→ Blood pressure in mmHg. Normal: 120/80. High: above 130/80.
→ Weather: inHg or kPa. 29.92 inHg = 1013.25 hPa at sea level.
→ Gauge vs absolute matters. Scientific work uses absolute; everyday uses gauge.
See also: Temperature · Energy · Weight · Speed