Core answer: 1 standard atmosphere = 101.325 kPa = 1.013 bar = 14.7 psi = 760 mmHg. Car tires run 2.2–2.5 bar (32–36 psi); road-bike tires 6–8 bar (90–120 psi); blood pressure 120/80 is in mmHg. Weather maps use hPa (= mbar): below 980 hPa means a serious storm.
The conversion table
| Unit | In kPa | Where you see it |
|---|---|---|
| 1 atm | 101.325 | chemistry reference |
| 1 bar | 100 | tires, industry |
| 1 psi | 6.895 | US tires, tools |
| 1 mmHg (Torr) | 0.133 | blood pressure, vacuum |
| 1 kgf/cm² | 98.07 | old gauges ("几公斤压力") |
| 1 m water column | 9.807 | pumps, diving |
Worked examples
Example 1 — Tire pressure. Door sticker says 230 kPa = 2.3 bar ≈ 33 psi. Gas stations in the US show psi; European gauges show bar — know both or underinflate by 15×.
Example 2 — Diving. Every 10 m of seawater adds ~1 atm: at 30 m your body feels 4 atm total; lungs' air compresses to 1/4 volume — the physics behind "never hold your breath ascending".
Example 3 — Blood pressure. 120/80 mmHg means peak arterial pressure could support a 12 cm column of mercury; hypertension starts at 140/90 (China/WHO) or 130/80 (US ACC/AHA 2017).
Example 4 — Espresso. Proper extraction needs 9 bar at the puck = 9× atmospheric; cheap machines claiming "15 bar pump" describe pump max, not brew pressure.
Example 5 — Typhoon intensity. A central pressure of 920 hPa vs normal 1,013 hPa is a ~9% atmosphere deficit — that pressure hole is what sucks winds inward at destructive speed.
Common mistakes and myths
- "公斤压力" ambiguity — old Chinese gauges say 公斤 (kgf/cm² ≈ 0.98 bar), not kg; a "5公斤" compressor is ~0.5 MPa.
- bar vs barg — gauges read relative (bar above atmosphere); absolute = gauge + 1. A 2.4 bar tire is 3.4 bar absolute — matters in physics, not at the pump.
- psi/bar slip — 32 psi (2.2 bar) vs 32 bar (464 psi): a 15× error that can explode a bicycle tube.
- Weather "pressure falling" — a 5 hPa/day drop means weather turning; 10+ hPa means batten down.
- Assuming altitude pressure is linear — it halves every ~5,500 m: at Everest's summit (8,849 m) it's ~1/3 atm, which is why water boils there at ~70 °C and oxygen masks exist.