Pressure reference levels.
Common pressure levels — atmospheric, blood pressure, tire pressure, hydraulic systems, deep-sea — all on one chart across psi, bar, kPa, atm, mmHg, and inH₂O.
The chart
| Pressure | psi | bar | kPa | Other / context |
|---|---|---|---|---|
| Vacuum (perfect) | 0 | 0 | 0 | 0 atm, 0 torr — theoretical only |
| Ultra-high vacuum (lab) | — | — | 10⁻⁷ to 10⁻¹¹ Pa | Used in semiconductor / surface science |
| Medium vacuum (process) | — | — | 0.1 to 100 Pa | Distillation, evaporation, vacuum pumps |
| Light vacuum (industrial) | −14 to −5 | −1 to −0.3 | −100 to −30 kPa | Vacuum cleaners, pneumatic gripping |
| Mt. Everest summit air | 4.9 | 0.34 | 34 | ≈ 1/3 atm, 250 mmHg |
| Commercial cabin (cruise) | 11.3 | 0.78 | 78 | Equivalent altitude ~7,000 ft (2,100 m) |
| 1 atm (sea level standard) | 14.696 | 1.01325 | 101.325 | ≡ 760 mmHg ≡ 29.92 inHg (defined exact) |
| Bicycle tire (low end) | 30-40 | 2.1-2.8 | 210-280 | MTB / fat tire |
| Car tire (typical) | 30-35 | 2.1-2.4 | 210-240 | Check driver's door placard for exact |
| Truck tire (light truck) | 50-80 | 3.4-5.5 | 340-550 | Higher load capacity |
| Bicycle tire (road) | 70-130 | 4.8-9 | 480-900 | Narrower tire, higher pressure |
| Shop compressed air | 90-125 | 6.2-8.6 | 620-860 | Typical pneumatic tools |
| Residential water (street) | 40-80 | 2.8-5.5 | 280-550 | Pressure-reducing valve to 50-60 psi for house |
| Residential gas (natural) | 0.25 | 0.017 | 1.7 | 7 inH₂O — most residential supply |
| Espresso machine (brew) | 130 | 9 | 900 | Standard espresso brewing pressure |
| Pressure washer (consumer) | 1500-3000 | 100-200 | 10,000-20,000 | Cleaning automotive, decks |
| Pressure washer (industrial) | 3000-7000 | 200-480 | 20-48 MPa | Surface prep, paint removal |
| Hydraulic system (mobile) | 2000-3500 | 140-240 | 14-24 MPa | Excavators, loaders, machine tools |
| Hydraulic system (high-perf) | 5000+ | 350+ | 35+ MPa | Press brakes, injection molding |
| Scuba tank (full) | 3000 | 207 | 20.7 MPa | Most commonly used dive tank pressure |
| Scuba tank (high-pressure) | 3442 | 237 | 23.7 MPa | DOT 3AA tanks, technical diving |
| CNG fuel tank | 3000-3600 | 207-248 | 20-25 MPa | Compressed natural gas vehicles |
| Blood pressure (normal systolic) | 2.3 | 0.16 | 16 | 120 mmHg — note: gauge pressure (above atm) |
| Blood pressure (normal diastolic) | 1.5 | 0.10 | 10 | 80 mmHg |
| Hypertension threshold (systolic) | 2.5 | 0.17 | 17 | ≥130 mmHg per current US guidelines |
| Intracranial pressure (normal) | 0.07-0.29 | 0.005-0.020 | 0.5-2 kPa | 5-15 mmHg |
| Boiler (residential) | 12-30 | 0.8-2 | 80-200 | Hydronic heating expansion tank |
| Boiler (commercial steam) | 15-150 | 1-10 | 100-1000 | Process steam |
| Ocean at 10 m depth | 14.5 | 1.0 | 100 | 1 additional bar per 10 m depth |
| Ocean at 100 m | 145 | 10 | 1.0 MPa | Recreational dive limit |
| Ocean at 1,000 m | 1,450 | 100 | 10 MPa | Submarine operating depth |
| Mariana Trench (deepest) | 16,000 | 1,100 | 110 MPa | Mariana Trench at ~11 km depth |
Gauge vs absolute. Most pressure measurements (tire, blood, hydraulic, etc.) are gauge pressure — relative to atmospheric. Absolute pressure is gauge + 14.7 psi (1 atm). The values above are gauge where that's the conventional measurement (tire, blood, hydraulics) and absolute where that's conventional (barometric, vacuum). Always check which your application expects.
Common pitfalls
- Tire pressure: cold vs hot. Manufacturer specs are 'cold' pressure (before driving). Tire pressure rises 2-5 psi after 20 minutes of highway driving. Don't bleed off when hot — wait for cooldown.
- Blood pressure 'over' is systolic / diastolic. 120/80 means 120 mmHg peak (systolic, heart contracting) and 80 mmHg trough (diastolic, heart relaxing). Both are gauge pressures.
- Hydraulic vs pneumatic confusion. Hydraulic systems use incompressible liquid (oil), pneumatic systems use compressible gas (air). Hydraulic operates at much higher pressure (140-350 bar typical) than pneumatic (6-8 bar typical).
- 1 bar of water depth ≈ 10 m / 33 ft. A useful diving rule. At 30 m depth: 1 atm surface + 3 atm water = 4 atm absolute = 60 psi.
- Sea-level air pressure varies ±5%. Standard atm is 1013.25 mbar. Weather systems push it up to 1050 (very high pressure / 'high') or down to 970 (storm). Hurricane pressure can drop below 920 mbar.
Common questions
Why does the weather report use different pressure units than my barometer?
Different fields kept their conventional units. US weather services use inches of mercury (29.92 inHg at sea level) and millibars (1013.25 mb). Europe uses hectopascals (1013.25 hPa = 1013.25 mb). Aviation uses inHg and hPa. Scuba uses bar or atmospheres. Engineering uses psi, kPa, or MPa. They all measure the same thing; just in different units.
Is 'gauge pressure' the same as 'absolute pressure'?
No. Gauge pressure is measured relative to atmospheric (atmospheric = 0 psig). Absolute pressure is measured from a vacuum (atmospheric = 14.7 psia). A tire reading 32 psig is actually 46.7 psia. Suffix matters: psig (gauge), psia (absolute), psid (differential). Confusing these has caused industrial accidents.
What's 'standard atmosphere' and why is it different from local?
Standard atmosphere is a global reference: 101.325 kPa (14.696 psi) at sea level. Local atmospheric pressure varies with weather (typically 980-1040 hPa) and elevation. Denver at 1600 m elevation has standard pressure of about 83 kPa, not 101 kPa. When precision matters, use local pressure, not standard.
How much pressure can a typical garden hose handle?
Standard residential hoses are rated 50-100 psi working pressure with burst pressures around 300-500 psi. Municipal water systems run 40-80 psi. If your house has unusually high pressure (over 80 psi), install a pressure regulator — it'll save hoses, washing machines, and water heaters.
Why does altitude affect cooking time?
Water boils at a temperature where its vapor pressure equals atmospheric pressure. Lower altitude = higher atmospheric pressure = higher boiling point = faster cooking. At sea level water boils at 100°C; at 1500 m elevation it boils at about 95°C; at 3000 m at 90°C. Recipes from sea level take longer at altitude — adjust by 10-20% per 1000 m.
Sources
- Standard atmosphere: ISO 2533. Standard sea-level pressure = 101,325 Pa (defined exactly).
- Blood pressure guidelines: American College of Cardiology / American Heart Association 2017 guidelines.
- Pressure unit definitions: The bar = 100,000 Pa exactly; atm = 101,325 Pa exactly; 1 psi = 6894.757 Pa.
- Hydraulic pressure ranges: Industry conventions from Eaton, Parker, Bosch Rexroth catalogs.
Disclaimer. Pressure measurements depend on temperature, altitude, and gauge vs absolute reference. For regulated work (gas, fire sprinkler, pressure vessel), always use the relevant code-mandated values.