Convert Pressure Units
Popular Conversions:
1 atm to PSI 1 bar to PSI 32 PSI to bar 760 mmHg to atm 101325 Pa to atm 100 kPa to PSIQuick Reference Table
| From | To | Multiply By | Example |
|---|---|---|---|
| PSI | Bar | 0.0689476 | 30 PSI = 2.07 bar |
| Bar | PSI | 14.5038 | 2 bar = 29.01 PSI |
| Atmosphere | PSI | 14.6959 | 1 atm = 14.70 PSI |
| Atmosphere | Pascal | 101,325 | 1 atm = 101,325 Pa |
| kPa | PSI | 0.145038 | 100 kPa = 14.50 PSI |
| mmHg | Pascal | 133.322 | 760 mmHg = 101,325 Pa |
| Bar | Pascal | 100,000 | 1 bar = 100,000 Pa |
| Torr | mmHg | 1 | 760 torr = 760 mmHg |
Common Pressure Values
| Description | PSI | Bar | kPa |
|---|---|---|---|
| Sea level atmospheric | 14.7 | 1.013 | 101.3 |
| Car tire (typical) | 32-35 | 2.2-2.4 | 220-240 |
| Bicycle tire (road) | 80-130 | 5.5-9.0 | 550-900 |
| Water faucet | 40-80 | 2.8-5.5 | 280-550 |
| Scuba tank (full) | 3000-3300 | 207-228 | 20,700-22,800 |
| Espresso machine | 130-150 | 9-10 | 900-1000 |
| Blood pressure (normal) | 2.3/1.6 | 0.16/0.11 | 16/11 |
| Vacuum (perfect) | 0 | 0 | 0 |
| Deepest ocean (Mariana) | ~16,000 | ~1,100 | ~110,000 |
Understanding Pressure Units
Pascal (Pa) & Kilopascal (kPa)
Definition: The SI unit of pressure. One pascal equals one newton of force per square meter (1 N/m²). Very small unit; kilopascal (1,000 Pa) is more commonly used in practical applications.
History: Named after French mathematician and physicist Blaise Pascal (1623-1662) who made significant contributions to fluid mechanics and pressure studies. Officially adopted as the SI pressure unit in 1971.
Current Use: Standard pressure unit in scientific work worldwide. Kilopascals (kPa) commonly used in Canada, Australia, and many other countries for tire pressure specifications, weather reports (barometric pressure), and engineering. Atmospheric pressure at sea level is approximately 101.3 kPa. Medical blood pressure sometimes expressed in kPa.
Pounds Per Square Inch (PSI or psi)
Definition: The pressure resulting from a force of one pound-force applied to an area of one square inch. Equal to approximately 6,895 pascals or 0.0689 bar.
History: Developed with the imperial system of measurements. Became standard in American and British engineering during the Industrial Revolution, particularly in steam engines and hydraulics.
Current Use: Dominant pressure unit in the United States for tire pressure (32-35 PSI typical for cars), air compressors, hydraulic systems, gas cylinders, pressure washers, and HVAC systems. Also used in UK alongside metric units. Standard in American plumbing and pneumatic tools.
Bar
Definition: A metric unit equal to exactly 100,000 pascals (100 kPa). Approximately equal to atmospheric pressure at sea level (1 bar ≈ 0.987 atm). Not an SI unit but widely accepted.
History: Introduced in 1909 from the Greek "baros" meaning weight. Designed to be close to one atmosphere for practical convenience while being based on metric units (10⁵ Pa).
Current Use: Standard pressure unit in Europe and most of the world for tire pressure, weather maps (millibars for barometric pressure), scuba diving depth calculations, and industrial equipment specifications. Most European car tire recommendations given in bar (2.0-2.5 bar typical). Meteorology uses millibar (mbar, 1/1000 bar) for atmospheric pressure.
Atmosphere (atm)
Definition: Defined as exactly 101,325 pascals. Originally based on average atmospheric pressure at sea level. Equal to 14.6959 PSI or 1.01325 bar.
History: Historically represented mean atmospheric pressure at sea level at 15°C. Standardized in 1954 for scientific calculations. Older engineering and scientific references often use atmospheres.
Current Use: Used in chemistry and physics for standard conditions (STP: 1 atm, 273.15 K). Scuba diving uses atmospheres to describe depth pressure (2 atm at 10 meters depth). High-pressure physics and engineering calculations. Less common in everyday applications than bar or PSI but important in scientific contexts.
Millimeters of Mercury (mmHg)
Definition: The pressure exerted by a column of mercury exactly 1 millimeter high at 0°C under standard gravity. Equal to approximately 133.322 pascals. One standard atmosphere = 760 mmHg.
History: Originates from mercury barometers invented by Evangelista Torricelli in 1643. Mercury's density and low vapor pressure made it ideal for measuring atmospheric pressure. "Inches of mercury" (inHg) also used historically.
Current Use: Primary unit for blood pressure measurements worldwide (120/80 mmHg is normal). Weather reports in some regions use mmHg or inHg for barometric pressure. Medical vacuum systems and respiratory equipment. Aviation altimeter settings sometimes given in inches of mercury (inHg).
Torr
Definition: Defined as exactly 1/760 of a standard atmosphere. Essentially equivalent to mmHg (defined slightly differently but practically identical: 1 torr ≈ 1 mmHg).
History: Named after Evangelista Torricelli, inventor of the mercury barometer. Defined precisely in 1954 to honor Torricelli while providing an exact fraction of one atmosphere.
Current Use: Commonly used in vacuum technology and high-vacuum physics. Vacuum pump specifications often given in torr or millitorr. Chemistry labs use torr for vapor pressure measurements. Medical applications sometimes use torr interchangeably with mmHg.
Inches of Mercury (inHg)
Definition: The pressure exerted by a column of mercury one inch high. Equal to approximately 3,386.39 pascals or 25.4 mmHg.
History: Traditional imperial measurement used with mercury barometers in English-speaking countries.
Current Use: Aviation in the United States uses inHg for altimeter settings. Weather reports in US sometimes give barometric pressure in inHg (standard: 29.92 inHg at sea level). HVAC systems occasionally use inches of mercury for vacuum measurements.
Millibar (mbar)
Definition: One thousandth of a bar, equal to 100 pascals or 1 hectopascal (hPa). Standard atmospheric pressure = 1013.25 mbar.
History: Derived from the bar unit. Became standard in meteorology because the numbers are convenient (around 1000 at sea level).
Current Use: Standard unit for atmospheric pressure in weather forecasting worldwide. Weather maps show pressure systems in millibars or hectopascals (equivalent). Aviation uses millibars/hectopascals internationally for altimeter settings. "High pressure" weather systems typically above 1020 mbar.
Pounds Per Square Foot (psf)
Definition: Pressure of one pound-force per square foot. Equal to 1/144 PSI or approximately 47.88 pascals.
History: Imperial unit used in structural and civil engineering calculations.
Current Use: Structural engineering for building loads (floor loading, wind pressure on buildings). Snow load calculations. Less common than PSI in mechanical applications.
Frequently Asked Questions
How do I convert PSI to bar?
Divide PSI by 14.5038 or multiply by 0.0689476. Formula: bar = PSI × 0.0689476. Example: 30 PSI × 0.0689476 = 2.07 bar. This is useful when inflating tires using European specifications. To convert bar to PSI, multiply by 14.5038.
What is normal atmospheric pressure?
Standard atmospheric pressure at sea level is defined as exactly 101,325 pascals (101.325 kPa), which equals 1 atmosphere (atm), 14.6959 PSI, 1.01325 bar, 760 mmHg, or 29.92 inches of mercury. Actual atmospheric pressure varies with weather and altitude.
What pressure should my car tires be?
Typical passenger car tires: 32-35 PSI (2.2-2.4 bar or 220-240 kPa). Check your vehicle's door jamb sticker or owner's manual for specific recommendations. Tire pressure varies by vehicle weight and tire size. Always check pressure when tires are cold for accuracy.
What does "gauge pressure" mean?
Gauge pressure measures pressure relative to atmospheric pressure. A tire gauge reading 32 PSI means 32 PSI above atmospheric pressure. Absolute pressure would be 32 + 14.7 = 46.7 PSIA (PSI absolute). Most pressure gauges read relative pressure. Scientific work often requires absolute pressure.
Why is blood pressure measured in mmHg?
Historical reasons: early blood pressure devices (sphygmomanometers) used mercury columns. The standard became entrenched in medicine. Normal blood pressure is 120/80 mmHg (systolic/diastolic), meaning 120 mmHg above atmospheric pressure during heartbeat and 80 mmHg between beats. Mercury devices are now being phased out, but the mmHg unit remains standard.
How does altitude affect pressure?
Atmospheric pressure decreases approximately 1.2 kPa (0.17 PSI) per 100 meters (328 feet) of altitude gain near sea level. At 5,000 feet elevation, atmospheric pressure is about 83 kPa vs 101 kPa at sea level. This affects cooking times, tire pressure readings, and breathing. Aircraft cabins pressurized to equivalent of ~6,000-8,000 feet.
What's the difference between PSI and PSIG?
PSI typically refers to gauge pressure (PSIG), measured relative to atmospheric pressure. PSIA means absolute pressure, including atmospheric pressure. PSIG is used for practical applications (tire pressure). PSIA is used in scientific and engineering calculations requiring absolute values. 0 PSIG = 14.7 PSIA at sea level.
How much pressure at ocean depth?
Pressure increases approximately 1 atmosphere (14.7 PSI or 1 bar) for every 10 meters (33 feet) of water depth. At 10m depth: 2 atm total (1 atm atmospheric + 1 atm water). At 100m: 11 atm. Mariana Trench (~11,000m): over 1,100 atmospheres or 16,000 PSI.
Common Uses for Pressure Conversion
- Automotive: Converting tire pressure between PSI, bar, and kPa
- Medical: Understanding blood pressure readings in mmHg
- HVAC: Converting system pressures for air conditioning and heating
- Scuba Diving: Calculating depth pressures in atmospheres or bar
- Aviation: Converting altimeter settings between inHg and millibars
- Weather: Understanding barometric pressure in different units
- Industrial: Hydraulic and pneumatic system specifications
- Science: Converting between units for experiments and calculations
Pressure Conversion Tips
- Remember benchmarks: 1 atm ≈ 14.7 PSI ≈ 1 bar ≈ 100 kPa ≈ 760 mmHg
- Tire pressure rule: Divide PSI by 14.5 to get bar (30 PSI ≈ 2 bar)
- Quick PSI to kPa: Multiply by 7 (close approximation: exact is 6.895)
- Gauge vs absolute: Most gauges show relative pressure, not absolute
- Altitude matters: Atmospheric pressure decreases with elevation
- Temperature affects pressure: Tire pressure increases when hot
- Use proper abbreviations: PSI, bar, Pa, kPa, mmHg, atm
Interesting Pressure Facts
- Space vacuum: ~10⁻¹⁷ bar (nearly perfect vacuum)
- Human bite force: up to 200 PSI
- Champagne bottle: 80-90 PSI (5-6 bar)
- Fire hose: 150-300 PSI
- Pressure cooker: 15 PSI above atmospheric (250°F water boiling point)
- Commercial aircraft cabin: 11-12 PSI (0.75-0.82 bar)
- Deepest ocean pressure: 1,086 bar (15,750 PSI) - would crush most objects
- Center of Earth: estimated 3.5 million bar
- Human lungs max: can generate ~1-2 PSI blowing force
- Hurricane eye wall: can drop to 900-920 mbar (very low pressure)
- Espresso pressure: 9 bar (130 PSI) for proper extraction
- Car wash pressure washer: 1,000-3,000 PSI
- Waterjet cutter: 60,000+ PSI (can cut steel)
Pressure in Different Applications
Automotive
Car tires: 32-35 PSI (2.2-2.4 bar). Truck tires: 50-80 PSI. Bicycle road: 80-130 PSI. Mountain bike: 30-50 PSI. Motorcycle: 28-40 PSI. Check when cold; driving heats tires and increases pressure by 2-4 PSI.
Medical
Blood pressure normal: 120/80 mmHg. Hypertension: ≥130/80 mmHg. Hypotension: <90/60 mmHg. Intracranial pressure: 7-15 mmHg normal. Intraocular (eye) pressure: 10-21 mmHg normal.
Weather
Standard sea level: 1013.25 mbar (29.92 inHg). High pressure: >1020 mbar (fair weather). Low pressure: <1000 mbar (storms). Hurricane: <920 mbar in eye. Record low: 870 mbar (Typhoon Tip, 1979).
Industrial
Air compressor: 90-150 PSI typical. Hydraulic systems: 1,000-5,000 PSI. Natural gas pipeline: 200-1,500 PSI. Steam boiler: 15-150 PSI. Scuba tank: 3,000-3,300 PSI when full.