Millimeter of Water Column to Bar Converter

What exactly is one bar?

One bar = 100,000 pascals (Pa) = 100 kilopascals (kPa).

Physical interpretation:

• One bar approximately equals atmospheric pressure at sea level (actual: 1.013 bar)
• One bar = pressure from a 10-meter (33-foot) column of water
• One bar = 10 newtons of force distributed over 1 cm² (10 N/cm²)

Why it's practical: The bar bridges the gap between the very small pascal (101,325 Pa for atmospheric pressure) and the non-metric atmosphere/PSI. Tire pressure of 2.2 bar is easier to remember and communicate than 220,000 Pa or 220 kPa.

How do I convert bar to PSI?

Formula: PSI = bar × 14.5038

Step-by-step example (2.2 bar to PSI):

1. 2.2 bar × 14.5038 = 31.9 PSI
2. Or use approximation: 2.2 bar × 14.5 ≈ 32 PSI

Quick approximation: Multiply bar by 15 for rough PSI (slightly high)

• 2 bar × 15 = 30 PSI (exact: 29.0 PSI)
• 2.5 bar × 15 = 37.5 PSI (exact: 36.3 PSI)

Common conversions:

• 1.0 bar = 14.5 PSI
• 2.0 bar = 29.0 PSI (bicycle/motorcycle tire)
• 2.2 bar = 32.0 PSI (typical car tire)
• 2.5 bar = 36.3 PSI (SUV tire)
• 10 bar = 145 PSI (compressed air)
• 200 bar = 2,900 PSI (scuba tank)

Bar to PSI converter →

How many bar is standard atmospheric pressure?

Standard atmospheric pressure = 1.01325 bar (exactly = 1 atmosphere)

Rounded: Often approximated as 1 bar for practical purposes (1.3% difference).

Weather variations:

• High pressure system: 1.020-1.060 bar (clear weather)
• Normal range: 1.000-1.020 bar
• Low pressure system: 0.980-1.000 bar (rain/clouds)
• Tropical storm: 0.950-0.980 bar
• Hurricane: <0.920 bar (severe storm)

Altitude variations:

• Sea level: 1.013 bar
• 1,000 m (3,280 ft): 0.900 bar
• Denver (1,600 m, 5,280 ft): 0.830 bar
• Mount Everest (8,849 m): 0.337 bar (1/3 sea level)

What is 2.5 bar in PSI?

2.5 bar = 36.3 PSI

Typical uses:

• SUV tires (loaded): 2.4-2.6 bar (35-38 PSI)
• Van/minivan tires: 2.5-2.8 bar (36-40 PSI)
• High-performance car tires: 2.5-3.0 bar (36-44 PSI)
• Light truck tires (front): 2.5-2.8 bar (36-40 PSI)
• Sports car (Porsche, BMW M): 2.5-3.0 bar (36-44 PSI)

Temperature note: Tire pressure increases ~0.1 bar when hot, so 2.5 bar cold may read 2.6-2.7 bar after highway driving.

Bar to PSI converter →

How do I convert bar to kPa?

Formula: kPa = bar × 100 (exactly—easiest conversion!)

Examples:

• 1 bar = 100 kPa
• 2 bar = 200 kPa
• 2.2 bar = 220 kPa (car tire)
• 10 bar = 1,000 kPa = 1 MPa
• 200 bar = 20,000 kPa = 20 MPa (scuba tank)

Mental math: Just multiply by 100 (add two zeros or move decimal point two places right).

Why this matters: International tire pressure specifications increasingly use kPa instead of bar or PSI. Same number, different unit: 220 kPa = 2.2 bar = 32 PSI.

Bar to kPa converter →

Why does Europe use bar and USA use PSI?

Historical reasons:

Europe/Asia → Bar:

• Metric system adoption: Bar fits decimal metric framework (1 bar = 100 kPa)
• SI-compatible: Exact relationship to pascal (1 bar = 10⁵ Pa)
• Intuitive scale: 1 bar ≈ atmospheric pressure, car tires 2-2.5 bar
• International cooperation: European standardization post-WWII promoted bar
• Current status: Dominant in Europe, Asia, Middle East, Africa, South America

USA/UK → PSI:

• Imperial system tradition: Pounds per square inch predates metric system
• Industrial legacy: Existing tools, gauges, training, documentation all in PSI
• Market size: US automotive/aerospace industry maintained PSI standards
• Cost of conversion: Retooling factories, retraining workforce expensive
• Current status: US predominantly PSI, UK transitioning to bar

Modern reality:

• Dual labeling: Tire sidewalls worldwide show both "32 PSI / 2.2 bar"
• Conversion apps: Smartphones make unit conversion trivial
• Professional equipment: High-end gauges display both units simultaneously
• Globalization: Engineers routinely work with both bar and PSI

What's the difference between bar and millibar?

Bar = 100,000 Pa = 100 kPa (industrial/engineering standard)

Millibar (mbar) = 0.001 bar = 100 Pa = 1 hectopascal (hPa) (meteorology standard)

Relationship: 1 bar = 1,000 mbar

Usage:

• Bar: Tire pressure (2.2 bar), hydraulics (200 bar), scuba tanks (300 bar)
• Millibar: Weather reports (1013 mbar), atmospheric pressure, aviation

Why different scales:

• Millibar provides human-scale numbers for weather: 980-1050 mbar range
• Bar avoids decimals for engineering: 2.2 bar tire vs 2,200 mbar (awkward)

Modern terminology: Meteorologists now use hectopascal (hPa) instead of millibar, but they're numerically identical:

• 1 mbar = 1 hPa (same number, SI vs non-SI name)
• "Pressure is 1015 mbar" = "Pressure is 1015 hPa"

Can I use bar for scientific research?

Generally no—use pascals (Pa/kPa/MPa) for scientific work.

Reasoning:

• SI standard: Pascal is the official SI unit of pressure
• Scientific journals: Require SI units for consistency and international comparability
• Peer review: Reviewers expect Pa/kPa/MPa in research papers
• Data archiving: Long-term data should use SI for future accessibility

Exceptions where bar is acceptable:

• Applied engineering research: Industrial collaborations, pressure vessel studies
• Geophysics/oceanography: Some fields traditionally use bar/decibar
• Historical comparisons: Citing older literature that used bar
• Supplementary notation: Can include bar in parentheses after SI value: "100 kPa (1 bar)"

Best practice:

• Report in SI (Pa/kPa/MPa) as primary unit
• Mention bar equivalents if relevant to application
• Follow journal guidelines: Check specific requirements before submission

How does pressure change with depth underwater?

Rule: Pressure increases 1 bar per 10 meters (33 feet) of depth.

Depth-pressure relationship:

• Surface: 1 bar (atmospheric pressure)
• 10 m (33 ft): 2 bar absolute (1 bar water + 1 bar atmosphere)
• 20 m (66 ft): 3 bar absolute
• 30 m (98 ft): 4 bar absolute—recreational diving limit
• 40 m (130 ft): 5 bar absolute—advanced open water limit
• 50 m (164 ft): 6 bar absolute
• 100 m (328 ft): 11 bar absolute—technical/commercial diving
• 200 m (656 ft): 21 bar absolute

Why it matters for diving:

• Air consumption: At 30 m (4 bar), diver breathes air 4× faster than surface
• Nitrogen absorption: Higher pressure = more nitrogen dissolves in blood/tissues
• Decompression requirements: Deeper/longer dives need staged ascent to avoid "bends"
• Regulator design: Delivers breathing gas at ambient pressure (2 bar at 10 m)

Formula: Pressure (bar absolute) = (Depth in meters ÷ 10) + 1

What pressure is a scuba tank filled to?

Standard scuba tank pressures:

Aluminum tanks (common in US/recreational diving):

• Aluminum 80 cu ft: 207 bar (3,000 PSI) when full
• Usable down to 50 bar (725 PSI) reserve

Steel tanks (common in Europe/technical diving):

• Steel 12L (European): 232 bar (3,365 PSI) standard, up to 300 bar (4,350 PSI) high-pressure
• Steel 100 cu ft (US): 237 bar (3,442 PSI)

Why such high pressure:

• Compact storage: More air fits in smaller volume at high pressure
• Extended dive time: Higher capacity = longer bottom time
• Safety margin: Start with 200-300 bar, surface with 50+ bar reserve

Tank safety:

• Hydrostatic testing: Every 5 years at 1.5× working pressure (300-450 bar)
• Visual inspection: Annual checks for corrosion, damage
• Burst disk: Releases at ~1.25× working pressure to prevent catastrophic failure

How do I measure pressure in bar?

Pressure measurement devices:

1. Tire Pressure Gauges:

• Dial gauge: Mechanical, reads in bar and/or PSI (accuracy ±0.1 bar)
• Digital gauge: LCD display, selectable units (accuracy ±0.05 bar)
• Pencil gauge: Simple, cheap, less accurate (±0.2 bar)
• Tire shops: Calibrated gauges (±0.03 bar)

2. Industrial Pressure Gauges:

• Bourdon tube gauge: Most common, 40-400 mm dial, ranges 0-1 to 0-1000+ bar
• Dial markings: Often dual-scale (bar outer ring, PSI inner ring)
• Accuracy classes: 0.6%, 1.0%, 1.6%, 2.5% of full scale

3. Digital Pressure Transducers:

• Electronic sensors: Piezoresistive, strain gauge, capacitive types
• Output: 4-20 mA current loop or 0-10V voltage proportional to pressure
• Display: Digital readout in user-selected units (bar, PSI, kPa, MPa)
• Accuracy: ±0.1% to ±0.5% full scale

4. Manometers (for low pressures):

• U-tube manometer: Water or mercury column (measures in mm H₂O or mm Hg, convert to bar)
• Digital manometer: Electronic, displays in bar, kPa, PSI, mbar

5. Barometers (atmospheric pressure):

• Aneroid barometer: Mechanical, often labeled in mbar and/or inHg
• Electronic barometer: Digital, weather stations, smartphones (MEMS sensor)

Calibration: Professional gauges calibrated against reference standards traceable to national metrology labs (NIST, PTB).

Is bar the same as barometric pressure?

No—they're related but different concepts:

Bar (unit):

• A unit of measurement for pressure
• 1 bar = 100,000 Pa = 100 kPa
• Can measure any type of pressure (tire, hydraulic, atmospheric, etc.)

Barometric pressure (measurement):

• The atmospheric pressure at a given location
• Measured and reported in various units: bar, mbar, hPa, mmHg, inHg, PSI
• Changes with weather and altitude
• Standard sea-level barometric pressure = 1.01325 bar = 1,013.25 mbar

Etymology connection:

• Both "bar" and "barometer" derive from Greek baros (βάρος) = weight
• Barometer = baros (weight) + metron (measure) = instrument measuring atmospheric weight/pressure
• William Napier Shaw chose "bar" to complement "barometer" terminology

Usage:

• "The barometric pressure is 1.015 bar" ✓ (atmospheric pressure measured in bar units)
• "The tire pressure is 2.2 bar" ✓ (tire pressure measured in bar units)
• Barometric pressure is just one type of pressure that can be expressed in bar