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📘 Overview
Pressure is force per area. The same physical value shows up in different unit “dialects” depending on the field: tires and hydraulics use psi, weather uses hPa/mbar, labs use torr/mmHg, and engineering often uses bar or high-range units like ksi.
This page focuses on definitions, interpretation, and common pitfalls—especially where units look similar but are not interchangeable. For more conversions, see the Unit Converters hub.
What each unit really means (quick intuition)
- Pa (pascal): SI unit. 1 Pa = 1 N/m². Practical ranges are often in kPa or MPa.
- bar: exactly 100,000 Pa. Convenient for engineering and scuba tanks.
- atm: standard atmosphere, exactly 101,325 Pa (sea-level reference).
- psi: pounds per square inch. Common in the U.S. (tires, hydraulics).
- mmHg / torr: mercury-column units used in medicine and vacuum work (nearly equal in practice).
- cmH₂O / inH₂O: water-column units (HVAC, respiration). Slightly sensitive to reference temperature.
Typical ranges you’ll recognize
| Scenario | Typical value | Approx. equivalents | Why it matters |
|---|---|---|---|
| Sea-level atmosphere | 1 atm | 101.325 kPa ≈ 1.01325 bar ≈ 14.70 psi ≈ 760 mmHg | Baseline for “absolute” pressure |
| Car tire (cold) | 30–36 psi | 207–248 kPa ≈ 2.07–2.48 bar | Common psi↔bar conversion |
| Blood pressure (systolic) | ~120 mmHg | ~16.0 kPa ≈ 0.160 bar ≈ 2.32 psi | mmHg is small in SI terms |
| HVAC duct static pressure | 0.2–1.0 inH₂O | ~50–250 Pa | Water-column units are “low pressure” |
| Scuba tank fill | ~200 bar | 20 MPa ≈ 2900 psi | High pressure: watch MPa/psi scale |
Related topic hubs (for context-specific tools): Energy, Health, Home & DIY.
📐 Formula & Methodology
Unit conversion is a two-step scaling problem. The most reliable approach is converting everything through the SI base: pascal (Pa).
Pa = value × factor(fromUnit → Pa) value_in_toUnit = Pa ÷ factor(toUnit → Pa)
Defined vs. conventional factors (where people trip up)
- Exactly defined: 1 bar = 100,000 Pa; 1 atm = 101,325 Pa.
- Conventional (practical standards): 1 mmHg ≈ 133.322 Pa; 1 torr = 1/760 atm ≈ 133.322368… Pa.
- Fluid-column units: mmH₂O/cmH₂O/inH₂O depend on density and standard gravity conventions (small variation across references).
Common math/logic mistakes (pressure-specific)
- Mixing gauge and absolute: “psi” on a tire gauge is typically psig (relative to atmosphere), not absolute pressure.
- Confusing bar and atm: 1 atm is about 1.3% higher than 1 bar—small, but noticeable in precise work.
- mbar vs bar: 1 bar = 1000 mbar. A “1013 mbar” weather reading is about 1 atm, not 1013 bar.
- inHg vs inH₂O: both use “in”, but mercury is ~13.6× denser than water—values differ by an order of magnitude.
- Huge-force units: kgf/mm² and ksi produce very large results; check the exponent/scale before concluding anything.
📊 Sample Conversion Table (→ Pascal)
| Unit | Symbol | Factor (→ Pa) | Definition / Note |
|---|---|---|---|
| Pascal | Pa | 1 | SI base unit |
| Atmosphere (standard) | atm | 101325 | Standard air pressure |
| Bar | bar | 100000 | Used in weather & industry |
| Pound per square inch | psi | 6894.757 | US customary unit |
| Torr | torr | 133.322 | Medical / lab pressure |
| Millimetre of mercury | mmHg | 133.322 | Medical / lab pressure |
| Atmosphere (technical) | at | 98066.5 | Engineering context |
| Barye (CGS unit) | barye | 0.1 | 1 barye = 0.1 Pa |
| Centimetre of mercury | cmHg | 1333.22 | 1 cmHg = 10 mmHg = 1333.22 Pa |
| Centimetre of water (4 °C) | cmH₂O | 98.0665 | Used in respiration |
| Foot of mercury | ftHg | 40636.7 | 1 ftHg = 12 inHg ≈ 40636.7 Pa |
| Foot of water (39.2 °F) | ftH₂O | 2989.07 | 1 ftH₂O ≈ 2989.07 Pa |
| Inch of mercury | inHg | 3386.389 | Barometric readings |
| Inch of water (39.2 °F) | inH₂O | 249.0889 | 1 inH₂O ≈ 249.0889 Pa |
| Kilogram‐force per square millimetre | kgf/mm² | 9806650 | Structural strength |
| Kip per square inch | ksi | 6894757.29 | High-pressure systems |
| Pound per square foot | psf | 47.8803 | 1 psf = 47.8803 Pa |
| Poundal per square foot | pdl/ft² | 1.48816 | 1 pdl/ft² = 1.48816 Pa |
| Long ton per square foot | tnf/ft² | 107992.86 | 1 long ton/ft² ≈ 1.0799286×10⁵ Pa |
| Short ton per square foot | ton_s/ft² | 95396.21 | 1 short ton/ft² ≈ 9.539621×10⁴ Pa |
| Millimetre of water | mmH₂O | 9.80665 | 1 mmH₂O = 9.80665 Pa |
| Micrometre of mercury | μmHg | 0.133322 | 1 μmHg = 0.133322 Pa |
| Pièze (MTS unit) | pź | 1000 | MTS system (historical) |
🧪 Examples
Example 1: 32 psi → bar (typical tire pressure)
- 32 psi × 6894.757 = 220,632 Pa
- 220,632 Pa ÷ 100,000 = 2.206 bar
Example 2: 1013 hPa → atm (weather station)
- 1013 hPa = 101,300 Pa
- 101,300 Pa ÷ 101,325 = 0.9998 atm
Example 3: 120 mmHg → kPa (blood pressure scale)
- 120 mmHg × 133.322 ≈ 15,998.6 Pa
- 15,998.6 Pa ÷ 1000 = 15.999 kPa
Example 4: 0.5 inH₂O → Pa (HVAC duct pressure)
- 0.5 inH₂O × 249.089 ≈ 124.5 Pa
- That’s only 0.001245 bar (very low vs tire pressure)
💡 Use Cases
- Automotive & cycling: psi ↔ bar for tire inflation and compressor settings.
- Meteorology: inHg ↔ hPa/mbar when comparing U.S. and metric weather reports (see Energy).
- Medicine & physiology: mmHg conversions for context and education (see Health).
- HVAC & building: inH₂O / cmH₂O duct pressures and fan curves (see Home & DIY).
- Engineering & materials: ksi and kgf/mm² for high stress/pressure regimes; verify your spec’s definition (absolute vs gauge).
Practical tips (math/logic, not UI)
- Preserve significant figures: don’t round mid-calculation; round only at the end.
- Check scale with a baseline: compare against 1 atm or 1 bar to spot 10× or 1000× mistakes.
- Be explicit about context: “psi” is ambiguous unless you know if it’s gauge (psig) or absolute (psia).
❓ FAQ
What’s the difference between atm and at (technical atmosphere)?
atm (standard atmosphere) is defined as 101,325 Pa. at is defined as 98,066.5 Pa (1 kgf/cm²). They differ by about 3.2%.
Are mmHg and torr exactly the same?
In everyday use they’re treated as equal: 1 mmHg ≈ 1 torr ≈ 133.322 Pa. Strictly, torr is defined as 1/760 of an atm, while mmHg comes from a mercury column definition, so tiny differences can exist.
Does this converter handle gauge pressure (psig) vs absolute pressure (psia)?
No—units are converted as pure numbers. If you have gauge pressure, add local atmospheric pressure (≈1 atm) before converting to an absolute unit, or subtract it after converting back.
Why is Pascal (Pa) used as the conversion base?
Pa is the SI unit (1 Pa = 1 N/m²). Converting via Pa keeps factors consistent across metric, imperial, and legacy units.
Is bar the same as atmosphere?
Not exactly: 1 bar = 100,000 Pa, while 1 atm = 101,325 Pa. So 1 atm ≈ 1.01325 bar and 1 bar ≈ 0.9869 atm.
What’s the relationship between hPa, mbar, and kPa?
hPa and mbar are numerically identical (1 hPa = 1 mbar = 100 Pa). Also, 1 kPa = 10 hPa and 1 bar = 100 kPa.
What’s the difference between inHg and inH₂O?
inHg is inches of mercury and is much larger: 1 inHg ≈ 3386.389 Pa. inH₂O is inches of water: 1 inH₂O ≈ 249.089 Pa (varies slightly with temperature).
Why do cmH₂O and mmH₂O depend on temperature?
Water-column units are based on fluid density and standard gravity. Different reference temperatures (e.g., 4°C vs ~20°C) slightly change the Pa per mmH₂O, so datasheets may use a specific convention.
What does ksi mean and when is it used?
ksi means kip per square inch (1 ksi = 1000 psi). It’s common in structural and materials engineering for high stresses and pressures.
How large is kgf/mm² compared with bar or psi?
It’s very large: 1 kgf/mm² = 9.80665 MPa ≈ 98.0665 bar ≈ 1422.33 psi. A small input can produce huge outputs.
Why can my result look different from a rounded ‘rule of thumb’?
Rules of thumb often round early (e.g., 1 atm ≈ 1 bar). This converter uses defined factors; keep enough significant figures before rounding your final answer.
What should I use for vacuum: torr, mmHg, or Pa?
For vacuum work, Pa (or kPa) is the SI choice and avoids ambiguity. torr and mmHg are widely used in labs; treat them as ≈133.322 Pa per unit unless your spec states otherwise.
Assumptions & reference notes:
Conversions use standard reference definitions (e.g., 1 atm = 101,325 Pa; 1 bar = 100,000 Pa; 1 psi = 6,894.757 Pa; mmHg/torr use conventional values near 133.322 Pa).
Water-column units (mmH₂O/cmH₂O/inH₂O) can vary slightly by temperature/convention in some specifications.
Educational disclaimer:
This converter provides unit conversions for informational use. For safety-critical work (medical devices, pressure vessels, diving, industrial systems), verify units, reference conditions, and gauge vs absolute definitions in the relevant standards and equipment documentation.
Last review: January 2026