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Convert Bar to Atmospheres – Pressure Converter

This tool converts a pressure value expressed in bar to the equivalent value in standard atmospheres (atm). The conversion is a fixed mathematical relationship based on SI definitions: 1 bar = 100000 pascals and 1 standard atmosphere = 101325 pascals.

Use this converter for engineering checks, documentation, unit reconciliation, and quick design estimations. For safety-critical decisions, use calibrated instrumentation and follow your organisation's measurement management procedures and relevant standards.

Be aware of gauge versus absolute pressure. This converter works with absolute values unless you subtract local reference (for gauge) first. See the FAQs and glossary for guidance on calibration, uncertainty, and regulatory considerations.

Updated Nov 4, 2025

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Methodology

The conversion is derived from the SI base unit pascal (Pa). Define 1 bar = 100000 Pa exactly and 1 standard atmosphere (atm) = 101325 Pa exactly by international convention.

To convert bar → atm, express the bar value in pascals and divide by the pascal value of 1 atm. This yields a precise, fixed factor with no dependence on temperature or material properties.

Accuracy considerations: the arithmetic conversion is exact given the unit definitions, but measurement uncertainty comes from the instrument used to obtain the input value. Follow measurement traceability and calibration best practices as described by NIST and ISO.

Worked examples

Example 1: 1.000 bar → 1.000 × 0.9869232667 = 0.986923 atm (rounded to 6 decimal places).

Example 2: 2.500 bar → 2.500 × 0.9869232667 = 2.467308 atm (rounded to 6 decimal places).

Example 3: 0.100 bar → 0.100 × 0.9869232667 = 0.098692 atm (rounded to 6 decimal places).

Key takeaways

Conversion between bar and atm is a fixed ratio derived from pascal definitions: multiply bar by 0.9869232667160062 to get atm.

Use full precision for calculations and round only for presentation. For regulatory, safety, or legal reporting, follow your organisation's specified rounding and uncertainty rules and rely on calibrated measurement equipment.

Further resources

Expert Q&A

What is the exact conversion factor from bar to atmospheres?

Using SI definitions: 1 bar = 100000 Pa and 1 atm = 101325 Pa, so 1 bar = 100000 / 101325 atm ≈ 0.9869232667160062 atm.

Does this converter assume gauge or absolute pressure?

The mathematical conversion is for absolute pressure units. If your reading is gauge pressure (relative to ambient), convert to absolute pressure by adding local atmospheric pressure before converting units.

How many significant figures should I report?

Report significant figures based on measurement uncertainty and the precision of your instrument. For typical lab and field work, 3 to 6 significant digits are common. Retain full precision in intermediate calculations and round only for final reporting.

Are there regulatory or calibration requirements I should follow?

Yes. Follow national and international guidance for measurement traceability and calibration, for example NIST recommendations and ISO measurement management standards. For safety-critical pressure equipment, follow applicable OSHA regulations and equipment-specific standards.

Will temperature or fluid type affect this unit conversion?

No. The bar-to-atm conversion is purely a unit conversion based on pascal definitions and does not depend on temperature or fluid. However, measured pressure values can be affected by temperature, head, or fluid properties; those measurement effects must be handled separately.

What about converting to other pressure units like psi or kPa?

Convert bar to pascals first (1 bar = 100000 Pa), then convert pascals to the target unit (for example, 1 psi ≈ 6894.757 Pa, 1 kPa = 1000 Pa). Many unit converters provide direct bar→psi or bar→kPa factors.

How should I handle uncertainty when converting units?

Propagate uncertainty from the measured value through the conversion factor. Because the conversion factor is exact from unit definitions, the relative uncertainty is the same as the input measurement's relative uncertainty. Document traceability to calibration standards.

Sources & citations