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Convert Atmospheres to Bar – Pressure Converter

This tool converts a pressure value expressed in atmospheres (atm) to bar using the internationally recognised fixed relationship between these SI-related units. It is intended for engineers, lab technicians, and anyone needing a reliable unit conversion for documentation or calculations.

Use the converter for single-value conversions where a simple, exact mathematical relationship applies. For instrument readings, process control, or safety-critical applications, read the methodology and accuracy notes below before relying on converted values.

Updated Nov 19, 2025

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Methodology

The conversion uses the internationally accepted definitions of the units: 1 standard atmosphere is defined as exactly 101325 pascals (Pa) and 1 bar is defined as exactly 100000 pascals. Converting between atm and bar therefore reduces to a fixed ratio derived from these definitions.

To support good engineering practice, the page explains measurement limits and uncertainty considerations. Where values come from instruments, ensure sensors are calibrated to traceable standards (NIST or equivalent), and include measurement uncertainty when using converted values in specifications, testing, or compliance reporting.

Worked examples

Example 1: 1 atm → 1 × 1.01325 = 1.01325 bar

Example 2: 0.5 atm → 0.5 × 1.01325 = 0.506625 bar

Example 3: 2 atm → 2 × 1.01325 = 2.0265 bar

Further resources

Expert Q&A

Is the conversion exact?

The mathematical conversion using the defined unit values (1 atm = 101325 Pa, 1 bar = 100000 Pa) is exact to the number of digits implied by those definitions: 1 atm = 1.01325 bar. However, when converting real instrument readings, include measurement uncertainty and calibration status; the numeric multiplication itself is an exact arithmetic operation.

When should I worry about precision or rounding?

Round to a number of significant digits appropriate for your application. For high-precision scientific or metrology work, maintain full precision through calculations and round only for reporting. For routine engineering tasks, 4–6 significant figures are usually sufficient. Always document rounding in formal reports.

Do I need to calibrate my pressure sensor before converting?

Yes. Converting a measured value does not remove sensor bias or drift. For traceable results, calibrate sensors regularly against a standard traceable to an accredited laboratory (for example, NIST-traceable standards) and record the calibration date and uncertainty.

Are there safety or regulatory considerations?

Yes. When converted values are used for safety limits, pressure vessel ratings, or compliance, follow applicable standards and regulations (OSHA, ISO, local codes). Use certified equipment and include uncertainty margins required by the relevant standard.

Can I convert bar to atm with this tool?

This configuration converts from atmospheres to bar. For the reverse conversion, use the complementary converter (bar to atm) or invert the factor: value_in_atm = value_in_bar ÷ 1.01325.

Sources & citations