Bar to Atm Conversion Calculator
Convert pressure between bar and standard atmospheres using exact pascal definitions.
Example workbook is ready.
Pressure input
Converted pressure
Nearby conversion table
| Multiplier | Pressure (bar) | Pressure (atm) |
|---|
How to use the bar to atm conversion calculator
What this calculator does
This calculator converts pressure between bar and the standard atmosphere, written atm. It uses the exact definitions 1 bar = 100,000 pascals and 1 atm = 101,325 pascals, so the conversion is an exact unit identity rather than an estimate. It is useful for translating equipment ratings, laboratory conditions, process specifications, tire or vessel documentation, and engineering notes that use different pressure units. It does not determine whether a pressure is safe for a particular vessel, regulator, tire, or process; always compare the converted value with the applicable manufacturer limits and engineering standards.
When to use it
Use the converter when a gauge or datasheet reports bar but a calculation, textbook, or procedure expects atm; when checking whether two pressure specifications describe the same condition; when converting laboratory reaction or gas-law inputs; or when preparing a consistent set of pressure values for a report or spreadsheet. The NIST Guide for the Use of the International System of Units provides authoritative background on SI units and conversion practice.
How to calculate
- The calculator opens with a ready-to-use example of 12 bar, and the Excel workbook is available immediately.
- Replace the value in Pressure value with your pressure. Use digits and an optional period for decimals; commas, scientific notation, unit text, negative values, and values above 1,000,000 are rejected.
- Choose bar or atm in the input unit selector. Changing the unit converts the current value so the physical pressure remains the same.
- Choose Displayed decimals to control on-page rounding. This does not reduce the precision stored in the model or exported workbook.
- Read the main converted result, the pascal and kilopascal equivalents, the formula line, and the nearby conversion table.
- Select Download Excel to create a current-state OOXML workbook. Select Reset to clear the demonstration and calculated data; export remains disabled until a complete valid value is entered again.
Input guide
Pressure value is required. Enter a plain finite decimal from 0 through 1,000,000, such as 12 or 2.75. A higher value produces proportionally higher results because pressure-unit conversion is linear. Zero is valid and converts to zero. A common mistake is pasting “12 bar” into the field; enter only 12 and choose bar separately. Input pressure unit is required and accepts bar or atm. Switching it converts the number already entered, rather than merely relabeling it. Displayed decimals is required and offers 2, 3, 4, or 6 decimal places. It changes only visible rounding, so two displayed values can look equal even when their underlying values differ slightly.
Output guide
Converted pressure is the exact equivalent expressed in the other unit, displayed with the selected number of decimals. Pascals and Kilopascals show the shared SI basis used to perform the conversion. The Formula summary exposes the factor applied to the current value. The summary pills show the fixed identities 1 bar = 0.986923 atm and 1 atm = 1.01325 bar. The Nearby conversion table lists one-half, one, one-and-one-half, and twice the current pressure, allowing quick comparison without entering multiple values. All outputs are conversions, not recommendations.
Worked example
The startup example uses 12 bar. First convert bar to pascals: 12 × 100,000 = 1,200,000 Pa. Then divide by the exact atmosphere definition: 1,200,000 ÷ 101,325 = 11.843079... atm. With three displayed decimals, the calculator shows 11.843 atm. The same state shows 1,200,000 Pa and 1,200 kPa, and the workbook stores the unrounded converted value. The BIPM SI Brochure explains the international measurement system that underlies pascal-based pressure units.
Why bar and atm are close but not equal
Both units represent pressures near ordinary atmospheric pressure, which is why their numerical values are close. A bar is defined as exactly 100 kilopascals. A standard atmosphere is exactly 101.325 kilopascals. Therefore one atmosphere is 1.01325 bar, while one bar is approximately 0.986923 atmosphere. Treating them as interchangeable can introduce an error of about 1.3%, which may matter in laboratory work, gas calculations, calibration, and pressure-rated systems.
atm = bar × 100,000 ÷ 101,325
bar = atm × 101,325 ÷ 100,000
Practical interpretation and common mistakes
Pressure values may be absolute or gauge values. Unit conversion does not change that reference convention: 2 bar absolute converted to atm remains an absolute pressure, while 2 bar gauge converted to atm remains a gauge pressure. Do not mix absolute and gauge readings without applying the appropriate atmospheric offset. Also verify whether a specification uses “atm” for the standard atmosphere or uses the word atmosphere informally. For calibration and formal reporting, retain enough significant digits for the measurement uncertainty rather than copying every calculator digit. NIST's SI guidance for pressure measurements offers additional context for pressure units and traceable reporting.