How to Convert Atmospheres to Bars
Converting atmospheres to bar is a frequently needed pressure conversion in science, engineering, and meteorology. The standard atmosphere (atm) is a reference pressure unit defined as the average atmospheric pressure at sea level, widely used in chemistry, physics, and diving calculations. The bar is a practical metric pressure unit used extensively in European industry, weather reporting, and compressed gas applications. One atmosphere equals exactly 1.01325 bar. This conversion is important for scientists translating literature values given in atm into bar for industrial equipment calibration, engineers converting between scientific and industrial pressure standards, meteorologists reconciling atmospheric data between the two units, and diving professionals working with equipment rated in bar who need to understand pressures expressed in atm. The near-equivalence of atm and bar (they differ by only about 1.3%) makes this a frequently encountered and sometimes carelessly approximated conversion.
Conversion Formula
To convert atmospheres to bar, multiply the pressure in atm by 1.01325. This factor derives from the exact definitions: one standard atmosphere is defined as 101,325 pascals, and one bar is defined as 100,000 pascals. Dividing 101,325 by 100,000 yields exactly 1.01325. The conversion is precise and involves no rounding, as both the atm and bar are defined in exact pascal values. This makes the atm-to-bar conversion one of the most precisely defined pressure conversions available.
bar = atm × 1.01325
5 atmospheres = 5.06625 bars
Step-by-Step Example
To convert 5 atm to bar:
1. Start with the value: 5 atm
2. Multiply by the conversion factor: 5 × 1.01325
3. Calculate: 5 × 1.01325 = 5.06625
4. Result: 5 atm = 5.06625 bar
Notice that the bar value is only slightly higher than the atm value, reflecting the close relationship between these two units.
Understanding Atmospheres and Bars
What is a Atmosphere?
The standard atmosphere traces its conceptual origins to Evangelista Torricelli's 1643 barometric experiment and Blaise Pascal's subsequent atmospheric pressure studies. The quantitative definition evolved over centuries as measurement precision improved. The modern standard atmosphere was defined as exactly 101,325 pascals, equivalent to 760 mmHg at 0 degrees Celsius under standard gravity (9.80665 m/s2). This definition was adopted by the International Union of Pure and Applied Chemistry (IUPAC) and other international bodies. The atmosphere serves as a fundamental reference in thermodynamics, where it defines standard pressure for gas law calculations, chemical equilibria, and phase diagrams. Despite the SI preference for the pascal, atm remains deeply rooted in scientific practice.
What is a Bar?
The bar was introduced in 1909 by British meteorologist Napier Shaw to provide a convenient metric pressure unit for atmospheric science. Named from the Greek "baros" (weight), the bar was defined as 10^6 dynes per square centimeter in the CGS system, translating to exactly 10^5 pascals (100,000 Pa) in the SI system. Its subunit, the millibar, rapidly became the standard for meteorological pressure charts. In the mid-to-late 20th century, industrial sectors across Europe adopted the bar as the primary pressure unit for manufacturing, automotive, and compressed gas applications. The bar gained legal recognition in EU directives as an acceptable non-SI unit for use with the International System. Today, it coexists with the pascal as one of the most widely used pressure units in the world.
Practical Applications
Chemical engineers converting reaction conditions specified in atm (common in academic literature) to bar for industrial reactor specifications use this conversion routinely. Scuba diving equipment is typically rated in bar, but dive tables and physiology references often use atm, requiring conversion for equipment setup. Meteorological data archives may store pressure in atm while current reporting systems use bar or millibar, necessitating conversion for data integration. Autoclave and pressure vessel manufacturers may receive specifications in atm from scientific clients but design equipment with bar-calibrated gauges. Gas supply companies converting cylinder pressure ratings between atm and bar serve customers across scientific and industrial sectors.
Tips and Common Mistakes
The most common mistake is treating atm and bar as identical units. While they are close (1 atm = 1.01325 bar), the 1.3% difference becomes significant at high pressures or in precision applications. At 100 atm, the difference is 1.325 bar, which is more than standard atmospheric pressure itself. Never assume 1 atm = 1 bar in engineering calculations. Another error is confusing the direction of conversion: since 1 atm is slightly more than 1 bar, converting from atm to bar should yield a slightly larger number. If your result is smaller, you have likely inverted the conversion. For quick mental math, add approximately 1.3% to the atm value to estimate bar.
Frequently Asked Questions
No, although they are very close. One standard atmosphere equals 1.01325 bar, which is about 1.3% more than 1 bar. In rough estimates, they are sometimes treated as equivalent, but for scientific and engineering work, the distinction matters. The difference arises because atm is based on atmospheric pressure (101,325 Pa) while bar is defined as a round number (100,000 Pa).