The standard atmosphere is an international reference pressure defined as 101,325 Pa and formerly used as unit of pressure (symbol: atm).^{[1]} For practical purposes it has been replaced by the bar which is 100,000 Pa.^{[1]} The difference of about 1% is not significant for many applications, and is within the error range of common pressure gauges.
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In 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1,013,250 dynes per square centimeter (101,325 Pa).^{[2]} This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France, and as a practical matter, truly reflects the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris).
In chemistry, the original definition of “Standard Temperature and Pressure” (STP) was a reference temperature of 0 °C (273.15 K) and pressure of 101.325 kPa (1 atm). However, in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “the standard pressure” should be defined as precisely 100 kPa (exactly 1 bar).^{[3]}
pascal (Pa) 
bar (bar) 
technical atmosphere (at) 
atmosphere (atm) 
torr (Torr) 
poundforce
per square inch (psi) 


1 Pa  ≡ 1 N/m^{2}  10^{−5}  1.0197×10^{−5}  9.8692×10^{−6}  7.5006×10^{−3}  145.04×10^{−6} 
1 bar  100,000  ≡ 10^{6} dyn/cm^{2}  1.0197  0.98692  750.06  14.5037744 
1 at  98,066.5  0.980665  ≡ 1 kgf/cm^{2}  0.96784  735.56  14.223 
1 atm  101,325  1.01325  1.0332  ≡ 1 atm  760  14.696 
1 torr  133.322  1.3332×10^{−3}  1.3595×10^{−3}  1.3158×10^{−3}  ≡ 1 Torr; ≈ 1 mmHg  19.337×10^{−3} 
1 psi  6.894×10^{3}  68.948×10^{−3}  70.307×10^{−3}  68.046×10^{−3}  51.715  ≡ 1 lbf/in^{2} 
Example reading: 1 Pa = 1 N/m^{2} = 10^{−5} bar = 10.197×10^{−6} at = 9.8692×10^{−6} atm, etc.
A pressure of 1 atm can also be stated as:
In the United Kingdom, scuba divers and others often use the word atmosphere loosely (the correct term is "ambient pressure") to mean 1 bar.
The old European unit technical atmosphere (at) is roughly equal to the gauge pressure under 10 m of water; 1 at = 98066.5 Pa.
