How this instrument works
Torricelli's barometer set this factor in motion. Mercury standing 760 mm tall at 0 °C, under standard gravity of 9.80665 m/s², presses down with 13595.1 kg/m³ × 9.80665 × 0.76 = 101325.0144 Pa. In 1954 the CGPM trimmed that trailing 0.0144 away and fixed one standard atmosphere at 101325 pascals exactly — a definition rather than a measurement, so nothing here carries uncertainty. Factorise 101325 and you get 3 × 5² × 7 × 193, which is why atmosphere figures so rarely land on tidy pascal values.
One pascal is one newton spread across one square metre, handed Blaise Pascal's name at the 1971 CGPM. It is startlingly small: one sheet of 80 gsm office paper lying flat presses on your desk with roughly 0.78 Pa. Since ordinary weather swings span about 96000 to 105000 Pa, meteorologists switched to hectopascals, where 1 hPa equals one old millibar exactly. Sea-level standard pressure therefore reads 1013.25 hPa — precisely what pilots dial into an altimeter above transition altitude, shown as 29.92 inHg on American instruments.
Atmospheres survive wherever a quantity is naturally sized against Earth's own air. Chemists still quote molar volume as 22.414 L/mol at 0 °C and 1 atm, though IUPAC shifted standard state to 1 bar in 1982, moving that figure to 22.711 L/mol. Divers and hyperbaric clinicians count in atmospheres absolute, prescribing oxygen at 2.0 to 2.8 ata. Vacuum engineers reach for torr, defined as precisely 1/760 atm, which is why residual-gas readouts speak of 10⁻⁶ rather than millipascals. Venus, for scale, sits near 91 atm at ground level.
- Type your reading into Atmospheres (atm); 1 atm is preloaded as a starting point.
- Read Pascals (Pa) directly beneath — it recomputes as you type, to six decimal places.
- Want kilopascals? Divide that result by 1000. For hectopascals or millibars, divide by 100 instead.
- Working backwards from a pascal figure, divide it by 101325 and enter that quotient as atmospheres.
- Check absolute versus gauge before entering: a gauge reading of zero corresponds to 1 atm absolute.
Worked example — bench-checking an absolute transducer
A technician verifies an absolute pressure transducer against a deadweight tester holding one standard atmosphere. Entering 1 in Atmospheres (atm) returns 101325.0 in Pascals (Pa), with nothing hiding beyond those decimals, because a factor of 101325 terminates immediately.
Suppose that transducer reports 101340 Pa. Its error is 15 Pa, near 0.015 %, comfortably inside a typical 0.05 % full-scale specification for a 0–2 atm sensor. Had our technician assumed one atmosphere meant a round 100000 Pa, apparent error would have ballooned to 1325 Pa and a perfectly sound instrument would have gone back to its maker.
Questions
Is 1 atm exactly 101325 Pa, or is that rounded?
Exactly. One standard atmosphere is defined as 101325 pascals, so this conversion introduces no error whatsoever — every digit comes from multiplication, never from measurement. Rounding happened once, in 1954, when the CGPM trimmed 101325.0144 Pa (760 mm of mercury under standard gravity, at 0 °C) down to a flat 101325 and fixed it there by agreement.
How does an atmosphere differ from a bar?
One bar is exactly 100000 Pa, so 1 atm equals 1.01325 bar — about 1.3 % higher. Both approximate sea-level air pressure, which is why they get muddled so often. Since 1982 IUPAC has specified 1 bar, not 1 atm, as standard state for thermodynamic tables, so older enthalpy and entropy values referenced to atmospheres differ slightly from modern published data.
Is one atmosphere the same as 760 mmHg?
It equals 760 torr exactly, because a torr is defined as 1/760 atm, or 20265/152 Pa. A conventional millimetre of mercury is a separate unit fixed at 133.322387415 Pa, making 760 mmHg equal 101325.0144 Pa — larger by roughly one part in seven million. No clinical or industrial gauge can resolve that gap, but metrology labs keep both symbols distinct on purpose.
Does this converter give absolute or gauge pressure?
Absolute — both quantities are reckoned upward from a hard vacuum. Tyre gauges, manifold gauges and most process instruments report gauge pressure instead, treating local air as zero. To convert a gauge figure, add one atmosphere before entering it: a tyre showing 2.2 bar gauge is actually holding about 3.17 atm absolute.
What is a technical atmosphere, and is it this same unit?
No, and confusing them is a classic trap in older European and Soviet engineering documents. A technical atmosphere, symbol at, is one kilogram-force per square centimetre, or 98066.5 Pa — roughly 3.2 % below a standard atmosphere. Where a legacy datasheet lists pressure in kgf/cm², multiply by 98066.5, not by 101325.
Why do weather forecasts use hectopascals rather than pascals?
Scale and continuity. Sea-level pressure hovers near 101325 Pa, an awkward five-digit quantity, while hectopascals shrink it to 1013.25 and match millibar readings on decades of older charts one-for-one. A deep North Atlantic low at 950 hPa works out to 0.94 atm; Typhoon Tip bottomed out near 870 hPa in 1979, roughly 0.86 atm.