How this instrument works
Evangelista Torricelli built the first mercury barometer in 1643, inverting a sealed glass tube over a mercury bath and watching the column settle near 760 mm — proof, though he did not yet call it that, that the atmosphere itself has weight and pushes back. That column height became the working definition of 'one atmosphere' for the next three centuries, until the 10th General Conference on Weights and Measures fixed it as an exact number in 1954: 101,325 pascals, or 101.325 kilopascals, with no reference to mercury, gravity or temperature left in the definition at all.
The kilopascal itself is a thousand pascals, and one pascal is one newton spread across one square metre — a unit named for Blaise Pascal and adopted into SI in 1971. It is far too small to write atmospheric or laboratory pressures in without a string of zeros, which is why almost every practical reading — barometric pressure, tyre pressure outside North America, soil bearing capacity, industrial process pressure — gets quoted in kilopascals rather than bare pascals.
Chemistry complicates the picture with two different 'standards.' Older textbooks and tables define STP, standard temperature and pressure, as 0 °C and exactly 1 atm — 101.325 kPa. IUPAC redefined standard pressure in 1982 to a flatter, SI-friendly 100 kPa exactly, so any gas-law calculation, molar volume or table built after that year may be quietly working from 100 kPa rather than 101.325 kPa. The two conventions differ by 1.325 kPa, small enough to slip past unnoticed until a third significant figure stops matching.
- Type your reading into Atmospheres (atm) — it opens at 1, the standard atmosphere itself.
- Read Kilopascals (kPa) beneath it, recalculated on every keystroke.
- Plugging into the ideal gas law with R = 8.314 J/(mol·K)? You need pressure in kPa or Pa, not atm — convert first.
- Only zero or positive readings are accepted, since an absolute pressure cannot run negative.
- Reversing the conversion, divide your kilopascal figure by 101.325 instead — that direction is the definition itself.
Worked example — 1 atm into an ideal-gas-law calculation
A general-chemistry student measures a gas sample at 1.00 atm on a mercury manometer and needs to run the ideal gas law with the SI gas constant, R = 8.314 J/(mol·K), which demands pressure in kilopascals rather than atmospheres. Type 1 into Atmospheres (atm) and Kilopascals (kPa) returns 101.325 exactly, since that is simply the defined value being restated.
One catch worth flagging before it costs a grade: if the assignment instead specifies IUPAC's modern standard pressure of 100 kPa exactly, rather than the older 101.325 kPa 'one atmosphere' convention, substituting the wrong constant shifts a molar-volume calculation by about 1.3 percent — enough to fail a three-significant-figure check even though every other step was correct.
Questions
Is 101.325 kPa an exact conversion, or a measured approximation?
Exact, and deliberately so. The 10th General Conference on Weights and Measures fixed the standard atmosphere at precisely 101,325 pascals in 1954, replacing the older practice of defining it through a 760 mm mercury column at standard gravity and 0 °C. Because both units are now definitions rather than measurements, multiplying atmospheres by 101.325 introduces no uncertainty of its own.
What is the difference between the old STP at 101.325 kPa and IUPAC's 100 kPa standard?
STP historically meant 0 °C and exactly 1 atm, 101.325 kPa. IUPAC redefined standard pressure in 1982 to a rounder 100 kPa exactly, chosen because it sits neatly on the SI scale rather than inheriting an old mercury-barometer convention. Textbooks, molar-volume tables (22.4 L/mol versus 22.7 L/mol) and lab software published after 1982 may assume either value, so check which convention a given source is using before comparing results.
Where did 101.325 kPa originally come from?
From a mercury column. Torricelli's 1643 barometer settled near 760 mm of mercury under typical sea-level conditions, and for centuries 'one atmosphere' simply meant that column height at 0 °C under standard gravity. The 10th CGPM converted that physical convention into a fixed number in 1954 — 101,325 Pa — so the definition no longer depends on mercury's density, local gravity or anything else that could drift between laboratories.
Why do chemists still use atmospheres when SI prefers pascals?
Mostly habit and convenient arithmetic. The gas constant is commonly tabulated both ways — 8.314 J/(mol·K), which needs pressure in pascals, and 0.08206 L·atm/(mol·K), which needs pressure in atmospheres — so a calculation set up around litres and atmospheres never has to touch kilopascals at all. Mixing the two constant forms without converting pressure consistently is one of the most common gas-law arithmetic errors in an introductory course.
How many kilopascals is a lab autoclave running at 2 atm absolute?
About 202.65 kPa. A typical benchtop autoclave sterilization cycle runs close to 121 °C at roughly 1 atm of gauge pressure above ambient — call it 2 atm absolute, 202.65 kPa — which is enough to raise water's boiling point well past 100 °C and kill heat-resistant spores that a simple boil would leave behind.
Does 1 atm mean the air pressure outside right now?
Only by coincidence, and rarely exactly. One standard atmosphere, 101.325 kPa, is a fixed reference value; actual sea-level barometric pressure wanders with weather, typically somewhere between about 98 and 104 kPa. Where a lab procedure or equation calls specifically for 'one atmosphere,' use the fixed 101.325 kPa value rather than whatever a barometer happens to read that day.
Can I enter fractional atmosphere values, like 0.5 atm for a vacuum setup?
Yes — the conversion applies identically at any positive value. A laboratory vacuum desiccator held at roughly half an atmosphere converts to 50.6625 kPa, a useful figure when a vacuum-pump gauge reads in kPa but a procedure or textbook specifies the target in atmospheres instead.