| Atmospheres (atm) | Newtons per square meter (N/m²) |
|---|---|
| 1 Atmosphere | 101325 N/m² |
| 2 Atmospheres | 202650 N/m² |
| 3 Atmospheres | 303975 N/m² |
| 4 Atmospheres | 405300 N/m² |
| 5 Atmospheres | 506625 N/m² |
| 10 Atmospheres | 1013250 N/m² |
| 20 Atmospheres | 2026500 N/m² |
| 25 Atmospheres | 2533125 N/m² |
| 50 Atmospheres | 5066250 N/m² |
| 100 Atmospheres | 10132500 N/m² |
| Reference | Atmospheres (atm) | Newtons per square meter (N/m²) |
|---|---|---|
| Atmospheric pressure at sea level | 1 atm | 101325 N/m² |
| Healthy blood pressure (120 mmHg) | 0.157908 atm | 16000 N/m² |
| A car tyre | 2.17123 atm | 220000 N/m² |
| A racing bicycle tyre | 5.92154 atm | 600000 N/m² |
The atmosphere is a unit of pressure defined as exactly 101,325 pascals. Its symbol is atm. It is not a measurement of the air at any particular place or moment but a fixed reference value, chosen in 1954 by the General Conference on Weights and Measures to sit close to the average pressure at sea level in temperate latitudes.
Its usefulness is that it turns pressure into a ratio. Saying that a vessel holds gas at six atmospheres says immediately that the pressure inside is six times what is outside, which is the fact that determines whether a container will burst, how much gas it holds and how fast it will empty. No other pressure unit carries that meaning in the number itself.
Chemistry built its early framework on it. Gas laws were written with pressures in atmospheres, the gas constant had a value tailored to litres and atmospheres, and standard conditions for tabulating thermodynamic data were defined at one atmosphere. In 1982 the International Union of Pure and Applied Chemistry changed the standard state to exactly one bar, so modern tables use bar and older ones use atmospheres, and the difference of 1.3 per cent matters in careful work.
Diving and hyperbaric medicine still count in it. Pressure underwater is described in atmospheres absolute, a diver at thirty metres experiencing about four, and hyperbaric oxygen therapy is delivered at two to three atmospheres absolute in a sealed chamber. Because gas volume and gas uptake by the body both scale with absolute pressure, the unit is doing real physiological work rather than merely reporting a reading.
The atmosphere also fixes the older mercury scales. One atmosphere is exactly 760 millimetres of mercury by definition, which is what ties the torr and the millimetre of mercury to the metric system, and it is 29.92 inches of mercury, the setting an American pilot dials into an altimeter as standard.
A related unit, the technical atmosphere of one kilogram-force per square centimetre, is close but not equal at 98,066.5 pascals. Documents from Japan, Korea and eastern Europe sometimes use it, and mistaking one for the other introduces an error of about three per cent.
One atmosphere equals 101,325 pascals, 1.01325 bar, 760 millimetres of mercury, or about 14.696 pounds per square inch.
The newton per square metre is a unit of pressure equal to one pascal. Its symbol is N/m². The two are not merely equivalent but identical: the pascal is the name given to this combination, and before the General Conference on Weights and Measures adopted that name in 1971 the SI unit of pressure had no name at all and was written out in full.
Both forms survive because they do different work on the page. The pascal is compact and reads as a unit in its own right, which suits a measurement. The newton per square metre shows its dimensions, which suits a calculation, because it makes visible that multiplying by an area in square metres will give a force in newtons.
Structural engineering leans on the second property constantly. Floor loads are specified in kilonewtons per square metre, with about 1.5 for a dwelling, 3 for an office and 5 for a place of assembly, and multiplying that figure by the floor area gives directly the load in kilonewtons that the beams must carry. Written as kilopascals the same numbers would be correct but would hide the step.
Snow and wind follow the same convention. Snow load is given in kilonewtons per square metre, from a few tenths in a mild climate to several in the mountains, and wind pressure on a facade likewise. Because those loads are combined with dead weight, which is naturally a force, keeping everything in newtons avoids the need to convert anything.
The construction repeats one prefix down. A newton per square millimetre is exactly one megapascal, which is why material strengths appear on drawings as N/mm² as often as MPa. The pattern is worth recognising: whenever a document writes force over area rather than naming a pressure unit, it is because the writer expects the reader to multiply.
Nothing else distinguishes the two forms. Any value in newtons per square metre can be written as pascals without change, and any conversion table treats them as one entry. The choice is a matter of what the number is about to be used for.
One newton per square metre equals 1 pascal, 0.01 millibars, 0.00001 bar, or about 0.000145 pounds per square inch.