How much is 1 atm? Pressure equivalence and conversion table

1 atmosphere conversion table

The standard atmosphere is not a measured value but a definition: 1 atm = 101,325 pascals exactly. Every other equivalence follows from that.

Pressure unit converter

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The P = ρgh equation: where 10.33 m and 760 mm come from

The pressure exerted by the mass of a fluid in a column is described by a single equation:

P = ρ · g · h

• P is pressure, in pascals (Pa).
• ρ (rho) is the fluid density, in kg/m³.
• g is the acceleration of gravity: 9.80665 m/s² on Earth.
• h is the column height, in metres.

Rearranged as h = P / (ρ · g) and fed one atmosphere, it yields the two classic figures directly:

Mercury is 13.6 times denser than water, so it needs a column 13.6 times shorter to balance the same pressure. That is the entire reason barometers were built with mercury: in 1641 Gasparo Berti had to erect a water tube more than ten metres tall against a building in Rome, while Evangelista Torricelli's mercury tube, in 1644, fitted on a bench.

Feet and metres of water

A foot of water is the pressure exerted by a one-foot-tall column of water under its own weight; a metre of water works the same way. Both are widely used in hydraulics, pumping, irrigation and HVAC, because they can be read straight off a drawing: if a tank sits 80 ft above a tap, that tap sees 80 ftH₂O of static pressure, whatever the pipe diameter or the flow rate.

Two rules of thumb follow, and both are worth remembering: 1 psi ≈ 2.31 ft of water, and 10 m of water ≈ 1 bar (error under 2 %).

A less obvious consequence: since the atmosphere can only hold up a 33.9 ft column of water, no suction pump can lift water higher than 33.9 ft, and in practice less than 26 ft once pipe friction losses and cavitation margin are accounted for. It is not a question of pump power — it is the ceiling the atmosphere imposes.

mmHg, inHg and Torr: the vacuum units

Vacuum and leak-test work revolves around three units: the millibar in Europe and Latin America, inches of mercury in the United States and the United Kingdom, and the Torr or mmHg in instrumentation and laboratory specifications.

The Torr and the millimetre of mercury are not strictly the same unit: the Torr is defined as 1/760 of a standard atmosphere, while the mmHg is defined by the height of a mercury column. The difference is one part in seven million — irrelevant in any industrial application, and detectable only in high-precision metrology.

In US practice vacuum is often quoted as inches of mercury of vacuum, counted down from atmospheric: 0 inHg of vacuum is atmospheric pressure and 29.92 inHg of vacuum is a perfect vacuum at sea level. That is a gauge scale, and mixing it up with absolute inHg is one of the most common sources of confusion in test specifications.

The real force on a vacuum chamber lid

Pressure units stop being abstract the moment you multiply them by an area. One bar is, in practical terms, the force of one kilogram on one square centimetre — or about 14.5 lb on one square inch.

A 20 × 20 cm square is 400 cm². Under one bar of differential pressure it carries 400 kg. The lid of one of our chambers measures 80 × 60 cm, that is 4,800 cm²: at sea level, with a full vacuum inside, the entire outside atmosphere bears on it and the total force reaches 4.8 tonnes, roughly the weight of two cars.

That is why the acrylic on our chambers runs up to 50 mm thick. It is not a commercial margin — it is the direct consequence of P × A.

Absolute vs gauge pressure: the costliest confusion

An industrial gauge does not measure the pressure that exists; it measures the difference from the atmosphere around it. That gives two different scales:

• Absolute pressure is counted from perfect vacuum. It is never negative, because no pressure is lower than the total absence of pressure. It is the scale of barometers and of vacuum pump datasheets.
• Gauge pressure is counted from local atmospheric pressure. It goes negative under vacuum, and its zero moves with the place where the instrument is installed.

The relation is simple: absolute pressure = local atmospheric pressure + gauge pressure. At sea level, a gauge vacuum of −750 mbar corresponds to 263.25 mbar absolute.

Gauge vacuum ↔ absolute pressure (at sea level)

Vacuum is also often expressed as a percentage of the atmosphere: a vacuum "at 80 %" means 20 % of the atmosphere is left inside, that is about 20 kPa absolute.

Atmospheric pressure changes with altitude

The 1,013.25 mbar of the standard atmosphere apply at sea level. The higher you are, the less air sits above you. In Torricelli's words, "We live submerged at the bottom of an ocean of air" — and the higher you go, the shallower that ocean becomes.

The quick rule: you lose roughly 100 mbar (3 inHg) for every 1,000 metres of elevation.

On the summit of Everest (8,848 m) the model gives 314 mbar, and real measurements sit closer to 337 mbar: about one third of the pressure at sea level.

This same effect is the subject of ASTM D6653, the altitude transport simulation standard: a package sealed at sea level and shipped over a 2,400 m pass swells because the outside pressure has dropped some 240 mbar while the inside pressure has not changed.

How much vacuum can a chamber pull at altitude?

This is the question we are asked most often. A pump datasheet quotes its reach in gauge pressure at sea level, but the pump's real physical limit is an absolute pressure: that is how far down it can evacuate, wherever it is installed.

Take a unit rated −900 mbar at sea level. Its absolute limit is 1,013 − 900 = 113 mbar absolute. That number does not change with altitude; what changes is what the gauge reads.

Worked example: the same unit in Bogotá

• Atmospheric pressure at sea level: about 1,013 mbar.
• Atmospheric pressure in Bogotá (2,650 m): about 750 mbar.
• Difference: 1,013 − 750 = 263 mbar.
• Maximum gauge vacuum in Bogotá: −900 + 263 = −637 mbar (equivalently, 113 − 750 = −637).

Three requests that break physics

We receive these regularly, and clearing them up is part of the job.

"A chamber that pulls 5 pounds of vacuum"

A pound is a force, not a pressure. It is like answering "80 miles" to the question "how fast are you going?" — the area is missing, just as the hour was. The correct unit is the psi, pounds per square inch. And even properly phrased, the request has a ceiling: since 1 atm = 14.696 psi, the greatest gauge vacuum possible at sea level is −14.696 psi. Anything beyond that is physically impossible.

"A vacuum of −2 bar"

At sea level gauge vacuum cannot go past −1.013 bar, because at that point the absolute pressure is already zero. Asking for −2 bar is asking for a negative absolute pressure.

"30 inches of vacuum"

A perfect vacuum at sea level is 29.92 inHg of vacuum, so 30 inHg is already slightly beyond the physical maximum — and in Denver, where the atmosphere only supplies about 24.6 inHg, it is far out of reach whatever the pump. "30 inHg of vacuum" is a very common line in specifications and almost never achievable as written.

By contrast, "1 Torr" is coherent

A Torr is an absolute pressure: 1/760 of an atmosphere, that is 0.0013 atm or 1.33 mbar. It describes a vacuum very close to absolute vacuum, typical of laboratory equipment and processes such as metallising — not of package leak testing, which works between −250 and −950 mbar gauge.

Video: pressure and vacuum explained

This video from our channel walks through the fundamentals: what pressure is, the Berti and Torricelli experiments, the units and their conversions, the British system, the difference between absolute and gauge pressure, and worked examples of vacuum at altitude.

Why these units matter in package leak testing

DVACI designs and builds vacuum chambers for leak detection in packaging. Everything above stops being theory the moment a test procedure is written:

• The vacuum level is the test parameter. ASTM D3078 does not fix a single value: it has to be defined, in mbar or in inHg. Confusing the absolute scale with the gauge scale changes the severity of the test completely.
• Water column sets the immersion depth. In a water-bath bubble test, every 10 cm (4 in) of water above the sample adds about 10 mbar of hydrostatic pressure opposing the escape of the bubble. That is why standards specify immersion depth and not only vacuum level.
• Site elevation limits the equipment. A −950 mbar specification only means something at sea level.
• Differential pressure is what drives flow through the leak. The absolute value matters less than the step: it is the pressure difference between the inside and the outside of the package that pushes air or liquid through the defect.

To define a specific test, see our CDV vacuum chamber range or the article on vacuum regulation and test time.

Frequently asked questions