Atmospheres to Pascals Converter

The atmospheres to pascals converter is a fast, exact pressure-conversion tool that moves any reading between atmospheres (atm) and pascals (Pa) in both directions, using the exact NIST SP 811 reference for the standard atmosphere (1 atm = 101 325 Pa exactly) or the CIPM 1954 reference for the technical atmosphere (1 atm_tech = 98 066.5 Pa exactly). Type a number into either field and the other updates live, with no rounding ambiguity and no measurement uncertainty, because the relationship between the two units is fixed by international definition rather than measured. It is built for the everyday situations where you are handed a pressure figure in one unit and need the other immediately: a chemistry textbook that quotes standard temperature and pressure as "1 atm" alongside the ideal gas constant R = 8.314 J/(mol·K), which is in pascal-cubic-metre units; a scuba table that lists depth in atmospheres absolute (ata) alongside a tank gauge in bar; a meteorology report that quotes sea-level pressure as 1013.25 hPa against a thermodynamic problem set that insists on pascals; an autoclave chart that prints working pressure in atmospheres against a calibration certificate that gives the same figure in pascals; an engineering drawing stamped "2.5 atm" on top of a pressure-rating note stamped "253 312 Pa" a few lines below; or a research-paper appendix that uses atmospheres throughout while the supplementary information gives everything in pascals. The page includes bonus read-outs for bar, kilopascals, psi, and mmHg / torr alongside the primary atm ⇄ Pa result, and a toggle that switches between the standard atmosphere (the modern, NIST-referenced definition) and the technical atmosphere (the older CIPM definition used in legacy European engineering and some diving tables).

The converter assumes the modern international standard atmosphere, which has been exactly 101 325 pascals since 1954. That year, the 10th General Conference on Weights and Measures (CGPM) adopted the figure originally proposed by the Comité International des Poids et Mesures (CIPM), and it has not changed since. NIST Special Publication 811 ("Guide for the Use of the SI Units"), the BIPM SI Brochure (9th edition, 2019, §2.3.3), and the IUPAC Green Book (3rd edition, 2007) all cite the same constant. The 1954 standard atmosphere is not a measurement of typical sea-level pressure, it is a definition, and today's actual atmospheric pressure at sea level happens to fall within a few tenths of a percent of it on most days. For legacy engineering work that uses the technical atmosphere (sometimes symbolised "at" rather than "atm"), the converter offers a toggle that switches the conversion factor to 98 066.5 Pa exactly without losing what you have already typed; the technical atmosphere was defined in 1954 by the CIPM as the pressure produced by one kilogram-force (the force of 1 kg under standard gravity 9.806 65 m/s²) applied to one square centimetre, and it survives in older German DIN standards, Soviet-bloc GOST standards, and some aeronautical tables.

How to use this converter

1. Type the numeric value you want to convert into the Atmospheres (atm) field or the Pascals (Pa) field. Decimal values are accepted, including sub-atmosphere values such as 0.001 atm (≈ 101.325 Pa) and sub-pascal values such as 1 Pa (≈ 9.87 × 10⁻⁶ atm).
2. For the Atm To Pa, The opposite field updates automatically as you type. For the Atm To Pa, The update is recomputed on every keystroke, so the panel always reflects the latest value.
3. Read the Result in atmospheres row to see the same pressure quoted in your chosen atmosphere definition, and the Result in pascals row to see it in SI pressure units.
4. Read the Also in bar, Also in kPa, Also in psi, and Also in mmHg (torr) rows for the four most common cross-unit read-outs at the same time. These bonus rows are always computed against the standard atmosphere reference because that is the convention used in NIST and BIPM documents.
5. Use the toggle at the top to switch between Standard (101 325 Pa) and Technical (98 066.5 Pa). The atmospheric reading re-derives from the current input under the new factor, so you can flip between definitions without retyping.
6. Use the ⇄ swap button when you want to flip the orientation: the value in the pascals field moves to the atmospheres field and vice versa.
7. Click Copy atm or Copy Pa to put the value on the clipboard in your chosen unit.
8. To find a partial pressure, vacuum level, or atmospheric reading at a specific altitude, enter the value in either field and read the result.

the Atm To Pa runs entirely in the browser. For the Atm To Pa, Inputs are not stored, not transmitted, and not associated with any account.

The Formulas

The conversion between atmospheres and pascals is defined rather than measured, so there is a single exact factor for each atmosphere definition:

Standard atmosphere (NIST SP 811, BIPM SI Brochure 9th ed.):

1 atm = 101 325 Pa exactly

To convert atm → Pa: multiply by 101 325.

Pa = atm × 101 325

To convert Pa → atm: divide by 101 325.

atm = Pa / 101 325

Technical atmosphere (CIPM 1954, 1 kgf/cm²):

1 atm_tech = 98 066.5 Pa exactly

To convert atm_tech → Pa: multiply by 98 066.5.

Pa = atm_tech × 98 066.5

To convert Pa → atm_tech: divide by 98 066.5.

atm_tech = Pa / 98 066.5

Between the two atmosphere definitions:

1 atm = 101 325 / 98 066.5 = 1.0332274 atm_tech (the standard atm is about 3.32% larger than the technical atm)

1 atm_tech = 98 066.5 / 101 325 = 0.9678411 atm (the technical atm is about 3.22% smaller than the standard atm)

Bonus read-outs (always quoted against the standard atmosphere):

1 atm = 1.01325 bar (1 bar = 100 000 Pa) 1 atm = 101.325 kPa (1 kPa = 1 000 Pa) 1 atm = 14.69594878... psi (1 psi = 6 894.757293... Pa, NIST SP 811) 1 atm = 760 mmHg (torr) (the textbook definition of the torr)

To go from atmospheres to any of these units, multiply the atm value by the factor above. To go from any of these units to atmospheres, divide by the factor above.

Worked examples

Example 1, Standard temperature and pressure in chemistry. A chemistry textbook asks for the molar volume of an ideal gas at STP (standard temperature and pressure). The IUPAC since-1982 definition of STP is 0 °C and exactly 1 atm. Using the ideal-gas constant R = 8.31446 J/(mol·K) and P = 1 atm = 101 325 Pa:

V = nRT / P = (1 mol × 8.31446 J/(mol·K) × 273.15 K) / 101 325 Pa ≈ 22.414 L / mol

The 22.414 L/mol figure is the canonical molar volume of an ideal gas at STP. If you had started from the same problem but worked in atm-litre units with R = 0.082057 L·atm/(mol·K), the answer would have been identical: V = (1 × 0.082057 × 273.15) / 1 = 22.414 L. The 101 325 Pa/atm conversion is what bridges the two forms of the ideal-gas constant.

Example 2, Scuba diving at 30 metres of seawater. A recreational scuba diver at 30 m of seawater experiences approximately 4 atm absolute (1 atm of air + 3 atm of hydrostatic pressure from the water column). Reading the calculator with the standard atmosphere:

P = 4 atm = 4 × 101 325 Pa = 405 300 Pa P = 4 atm = 405.300 kPa = 4.0530 bar = 58.7838 psi = 3 040 mmHg

The bar reading (4.0530) is what a typical dive gauge displays. If an older dive table instead quotes the depth in "ata" using the technical atmosphere, then 4 ata would be 4 × 98 066.5 = 392 266 Pa, a 3.3% difference that does not matter for a 30 m dive but matters at the precision required to plan decompression stops.

Example 3, Boiler pressure rating in engineering. A boiler's maximum working pressure is stamped "10 atm" on the European nameplate and "1 013 250 Pa" on the US specification sheet.

10 atm = 10 × 101 325 = 1 013 250 Pa ✓

The two numbers are identical because "1 atm = 101 325 Pa" is an exact definition. If the European nameplate instead used technical atmospheres (the older DIN convention), then 10 atm_tech would equal 980 665 Pa (about 3.3% smaller), exactly the kind of cross-unit error the toggle is designed to prevent.

Example 4, Vacuum and rough pressure. A laboratory roughing pump pulls a chamber down to 0.001 atm:

P = 0.001 atm × 101 325 = 101.325 Pa exactly = 0.76 mmHg (torr)

This is the canonical "1 millitorr" target for an oil-sealed rotary roughing pump; the calculator's bonus mmHg row makes the cross-check immediate.

Example 5, Round-trip identity. Any pressure expressed in pascals and converted to atm and back, with no intermediate rounding, returns the original number exactly:

P = 50 000 Pa → atm = 50 000 / 101 325 ≈ 0.49346 atm → Pa ≈ 50 000 Pa

The defining property of an exact conversion: there is no measurement uncertainty in either direction, only the precision of the floating-point representation on screen.

Where atm ⇄ Pa shows up

Chemistry. IUPAC standard temperature and pressure (STP) since 1982 is defined as 0 °C and 1 atm, giving a molar volume of 22.414 L/mol. The ideal-gas constant R = 8.31446 J/(mol·K) is in Pa-m³ units; the same constant can be written R = 0.082057 L·atm/(mol·K). The 1 atm = 101 325 Pa factor bridges the two forms.

Meteorology. Surface pressure is reported in hectopascals (hPa, identical to millibars); standard sea-level pressure is 1013.25 hPa = 101 325 Pa = 1 atm.

Scuba and hyperbaric. Ambient pressure at depth is quoted in atmospheres absolute (ata), 10 m of seawater ≈ 2 ata, 30 m ≈ 4 ata. Divers converting to bar (their gauge unit) or to pascals use the 101 325 Pa/atm factor.

Engineering. Pressure-rating certificates mix units across decades and borders. A "2.5 atm" European autoclave and a "253 312 Pa" US spec sheet are the same physical pressure. A "10 atm" boiler rated to the technical atmosphere is 980 665 Pa, not 1 013 250 Pa, the 3.3% difference is what the toggle is designed to prevent.

Vacuum science. 1 torr = 1/760 atm exactly, so 1 Pa = 7.5006 millitorr. The calculator's mmHg row gives the cross-check immediately.

Aviation. Altimeter readings use feet, but the underlying pressure measurement is in pascals or inches of mercury. 101 325 Pa is the reference against which pressure altitude is computed.

Common mistakes when converting atm and Pa

Mistake 1: Using the standard atmosphere conversion when the source text means the technical atmosphere. A 1954 CIPM technical atmosphere is 98 066.5 Pa, about 3.32% smaller than the standard 101 325 Pa. If you validate a "1 atm" reading from a 1960s DIN or GOST table against 101 325 Pa you will be out by 3.32%, enough to fail a 5% engineering tolerance. Use the toggle.

Mistake 2: Treating "1 atm ≈ 1 bar" as exact. Standard atmosphere pressure is 101 325 Pa, but 1 bar is exactly 100 000 Pa. The difference (1.325%) shows up in any NIST-traceable calibration: a 1 atm barometer reads about 1.01325 bar, not exactly 1 bar. The calculator's bar row makes the difference explicit.

Mistake 3: Confusing "at" with "atm". The symbol "at" (lowercase, no final m) is the deprecated symbol for the technical atmosphere. The symbol "atm" is the standard atmosphere. Many legacy European tables use "at" exclusively; treating it as a synonym for "atm" introduces the 3.32% error from Mistake 1.

Mistake 4: Using "atm" as a unit of force. Pressure is force per unit area. "Atmospheres" is a unit of pressure, not a unit of force. A force of "1 atm" makes no sense; what you usually want is a pressure of "1 atm" applied over an area to compute force (F = P × A).

Mistake 5: Forgetting to convert SI prefixes. The pascal is the SI unit, but engineering data often quotes kilopascals (kPa), megapascals (MPa), or hectopascals (hPa). Converting "1013.25 hPa" to pascals requires multiplying by 100 (since 1 hPa = 100 Pa), and only then dividing by 101 325 to get atm. A common error is to skip the 100 and get "1 atm ≈ 1013 hPa", which is off by a factor of 100.

Mistake 6: Reading the meter at the wrong moment. A pressure that "decays from 1 atm to 0.5 atm" over a vacuum pump cycle is a halving of pressure, not of concentration. The corresponding pascal reading is 101 325 Pa → 50 662.5 Pa, exactly half. The calculator shows this directly.

Mistake 7: Round-trip rounding error. Converting 100 000 Pa to atm gives 0.986923... atm, and converting back gives 100 000 Pa exactly because 101 325 is exact. But rounding the intermediate atm figure to "0.987" before converting back gives 99 998.78 Pa, a 1.22 Pa error that grew from a three-decimal rounding choice. Use the full stored value at every step.

Frequently Asked Questions

What is the exact relationship between atmospheres and pascals? The international standard atmosphere is defined as exactly 101 325 pascals. NIST Special Publication 811 and the BIPM SI Brochure (9th edition, §2.3.3) both cite this exact figure. Therefore 1 atm = 101 325 Pa is a definition rather than a measurement, and the reciprocal 1 Pa = 1/101 325 atm ≈ 9.869 232 67 × 10⁻⁶ atm is also exact.

What is the technical atmosphere, and how is it different? The technical atmosphere (sometimes symbolised "at") is an older pressure unit defined by the CIPM in 1954 as exactly 98 066.5 Pa, the pressure produced by one kilogram-force per square centimetre. It is about 3.32% smaller than the standard atmosphere and survives primarily in legacy European engineering tables, older DIN standards, and some aeronautical and diving references. The toggle on this page switches the conversion between the two definitions.

Why are there two different definitions of "atmosphere"? Because two different but equally defensible pressure references were in circulation in 1954. The standard atmosphere (101 325 Pa) was defined to match typical mean sea-level air pressure and became the convention in physics, chemistry, and meteorology. The technical atmosphere (98 066.5 Pa) was defined to match the kilogram-force per square centimetre already in use in European mechanical-engineering practice. Both were codified in the same year. Modern usage defaults to the standard atmosphere; the technical atmosphere survives in legacy documents.

Where does the figure 101 325 Pa come from? It is not measured, it is defined. The Comité International des Poids et Mesures proposed the figure in 1954, the 10th General Conference on Weights and Measures (CGPM) adopted it, and it has been the international standard atmosphere ever since. The figure is close to the average air pressure at sea level in temperate latitudes, but it is a definition rather than an empirical average; today's actual sea-level pressure fluctuates around 101 325 Pa by up to a few percent on extreme weather days.

Can I use this converter for vacuum measurements? Yes. The pascal is the SI unit of pressure, and it covers the full range from ultra-high vacuum (10⁻⁹ Pa or below) to gigapascal geology. Below about 1 Pa the torr is the more practical unit, and 1 torr = 1/760 atm exactly, so 1 Pa ≈ 7.5006 millitorr. The calculator's mmHg (torr) read-out gives the same figure at standard resolution.

Should I round the result to a sensible number of decimal places? It depends on the application. For a chemistry textbook problem, four or five significant figures is appropriate. For NIST-traceable calibration, use the full stored value at every step, even a three-decimal round-trip accumulates a pascal of rounding error. For scuba and hyperbaric applications, two decimal places in bar is usually enough. The calculator shows eight decimal places in atm and the full integer figure in Pa, so you can pick the precision your work needs.

Is 1 atm the same as 1 atm absolute (ata)? For surface conditions, yes. Scuba and hyperbaric work distinguishes atm absolute (ata) from atm gauge (atg), gauge pressure reads zero at the surface (so 1 atg is 1 atm above atmospheric, or 2 ata total), while absolute pressure reads 1 atm at the surface. The converter here is a pure unit-conversion tool and works on whichever reference you supply; subtract atmospheric pressure from a gauge reading before converting if you want the absolute figure.

When did the standard atmosphere change from 1 atm = 101 325 Pa? It has not changed. The exact definition was set in 1954 and has remained constant. The technical atmosphere was also defined in 1954 and is unchanged. The only modern equivalent is the adoption of the bar in some industrial standards (1 bar = 100 000 Pa exactly), but the standard atmosphere has not been redefined.

How do I convert between atm and bar? 1 atm = exactly 1.01325 bar. Therefore to convert atm to bar, multiply by 1.01325; to convert bar to atm, divide by 1.01325 (or equivalently, multiply by 0.986923...). The two units differ by about 1.3%, and a 1 atm reading is 1.01325 bar, not exactly 1 bar.

How do I convert atm to psi (pounds per square inch)? 1 atm = 14.69594878... psi. This is the conversion used in the US for tyre pressure (atmosphere vs. gauge), engineering pressure ratings, and older aviation documents. The calculator's bonus psi row shows this conversion at six-decimal precision.

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