Atmosphere Pascal Converter

1 atm equals 101,325 Pa

One standard atmosphere (1 atm) is exactly 101,325 pascals. In 1954 the 10th General Conference on Weights and Measures defined standard atmospheric pressure as 101,325 N/m², and since 1 N/m² is 1 Pa, nothing is rounded. In other units that is 1,013.25 hPa, 101.325 kPa and 1.01325 bar. Going the other way, 1 Pa is 1 ÷ 101,325 ≈ 0.00000986923 atm, or 9.86923×10⁻⁶ atm in scientific notation.

The 12 pressure units in the converter above are linked, so typing a number into any box updates the other eleven. Values that come out exact by definition are shown as they are; rounded values are marked with '≈'. The cross-section chart on the right shows where the pressure you entered occurs: how many meters above sea level in a standard atmosphere, or how many meters deep in the sea.

When you need to convert pressure units

Each field has its own habitual pressure unit, so the same pressure shows up as different numbers in different sources. Converting is what makes comparison and calculation possible in situations like these.

• Chemistry and physics classes: textbooks move between 1 atm, 760 mmHg, 760 Torr and 101.325 kPa, and the ideal gas law PV = nRT needs a pressure unit that matches the gas constant R. With R = 0.08206 L·atm/(mol·K) the pressure goes in atm; with 8.314 J/(mol·K) it goes in Pa and the volume in m³.
• Weather and flying: U.S. weather reports and aircraft altimeter settings commonly give pressure in inches of mercury, while maps and meteorology elsewhere use hPa or mbar. 1,013.25 hPa and 29.92 inHg are nearly the same pressure.
• Tires and shop tools: tire placards and gauges in the U.S. are usually in psi, while many car manuals also list kPa or bar. If an air compressor reads in a different unit than the placard, convert before filling.
• Vacuum work and labs: vacuum levels are often written in Torr or Pa, with tiny values such as 1×10⁻⁶ Torr. In the converter you can type 1e-6 or 1×10⁻⁶.
• Blood pressure and medical equipment: blood pressure is measured in mmHg. A systolic 120 mmHg is about 16.0 kPa. Ventilators and similar devices sometimes use cmH₂O.
• Older gauges and specs: older European or imported equipment may be marked in kgf/cm² or bar. When you carry such a figure into a spec sheet, converting to psi or kPa avoids mistakes.

How to use the converter

1. Type your number into the box for the unit you have. The box you typed in gets a darker border and every other box is recalculated from it. Typing in another box makes that box the new reference.
2. The result strip at the top shows the value converted to pascals (Pa) with the formula. If you typed in the Pa box, it shows standard atmospheres (atm) instead.
3. Choose 4, 6 or 8 'significant digits'. The whole-number part is never cut off, and boxes rounded to your chosen digits are marked '≈'.
4. Values below 0.0001 or at 1 billion and above appear in scientific notation, such as 1.33322×10⁻⁴. You can enter 1.5e-6, 1.5×10^-6 or 1.5×10⁻⁶.
5. If a number leaves out atmospheric pressure, as tire pressure does, choose 'Gauge' above the chart. Negative values down to −1 atm (a perfect vacuum) are then accepted, and the chart draws the absolute pressure with atmospheric pressure added.
6. 'Copy result' copies all 12 values on one line. 'Copy link' copies a URL that holds the current value, unit, basis and digits.

How it works: every unit goes through pascals

Every pressure unit is defined as a fixed number of pascals. To convert a value from unit A to unit B, multiply by (Pa in one A) and divide by (Pa in one B). For example, 35 psi in kPa is 35 × 6,894.757… ÷ 1,000 ≈ 241.317 kPa. The converter keeps each factor as an unrounded fraction and rounds only once, at the end.

For quick comparison, here is what one of each common unit equals in the others. Read across: one unit of the row, expressed in the unit of the column.

How NIST treats bar, mmHg, atm and Torr

The U.S. National Institute of Standards and Technology (NIST) publishes the Guide for the Use of the International System of Units (SP 811). Chapter 5 sorts units outside the SI into groups, and several of the units in this converter appear in it. The pascal is the SI unit of pressure, and kPa, MPa and hPa are pascals with SI prefixes.

• bar and mmHg: Table 9 of the chapter lists them among non-SI units accepted for use with the SI, with 1 bar = 10⁵ Pa and 1 mmHg ≈ 133.322 Pa. The guide says they must not be introduced in fields where they are not already used, and it strongly discourages their continued use by NIST authors except when absolutely necessary.
• atm and Torr: Table 11 lists the standard atmosphere and the Torr among 'other unacceptable units', and the guide says such units must strictly be avoided.
• Presentation: when a NIST author does use a non-SI unit, the SI value comes first and the non-SI unit follows in parentheses, for example 101.325 kPa (1 atm).

This is NIST's guidance for its own publications and for SI usage, not a statute, and it does not forbid psi, inHg or mmHg in everyday U.S. practice such as tires, weather reports or blood pressure. It is a good reason, though, to put the kPa value first when you write a technical document.

Pressure and altitude: how fast it drops as you climb

The sky half of the chart follows the 1976 U.S. Standard Atmosphere. It starts at 15 °C and 101,325 Pa at sea level, lets the temperature fall 6.5 °C per kilometer up to 11 km, and computes pressure from the weight of the air above. In this model the pressure is half of the sea-level value at about 5.5 km.

Airliners fly higher than this, but the cabin is pressurized. The U.S. Federal Aviation Regulations, 14 CFR 25.841(a), require the cabin pressure altitude in normal operation to be no more than 8,000 ft (2,438 m), which in a standard atmosphere is about 753 hPa. The chart draws that line as the airliner cabin limit. Denali's height of 20,310 ft (6,190 m) is the figure from the USGS announcement of its new official height.

Real pressure departs from this table with the weather. According to the World Meteorological Organization (WMO) records, the lowest sea-level pressure is 870 hPa (about 0.859 atm) measured in the eye of Typhoon Tip on 12 October 1979. The highest is 1,083.8 hPa at Agata, Russia, on 31 December 1968 for stations below 750 m, and 1,089.1 hPa at Tosontsengel, Mongolia, on 30 December 2004 for stations at 750 m or above. 870 hPa is the standard-atmosphere pressure at about 1,270 m, but the weather alone pushed the sea-level reading that low.

Pressure underwater: about 1 atm for every 10 m of seawater

Underwater, the weight of the water above is added to the atmospheric pressure at the surface. At h meters in water of density ρ, absolute pressure is 101,325 Pa + ρ × 9.80665 × h. The chart assumes seawater at 1,025 kg/m³, so pressure rises 1 atm about every 10.08 m. In fresh water (1,000 kg/m³) it is about every 10.33 m, which is where 1 atm ≈ 10.3323 mH₂O comes from.

Absolute pressure and gauge pressure

Absolute pressure is measured from a perfect vacuum, and gauge pressure is measured from the local atmospheric pressure. Most pressure gauges and all tire pressures are gauge readings, so a flat tire reads 0 psi even though the air inside is at atmospheric pressure. The two are related by absolute = gauge + atmospheric.

• A 35 psi tire (gauge) is about 241.317 kPa, or 241.317 + 101.325 ≈ 342.6 kPa absolute, about 49.7 psi. Choose 'Gauge' in the converter and enter 35 psi, and the chart places that absolute pressure at about 24.0 m underwater.
• A vacuum pump gauge reading −80 kPa (gauge) is 101.325 − 80 = 21.325 kPa absolute. Gauge pressure cannot go below −101.325 kPa, a perfect vacuum.
• In English-language specs, psia means absolute and psig means gauge. The conversion factors are the same for both, so the converter keeps the basis label on the converted values.

Matching pressure units in the ideal gas law

In PV = nRT, the gas constant R is exactly 8.314462618 J/(mol·K), and 1 J = 1 Pa·m³ = 1 kPa·L. If you use another pressure unit, use R expressed in that unit.

Common mistakes

• Treating bar and atm as equal: 1 atm is 1.01325 bar. The 1.3% gap is fine for rough work but changes the answer on an exam or in a design calculation.
• Moving kgf/cm² straight to bar: 1 kgf/cm² is 0.980665 bar, so 10 kgf/cm² is 9.80665 bar, about 0.2 bar off.
• Mixing gauge and absolute: gas-law and boiling-point calculations need absolute pressure. Forgetting to add atmospheric pressure to a gauge reading is off by 1 atm.
• Mixing up hPa and kPa: 1 kPa is 10 hPa. Writing 1,013 hPa as 1,013 kPa makes it ten times too big.
• Which inHg: some U.S. data give inHg as mercury at 32 °F (3,386.38 Pa) rather than the conventional value (3,386.389 Pa). The difference is about 0.0003%, which usually does not matter.
• Assuming mmHg and Torr are identical by definition: they differ by about 0.000014%. You can treat them as equal in practice, but 760 mmHg is not exactly 1 atm.

Limits and cautions

• Unit conversion uses exact defined values, but the screen rounds to the digits you choose. Rounding does not make a measurement more precise, so keep the digits of the original reading in mind.
• Altitudes on the chart come from the standard atmosphere model. Real pressure changes by a few hPa within a day with the weather, and by tens of hPa in a typhoon or strong high.
• Depth assumes seawater at 1,025 kg/m³. Salinity, temperature and fresh water can shift it by a few percent, so do not use it for dive planning.
• When converting gauge to absolute, atmospheric pressure is taken as the standard 1 atm. At altitude or in real weather it can differ.
• This converter only changes units. For the safe working pressure of a vessel, pipe or tire, check the manufacturer's rating and the applicable requirements.

Basis: the unit factors are the definitions in the CGPM resolution and NIST SP 811 Appendix B.8, so they do not change. Altitude uses the 1976 U.S. Standard Atmosphere and weather extremes use the WMO records table (July 2025). NIST's treatment of bar, mmHg, atm and Torr comes from the current NIST SP 811 chapter 5 page. All of these were checked on 20 October 2026.

Sources

• 01BIPM — Resolution 4 of the 10th CGPM (1954)1 standard atmosphere = 101 325 newtons per square metre
• 02NIST SP 811 — Appendix B.8 Factors for units listed alphabeticallyConversion factors for atm, bar, mbar, mmHg and inHg (conventional), inHg (32 °F), torr, kgf/cm², psi, cmH₂O (conventional)
• 03NIST SP 811 — Chapter 5, Units outside the SITable 9: bar and mmHg accepted with the SI; Table 11: atm and Torr listed as unacceptable units
• 04NASA NTRS — U.S. Standard Atmosphere, 1976NOAA-S/T 76-1562, NASA-TM-X-74335. Pressure-altitude model
• 05eCFR — 14 CFR 25.841 Pressurized cabinsCabin pressure altitude of not more than 8,000 feet in normal operation
• 06WMO — Weather and Climate Extremes Archive, records table (July 2025)Lowest sea-level pressure 870 hPa (Typhoon Tip, 1979-10-12); highest 1,083.8 hPa (Agata, below 750 m) and 1,089.1 hPa (Tosontsengel, 750 m or above)
• 07NIST CODATA — molar gas constant R8.314 462 618… J mol⁻¹ K⁻¹ (exact)
• 08NASA GES DISC — Glossary: DensitySeawater of density 1,025 kg/m³ has sigma-t = 25 (the density assumed on the chart)
• 09USGS — New elevation for the nation's highest peakDenali, new official height 20,310 feet
• 10Ministry of Foreign Affairs of China — Nepal country profileMount Everest (Qomolangma) height 8,848.86 m

Frequently asked questions

Related tools