Atmospheres to Pascals
Quick answer: 1 atm = 101,325 Pa. To convert, multiply the atm value by 101,325; to go back, divide.
The standard atmosphere is defined as exactly 101,325 pascals (101.325 kPa, 1.01325 bar, 14.696 psi), so converting is a single multiplication and the result is exact. Atmospheres are the everyday unit of diving, pressure cookers and chemistry; pascals are the SI unit used in engineering and weather.
The formula
Pa = atm × 101,325 · atm = Pa ÷ 101,325
Common values
| atm | Pa | also |
|---|---|---|
| 0.5 atm | 50,662.5 Pa | 50.663 kPa · 0.5066 bar · 7.35 psi |
| 1 atm | 101,325 Pa | 101.325 kPa · 1.0132 bar · 14.7 psi |
| 1.5 atm | 151,987.5 Pa | 151.988 kPa · 1.5199 bar · 22.04 psi |
| 2 atm | 202,650 Pa | 202.65 kPa · 2.0265 bar · 29.39 psi |
| 2.5 atm | 253,312.5 Pa | 253.313 kPa · 2.5331 bar · 36.74 psi |
| 3 atm | 303,975 Pa | 303.975 kPa · 3.0397 bar · 44.09 psi |
| 4 atm | 405,300 Pa | 405.3 kPa · 4.053 bar · 58.78 psi |
| 5 atm | 506,625 Pa | 506.625 kPa · 5.0663 bar · 73.48 psi |
| 10 atm | 1,013,250 Pa | 1,013.25 kPa · 10.1325 bar · 146.96 psi |
2.5 atm to Pa: 253,312.5 Pa
2.5 atm = 253,312.5 Pa — 253.313 kPa · 2.5331 bar · 36.74 psi. Working: 2.5 × 101,325 = 253,312.5.
2.5 atmospheres is 253,312.5 Pa — the pressure at about 15 m (50 ft) of sea-water depth (1 atm of air plus 1.5 atm of water), a typical hyperbaric-chamber treatment pressure, close to a car tyre (2.2–2.5 bar), and more than a domestic pressure cooker (about 1.7–2 atm absolute).
The other way: Pa to atm
| Pa | atm |
|---|---|
| 1 Pa | 0.00001 atm |
| 2 Pa | 0.00002 atm |
| 5 Pa | 0.000049 atm |
| 10 Pa | 0.000099 atm |
| 12 Pa | 0.000118 atm |
| 15 Pa | 0.000148 atm |
| 20 Pa | 0.000197 atm |
| 25 Pa | 0.000247 atm |
| 50 Pa | 0.000493 atm |
| 100 Pa | 0.000987 atm |
Common mistake
Mixing absolute and gauge pressure: a tyre gauge reading 2.5 bar is 2.5 bar above atmospheric — about 3.5 atm absolute.
Where the number comes from
101,325 is not a rounded measurement — it is a definition. The standard atmosphere was fixed by the 10th General Conference on Weights and Measures in 1954 as exactly 101,325 pascals, chosen to approximate mean sea-level air pressure.
The pascal itself is the SI unit of pressure: one newton of force spread over one square meter. It is a very small unit for everyday pressure, which is why atmospheric readings are almost always quoted in hectopascals (hPa) or kilopascals (kPa) rather than raw pascals.
Because 1 atm is defined rather than measured, the conversion is exact to as many digits as you need.
The same pressure in every common unit
Pressure suffers from more competing units than almost any other quantity, and you will meet several of them for the same physical thing.
| Unit | 1 atm equals | Where you meet it |
|---|---|---|
| Pascal (Pa) | 101,325 Pa | SI base unit; scientific work |
| Hectopascal (hPa) | 1,013.25 hPa | Weather forecasts and aviation |
| Kilopascal (kPa) | 101.325 kPa | Engineering; tyre pressure in some countries |
| Bar | 1.01325 bar | Diving, industry, European tyre pressure |
| Millibar (mbar) | 1,013.25 mbar | Older weather reporting |
| psi | 14.696 psi | US and UK tyre pressure, plumbing |
| Torr / mmHg | 760 Torr | Vacuum work, blood pressure |
Two useful things fall out of that table. A bar is almost exactly one atmosphere — within 1.3% — which is why the two get used loosely as if they were the same. And a hectopascal is identical to a millibar, so a forecast reading "1013 hPa" and an older one reading "1013 mbar" are quoting the same number, not converting between anything.
Gauge pressure versus absolute pressure
This distinction causes more real errors than the arithmetic ever will, and it is the reason a converted number can be right and still wrong.
Most everyday gauges — tyre gauges, dive computers, pressure vessels — read gauge pressure, which is pressure above the surrounding atmosphere. A tyre showing 32 psi is 32 psi above the air around it, so its absolute pressure is about 46.7 psi.
- Absolute pressure is measured from a perfect vacuum. Zero means no pressure at all.
- Gauge pressure is measured from local atmospheric pressure. Zero means "same as outside".
- The relationship:
absolute = gauge + atmospheric.
Converting units never changes which of the two you have. If a figure came from a tyre gauge, it stays gauge pressure in pascals, bar or psi alike. Scientific and thermodynamic work almost always wants absolute; anything you read off a workshop dial is almost always gauge.
Common values
| atm | Pa | kPa | bar | psi |
|---|---|---|---|---|
| 0.5 atm | 50,662.5 | 50.66 | 0.507 | 7.35 |
| 1 atm | 101,325 | 101.33 | 1.013 | 14.70 |
| 1.5 atm | 151,987.5 | 151.99 | 1.520 | 22.04 |
| 2 atm | 202,650 | 202.65 | 2.027 | 29.39 |
| 2.5 atm | 253,312.5 | 253.31 | 2.533 | 36.74 |
| 3 atm | 303,975 | 303.98 | 3.040 | 44.09 |
| 5 atm | 506,625 | 506.63 | 5.066 | 73.48 |
| 10 atm | 1,013,250 | 1,013.25 | 10.133 | 146.96 |
Getting a feel for the scale
- 1 atm — sea level. The air above you presses down at about 10 tonnes per square meter; you do not notice because the same pressure acts inside you.
- 0.8 atm — a commercial aircraft cabin, pressurised to roughly 2,400 m of altitude.
- 0.5 atm — around 5,500 m up, near Everest base camp.
- 2 atm — 10 m underwater. Every additional 10 m of seawater adds about one atmosphere, which is the basis of dive tables.
- 2.5 atm — roughly 15 m down, a common recreational dive depth.
- 3 atm — a typical car tyre, measured as absolute pressure (about 2 bar gauge).
The diving rule is the most useful one to carry: pressure roughly doubles at 10 m and triples at 20 m, which is why gas consumption rises so steeply with depth.
Estimating in your head
- atm to kPa: multiply by 100. 2.5 atm → 250 kPa, against a true 253.3 — within 1.3%.
- atm to bar: they are nearly identical. Treat 1 atm as 1 bar unless precision matters.
- atm to psi: multiply by 15 and take a little off. 2.5 atm → 37.5, true value 36.74.
Both units belong to the same family: our pressure conversion guide covers bar, psi and the rest with their exact factors, and psi to bar handles the pairing most often needed alongside atmospheres.
Frequently asked questions
Is the conversion exact?
Yes — the factor follows from the legal definition of the units (1 atm = 101,325 Pa), so the only rounding is in the last decimal place shown.
How do I convert a different value?
Use the atm to Pa converter for any number, or the pressure conversion guide for every pressure unit and formula in one place.
Is 1 atm the same as 1 bar?
Almost, but not exactly. 1 atm is 101,325 Pa and 1 bar is 100,000 Pa, so an atmosphere is about 1.3% higher. They are often treated as interchangeable in casual use.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to the surrounding air, absolute pressure from a vacuum. A tyre gauge reading 32 psi means 32 psi above atmospheric, so about 46.7 psi absolute. Converting units does not change which one you have.
Why is atmospheric pressure given in hPa on weather forecasts?
Because a hectopascal is numerically identical to the older millibar, so 1013 hPa and 1013 mbar are the same reading. It let forecasters adopt SI units without changing the numbers people were used to.
Comments