How to use
- Type the pressure you have.
- Choose its unit: Pa, kPa, bar, psi, atm or mmHg.
- The other five appear underneath, each with a copy button. Nothing is shown until the box has a number in it.
💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
Every unit here is a fixed multiple of the pascal, so a conversion is one multiplication and one division. What is worth knowing is which of those multiples are definitions and which are measurements rounded to seven figures — because that decides how many digits of your answer mean anything.
The factors, and where they come from
| Unit | In pascals | Exact? | Source |
|---|---|---|---|
| bar | 10⁵ | definition | NIST SP 811 B.8 · JIS Z 8000-4 4-15.a |
| atm | 101 325 | definition | NIST SP 811 B.8 · JIS Z 8000-4 4-15.C.a |
| psi | 6 894.757 | 7 figures | NIST SP 811 B.8 |
| mmHg | 133.3224 | 7 figures | NIST SP 811 B.8 · JIS Z 8000-4 4-15.C.e |
Torr and mmHg share this row
Torr and mmHg are defined differently — 1 Torr is exactly atm/760, while a millimetre of mercury is a column of mercury under standard gravity — so it is true that they are not the same unit. It is also true that no table can tell them apart: they differ by about 1.4 parts in ten million, and JIS Z 8000-4 4-15.C.f writes the relation as 1 Torr = (1/760) atm ≈ 1 mmHg ≈ 133.322 4 Pa, giving both the same seven-figure value. The line below divides by the defining value of the millimetre of mercury — 13 595.1 kg/m³ × 9.806 65 m/s² × 1 mm = 133.322 387 415 Pa — while the table above carries NIST's rounding of it to seven figures, 133.3224 Pa, which is the number this tool actually computes with. The two agree to all seven figures; which one you divide by first shows in the eighth.
- One atmosphere in millimetres of mercury
- 1 atm = 101 325 ÷ 133.322 387 415 = 759.999 89 mmHg
- What it would be in Torr
- 1 atm = 760 Torr, exactly, by definition
⭐ So a value in Torr goes in the mmHg box, and the answer in that row is your answer in Torr. That is why the selector names both instead of offering two rows: two rows printing the same number would teach a reader that the units are interchangeable, which is the opposite of what is true — one row that names both, and this paragraph, is the honest version. Type 1 atm in and the tool answers 760 mmHg, incidentally: it shows six significant figures and the discrepancy lives in the seventh, which is exactly how far down it is.
Practical notes
- bar and atm are exact. 1 bar is 10⁵ Pa and 1 atm is 101 325 Pa, both by definition — NIST SP 811 Appendix B.8 and JIS Z 8000-4:2014 4-15 agree.
- psi and mmHg are not. NIST publishes 6 894.757 Pa and 133.3224 Pa, seven figures each, and this tool uses exactly those. Quoting a psi result to ten digits is quoting the arithmetic, not the pressure.
- 1 bar is not 1 atm. They are 1.3 % apart, which is enough to matter in a pressure-driven filtration or a gas line and small enough to slip past a glance.
- This converts absolute pressures. A gauge reads the difference from the surrounding air, so a gauge value converts by the same factors but means something else — 1 bar on a gauge is about 2 bar absolute.
- Negative input is refused rather than converted, because the sign belongs to the reading and not to the unit. Convert the magnitude and carry the sign yourself.
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