Atmospheres to Inches of Mercury Converter
Common Conversions
| atm | inHg |
|---|---|
| 0.01 | 0.299 |
| 0.1 | 2.992 |
| 0.25 | 7.48 |
| 0.5 | 14.961 |
| 1 | 29.921 |
| 2 | 59.843 |
| 5 | 149.607 |
| 10 | 299.213 |
| 25 | 748.033 |
| 50 | 1496.07 |
| 100 | 2992.13 |
| 1000 | 29921.3 |
Why this conversion matters in chemistry
Inches of mercury is the unit a US weather report uses for barometric pressure (sea-level standard is 29.92 inHg) and the one HVAC engineers reach for when sizing duct pressures. In a chemistry lab the practical use case is barometric corrections to boiling-point work. A barometer reading of 28.5 inHg, equivalent to 0.952 atm, drops water's observed boiling point to about 98.6 °C — a noticeable shift when calibrating a thermometer against the steam point. Multiplying by 29.9213 is the conversion that lets a chemistry-textbook atm value land on the inHg gauge that's actually in front of you.
Formula
Where the factor comes from
The inch of mercury has no independent standing in SI; it is assembled in three moves. Start with the conventional millimeter of mercury, which assigns mercury a density of 13595.1 kg/m³ and gravity the standard value 9.80665 m/s², making 1 mmHg = 133.322387 Pa. Multiply by the international inch, exactly 25.4 mm, and 1 inHg = 3386.3886 Pa. Divide the atmosphere by that: 101325 ÷ 3386.3886 = 29.92126 inHg. The inch is exact and standard gravity is exact, but the density figure is an assigned convention rather than a measurement of the mercury sitting in anyone's barometer. So the factor is exact in the sense that everyone agreed on the same reference fluid — not in the sense that a real column obeys it.
Precision and significant figures
Six figures, 29.9213, already exceeds what any barometer justifies, and the seventh is genuinely ambiguous: building the inch from the conventional mercury millimeter gives 29.921256, while building it from the torr gives 29.921260. Nothing at a bench distinguishes those. Temperature swamps all of it. Mercury expands roughly 1.8 parts in 10⁴ per kelvin, so a barometer sitting in a 25 °C room carries about 0.13 inHg of uncorrected error against its 0 °C reference — four orders of magnitude past the definitional ambiguity, which is why mercury barometry is always quoted with a temperature correction attached. Aneroid and digital barometers report to 0.01 inHg, and that hundredths place is the honest working resolution.
Worked Examples
Standard sea-level atmospheric pressure — the value behind every barometric correction in a calorimetry or distillation calibration.
Half an atmosphere — about ambient pressure at 5,500 m elevation, also a moderate vacuum-distillation setting.
About the pressure inside a pressurized reaction vessel during a small-scale hydrogenation charge.
A modest vacuum, the kind a sealed desiccator might hold with a freshly charged anhydrous calcium sulfate desiccant.
Common mistakes
Weather-report inHg is sea-level corrected
A published barometric pressure has usually been reduced to sea level, so it describes a fictitious column beneath your building rather than the air in your fume hood. A lab at 1500 m sits near 25.0 inHg of actual station pressure while the local report still reads close to 29.9. Boiling-point corrections need the station value.
US vacuum gauges read depth, not absolute
Many shop-style gauges are scaled 0 to 30 inHg of vacuum, counting downward from ambient rather than upward from zero. A needle at 25 on such a face means roughly 4.9 inHg absolute, about 0.16 atm — not 25 inHg absolute. Converting the dial number directly overstates the pressure by a factor of five.
Local gravity is not standard gravity
The factor assumes 9.80665 m/s². Real gravity varies with latitude and elevation by a couple of parts per thousand, which on a 29.92 inHg reading is a few hundredths of an inch. Irrelevant for a distillation, but it is why precision barometry quotes a station gravity correction alongside the temperature one.