Inches of Mercury to Atmospheres Converter
Common Conversions
| inHg | atm |
|---|---|
| 0.1 | 0.003342 |
| 1 | 0.033421 |
| 5 | 0.167106 |
| 10 | 0.334211 |
| 15 | 0.501316 |
| 20 | 0.668421 |
| 25 | 0.835527 |
| 29.921 | 1 |
| 30 | 1.003 |
| 50 | 1.671 |
| 100 | 3.342 |
| 1000 | 33.421 |
Why this conversion matters in chemistry
US weather reports quote barometric pressure in inches of mercury — sea-level standard is 29.92 inHg, equivalent to 1 atm. Chemistry calculations want atm. The factor between them is 0.033421 atm per inHg, which falls out of 29.9213 inHg per atm. Local barometric readings are what feed into a boiling-point correction or a gas-volume calculation at actual conditions. A 30.15 inHg reading on a high-pressure morning is 1.008 atm, which is to use in correcting an experimental boiling point against the literature value at exactly 1 atm.
Formula
Where the factor comes from
Almost nobody actually multiplies by 0.033421; they divide by 29.9213 instead, and the reason shows in how the number is put together. The standard atmosphere sits at exactly 101325 Pa, fixed by international agreement rather than measured. The inch of mercury is not pinned directly but inherits a chain of conventions: mercury assigned a density of 13595.1 kg/m³, gravity set at exactly 9.80665 m/s², and the international inch defined as exactly 25.4 mm, which together give 3386.388640 Pa. The factor is the second divided by the first, 3386.38864 ÷ 101325 = 0.0334210574. Every input was agreed rather than determined in a laboratory, so nothing uncertain propagates — but the quotient runs on without closing, which is precisely why the reciprocal is the value people commit to memory.
Precision and significant figures
Let the barometer decide the digits. A digital unit displaying 30.15 inHg offers four significant figures, supporting 1.008 atm and not one place beyond, whatever the 1.0076449 on the calculator implies. Five figures on the factor already exceeds what any barometer earns and six is decoration. The physical corrections dwarf the arithmetic regardless: a mercury column stands taller when warm, by roughly 0.005 inHg per kelvin above its 0 °C reference, so an uncorrected instrument in a 25 °C room reads about 0.13 inHg high. That is 0.004 atm — comfortably larger than the trailing figure anyone was arguing over.
Worked Examples
Standard atmospheric pressure expressed in both unit conventions — the calibration anchor.
A typical sea-level barometric reading, slightly above standard pressure.
The factor itself — useful as a quick mental check on a barometer reading.
About half an atmosphere — the kind of pressure a moderate vacuum-distillation operation might hold.
Common mistakes
Rounding 29.92 up to a flat 30
Thirty inches is a comfortable number and 0.26 percent wrong. On a gas-volume or vapor-pressure calculation that sits below most experimental scatter and stays invisible, but it is a bias rather than noise and it points the same way every time. Divide by 29.9213, not by 30, even when the answer only needs three figures.
Barometric atm is not standard-state p°
Converting a barometer reading yields a measured pressure — 1.008 atm on a high-pressure morning. The p° in an equilibrium expression or a tabulated ΔG° is a defined reference of exactly 1 atm, or 1 bar in post-1982 compilations. Substituting the day's barometric value for that reference makes a dimensionless K drift with the weather.
Temperature corrections belong to real columns
A mercury barometer stands taller in a warm room and needs reducing to its 0 °C reference before anything else happens. A digital sensor reporting inHg has already computed a pressure through the conventional column definition, so applying the same correction again subtracts roughly a tenth of an inch that was never in the reading.