Inches of Mercury to Kilopascal Converter
Common Conversions
| inHg | kPa |
|---|---|
| 0.1 | 0.339 |
| 0.5 | 1.693 |
| 1 | 3.386 |
| 2 | 6.773 |
| 5 | 16.932 |
| 10 | 33.864 |
| 15 | 50.796 |
| 20 | 67.728 |
| 25 | 84.66 |
| 29.921 | 101.325 |
| 50 | 169.32 |
| 100 | 338.639 |
Why this conversion matters in chemistry
US barometric pressure runs in inches of mercury — sea-level standard is 29.92 inHg, equivalent to 101.325 kPa. International METAR weather data and SI-aligned chemistry calculations use kPa or hPa. The factor is 3.38639 kPa per inHg, exact through the SI definitions of both. The conversion comes up at any boundary between US barometric data and an SI-aligned calculation — a barometric correction to a boiling-point measurement, a pressure-sensor calibration against local atmospheric data, or any vapor-pressure work that crosses unit conventions.
Formula
Where the factor comes from
Of the mercury conversions this is the shortest path into SI, because the inch of mercury is itself defined as a fixed number of pascals and kilo is exactly 10³ — one division finishes the job. The pascal figure is built from conventions, not experiments. Mercury is assigned a density of 13595.1 kg/m³, standard gravity is fixed at exactly 9.80665 m/s², and their product with one millimeter gives 133.322387 Pa; the international inch of exactly 25.4 mm scales that to 3386.388640 Pa, or 3.38639 kPa. Read carefully, calling this exact means everyone agreed on the same reference fluid and the same gravity. It does not mean the mercury in a particular barometer has that density at the temperature it happens to be sitting at.
Precision and significant figures
3.38639 is six figures of a finite decimal, 3.386388640, so extending it is possible and pointless — nothing on a bench resolves a part in a million. The instrument sets the honest limit. A digital barometer reading to 0.01 inHg maps to about 0.034 kPa, so a converted result quoted to three decimals of a kilopascal claims resolution the source never had. The sixth figure also depends faintly on route: building the inch from the torr rather than from the conventional millimeter gives 3.3863882 kPa, a difference of roughly one part in seven million, well beneath anything a transmitter or barometer can distinguish.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
The factor itself — useful as a quick mental check on a barometric reading.
A low-pressure measurement — about a third of an atmosphere, the kind of reduced pressure a vacuum operation might hold.
About half an atmosphere — the working range of vacuum distillation for moderately volatile solvents.
Common mistakes
Two inches of mercury are in circulation
The 3.38639 factor describes a mercury column referenced to 0 °C. Parts of the US gas and HVAC trade use an inch of mercury referenced to 60 °F, where warmer and less dense mercury gives about 3.3769 kPa — 0.28 percent lower. On a 30 inHg figure that is nearly 0.3 kPa, and datasheets very often decline to say which reference they mean.
kPa and inHg gauges rarely share a zero
Process readings in kilopascals are frequently gauge pressure, zeroed at ambient, while a barometric inHg figure is absolute. Convert 5 inHg to 16.93 kPa and feed it to a gauge-referenced calculation and the absolute pressure is understated by a full atmosphere. Establish what each instrument counts from before the factor goes anywhere near the numbers.
Vapor-pressure tables may not be in kPa
Correcting a boiling point is the usual reason to convert a barometric inHg reading, and a great many of the tables involved are still printed in mmHg or torr. Landing the barometer in kilopascals and then subtracting a torr-valued vapor pressure mixes two scales that differ by a factor of about 7.5, in a step that looks arithmetically innocent.