Kilopascal to Inches of Mercury Converter
Common Conversions
| kPa | inHg |
|---|---|
| 1 | 0.2953 |
| 5 | 1.4765 |
| 10 | 2.953 |
| 25 | 7.383 |
| 50 | 14.765 |
| 100 | 29.53 |
| 101.325 | 29.921 |
| 200 | 59.06 |
| 500 | 147.65 |
| 1000 | 295.3 |
| 5000 | 1476.5 |
| 10000 | 2953 |
Why this conversion matters in chemistry
Modern process equipment carries kPa pressure ratings; older US-built field instruments and US meteorological data display in inches of mercury. A 101 kPa relief-valve setting is 29.83 inHg on a legacy Bourdon gauge. The constant of 0.2953 inHg per kPa falls cleanly out of 29.92 inHg per atmosphere divided by 101.325 kPa per atmosphere. In practice you reach for it when an SI-spec process datasheet meets a US-spec gauge or a barometric report calibrated against a mercury column.
Formula
Where the factor comes from
Everything in this direction hangs on a reciprocal. The forward constant, 3386.388640 Pa per inch of mercury, is a finite decimal — the product of three agreed values and nothing measured, those being mercury's assigned density of 13595.1 kg/m³, standard gravity of exactly 9.80665 m/s², and the international inch of exactly 25.4 mm. Turn that constant upside down and the finiteness evaporates: 1000 ÷ 3386.38864 = 0.29529983301… with no closing digit anywhere. The 0.2953 printed on wall charts is therefore a truncation, whereas the 3.38639 quoted going the other way is a rounding of a number that genuinely stops. At four figures the distinction changes no answer, but it is why the two directions get quoted to different precisions in different places.
Precision and significant figures
0.2953 is good to about six parts in ten million, far past what any inHg instrument delivers, so the chart value is adequate for every practical purpose. Do not casually shorten it to 0.295, which costs 0.1 percent, and certainly not to 0.3, which is 1.6 percent off and turns 101.325 kPa into 30.4 inHg instead of 29.92. The figures in your answer belong to the source reading, so 100 kPa off a two-figure gauge does not become 29.530 inHg. Legacy Bourdon dials graduated in inches of mercury resolve perhaps half an inch, and even a decent aneroid stops at 0.01.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
The factor itself — a useful quick reference when scanning a kPa value.
Exactly 1 bar — the IUPAC reference pressure for tabulated thermodynamic data, expressed in inHg.
About half an atmosphere — the kind of reduced pressure a vacuum operation might hold.
Common mistakes
Three tenths is not close enough
0.3 inHg per kPa is tempting for mental arithmetic and 1.6 percent high. On a barometric figure it turns 29.92 into 30.40, which reads as a genuinely different weather day, and on a 100 kPa setpoint it moves the target by nearly half an inch. Keep 0.2953 even when the answer is only an estimate.
Inches of water are not inches of mercury
Low differential pressures in ventilation and filtration work are quoted in inches of water, and both units get abbreviated to a few characters on the same datasheet. One inch of water is about 0.249 kPa against 3.386 for mercury — a ratio near 13.6, which is simply the density ratio. Converting a kPa figure into the wrong column fluid is a thirteenfold error.
100 kPa is 29.53 inHg, not 29.92
The two anchors sit close together and get swapped constantly: 101.325 kPa is the standard atmosphere at 29.921 inHg, while a round 100 kPa is one bar at 29.530 inHg. Reaching for 29.92 whenever a number looks vaguely atmospheric introduces a 1.3 percent error, and it lands in the same direction every single time.