Kilopascals to mmHg Converter
Common Conversions
| kPa | mmHg |
|---|---|
| 0.133 | 1 |
| 1 | 7.501 |
| 5 | 37.503 |
| 10 | 75.006 |
| 13.332 | 100 |
| 25 | 187.52 |
| 50 | 375.03 |
| 100 | 750.06 |
| 101.325 | 760 |
| 200 | 1500.12 |
| 500 | 3750.31 |
Why this conversion matters in chemistry
Digital pressure gauges mostly read in kPa these days, but the vapor-pressure literature that feeds rotovap and freeze-drying protocols is full of mmHg values. 2.7 kPa on a Schlenk-line gauge is about 20 mmHg; 3.17 kPa is 23.8 mmHg — the vapor pressure of water at 25°C. Multiplying by 7.50062 does the conversion. The factor drops directly out of 101.325 kPa being 760 mmHg, so you can reconstruct it if you forget. The step comes up whenever a modern instrument reading has to match an older purification recipe or a tabulated vapor-pressure value.
Formula
Where the factor comes from
The standard atmosphere never appears in the statement of this conversion, yet it supplies the entire number. A kilopascal is exactly 1000 Pa. A millimeter of mercury, as chemistry uses the symbol, is the torr — one seven-hundred-and-sixtieth of an atmosphere that has itself been fixed at exactly 101325 Pa since 1954. The factor is therefore 1000 ÷ (101325/760) = 7.500616827… mmHg per kilopascal, exact in the sense that no measurement produced it and endless in the sense that the decimal never closes. Read the symbol strictly instead — as a column of mercury of assigned density standing under standard gravity — and the same conversion gives 7.5006158. The two disagree in the seventh figure, which is why chemistry treats mmHg and torr as one unit and moves on.
Precision and significant figures
Six figures is generous here and eight is theatre. 7.50062 reproduces the exact ratio to better than a part in two million, and the working shortcut of 7.5 costs 0.008 percent — on a full barometric reading of 760 mmHg that is six hundredths of a millimeter. Significant figures should come from the gauge rather than from the factor. A Schlenk-line transducer displaying 2.7 kPa carries two of them, so 20 mmHg is the honest answer and 20.3 is arithmetic wearing a lab coat. Because mmHg is the numerically larger unit, converted values grow digits, and that inflation is easy to let stand unchallenged in a notebook.
Worked Examples
Sea-level atmospheric pressure — the defining anchor for both units.
1 bar — IUPAC standard pressure since 1982. Slightly below 1 atm, which is where the 760 versus 750 mmHg discrepancy between old and new conventions originates.
Water vapor pressure at 25°C. The correction applied when collecting a gas over water to get the dry-gas partial pressure.
A clean reference point. 100 mmHg is also the upper clinical threshold for diastolic blood pressure, so the number appears in several non-chemistry contexts too.
Common mistakes
Copied data carries no unit with it
Online thermophysical data sources let you switch a pressure column between kPa, bar, mmHg and atm, and the figure you copy out keeps no memory of which was selected. A vapor pressure pasted into a sheet as 3.17 and later multiplied by 7.50062 is only right if the source was set to kilopascals. Put the unit in the cell beside the value, not in the column heading.
Rule-of-thumb vacuum formulas hide their unit
Convenient expressions for mean free path, molecular-flow conductance and pumpdown time are published with a pressure unit baked into the leading coefficient, and that unit is usually torr or millibar rather than kilopascals. Feed a converted kilopascal figure into one without checking which unit the coefficient assumed and the answer scales by whatever separates the two, arriving with units that still look correct.
Ultimate vacuum specs assume a clean dry system
Pump datasheets quote an ultimate pressure in mmHg or mbar, measured on a blanked-off, dry, room-temperature port. A Schlenk line carrying solvent vapor, stopcock grease and a meter of tubing sits well above that. A manifold gauge reading 0.05 kPa converts to 0.375 mmHg, and finding it hundreds of times the pump's rating is ordinary rather than evidence of a fault.