Inches of Mercury to mmHg Converter
Common Conversions
| inHg | mmHg |
|---|---|
| 0.1 | 2.54 |
| 0.5 | 12.7 |
| 1 | 25.4 |
| 2 | 50.8 |
| 5 | 127 |
| 10 | 254 |
| 15 | 381 |
| 20 | 508 |
| 25 | 635 |
| 29.921 | 760 |
| 50 | 1270 |
| 100 | 2540 |
Why this conversion matters in chemistry
The conversion is exact: both units measure the same physical mercury column, just in different length units. Since 1 inch is defined as exactly 25.4 mm, 29.921 inHg of standard atmospheric pressure becomes 760 mmHg without any rounding. The factor comes up most when a US barometer reading has to land on a vapor-pressure table written in mmHg, or when a boiling-point correction needs the local barometric pressure expressed in chemistry units. Both ends of the conversion describe the same physics — a column of mercury at standard gravity — so there's no fudge in the factor.
Formula
Where the factor comes from
Both units describe the same column of mercury and differ only in the ruler laid against it, which is why it is the cleanest of the mercury conversions, provided both figures refer to a column at the same reference temperature. The international yard and pound agreement of 1959 fixed the inch at exactly 25.4 millimetres, and that single relation does all the work. Whatever convention you hold about mercury — the assigned density of 13595.1 kg/m³ with standard gravity behind the conventional millimeter, or the torr's definition as one seven-hundred-and-sixtieth of the standard atmosphere — it appears identically on both sides and cancels without trace. The seventh-figure ambiguity that dogs every other mercury conversion never arises here. The factor is exact, and the standard atmosphere at 29.9213 inHg lands on 760.00 mmHg.
Precision and significant figures
Multiplying by a definition neither adds nor removes significant figures: whatever the barometer gave you comes through intact. A reading of 29.92 inHg supports 760.0 mmHg and stops there, so writing 759.968 because the calculator offered it manufactures three digits from nothing. The real precision question sits upstream, in the column. Mercury expands by roughly 1.8 parts in 10⁴ per kelvin, so a manometer at 20 °C rather than its 0 °C reference stands about 2.8 mmHg high on a 760 mm column — thousands of times larger than any rounding in the factor, and the reason serious mercury barometry always travels with a temperature correction.
Worked Examples
Standard atmospheric pressure — the calibration reference in both unit conventions.
The defining identity — exactly 25.4 mm to an inch, locked by the 1959 international yard and pound agreement.
A reduced barometric reading you might see on a high-altitude weather report or in a partially evacuated apparatus.
A low-pressure measurement, the kind shown on a vacuum gauge during a moderate-vacuum operation.
Common mistakes
Mercury's meniscus is read at the crown
Mercury does not wet glass, so its surface bulges upward and the height is taken at the top of the dome. Water and oil manometers curve the other way and are read at the bottom of the trough. Carrying the wrong habit across shifts a reading by a millimeter or two before the exact 25.4 factor ever touches it.
Millimetres of oil are not millimetres of mercury
Low-pressure manometers are often filled with a light oil precisely because it gives a taller and more readable column. A fluid near 1.0 g/cm³ produces roughly one-thirteenth the pressure per millimeter that mercury does. A height in millimetres taken off such an instrument is simply not mmHg, and converting it as though it were overstates the pressure by more than an order of magnitude.
An exact factor cannot repair a biased reading
Because 25.4 is definitional, the converted number feels authoritative. It carries forward everything already wrong with the inHg figure — an uncorrected column temperature, a weather value reduced to sea level, a dial zeroed at ambient rather than at vacuum. The conversion is exact; the quantity being converted usually is not, and only the second one is yours to fix.