mmHg to Inches of Mercury Converter
Common Conversions
| mmHg | inHg |
|---|---|
| 1 | 0.03937 |
| 5 | 0.19685 |
| 10 | 0.3937 |
| 25 | 0.9843 |
| 25.4 | 1 |
| 50 | 1.9685 |
| 100 | 3.937 |
| 250 | 9.843 |
| 500 | 19.685 |
| 760 | 29.921 |
| 1000 | 39.37 |
| 2000 | 78.74 |
Why this conversion matters in chemistry
Lab mercury manometers are graduated in mmHg by definition; older US vacuum-regulator dials are marked in inHg. A 760 mmHg atmospheric reading is 29.92 inHg on a legacy dial — the bridge value used in qualification runs before retiring an old dial gauge in favor of a digital pressure transducer. A factor of 1/25.4 inHg per mmHg reduces to the inch's exact definition (25.4 mm). It's the unit step at the boundary of SI-aligned mercury manometry and US legacy pressure equipment.
Formula
Where the factor comes from
This is the one pressure pair where mercury drops out entirely. Both units describe the same conventional column — the same assigned density, the same standard gravity — and differ only in how its height is measured. Write each as ρgh and everything except h cancels, leaving nothing but the ratio of an inch to a millimeter. The international inch is exactly 25.4 mm, so the factor is exactly 1/25.4, or 5/127 as a fraction. Because 127 is prime and carries no factor of two or five, the decimal does not terminate: 0.0393700787… repeating with a period of 42 digits, while the reverse direction is a clean, terminating 25.4. The older US survey inch differed by roughly two parts in 10⁶, well below anything a mercury column resolves.
Precision and significant figures
The corrections that dominate every other mercury conversion cancel here. A column standing at 25 °C rather than its 0 °C reference reads high, local gravity departs from the standard value, and the assigned density may not match the mercury in the tube — all three affect both units identically, so the ratio survives untouched. Only the reading's own figures matter, and the two scales are mismatched. One millimeter of column moves the inHg value by 0.0394, so a second decimal place in inHg is already finer than a millimeter scale supports. Going the other way, an inHg display resolving 0.01 corresponds to 0.254 mmHg: tenths of a millimeter are the honest limit.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
The exact conversion anchor — one inch of mercury in mm.
About a typical low-vacuum gauge reading.
About 2/3 of standard atmospheric pressure.
Common mistakes
Dividing by 2.54 instead of 25.4
The centimeter-per-inch factor is the one everybody knows, and reaching for it here inflates the result tenfold. What makes it dangerous is the shape of the answer: 760 divided by 2.54 gives 299.2, which reads as 29.92 with a slipped decimal and survives a glance at the page. Confirm the factor is millimeters per inch.
Applying a temperature correction twice
US references specify inHg at 32 °F and metric ones specify mmHg at 0 °C, and those are the same temperature. No offset separates the two references and there is nothing to correct for. Applying a mercury expansion correction on top of the conversion, on the assumption that the conventions differ, introduces an error where none existed.
Inches of water are not inches of mercury
Low-pressure US instrumentation is frequently scaled in inches of water column, and both scales get shortened to inches on a label or in a specification. A water column stands about 13.6 times taller than mercury for the same pressure, so 10 inH₂O is 0.735 inHg. Converting a water-column figure with the mercury factor overstates the pressure by more than an order of magnitude.