mmHg to Torr Converter
Common Conversions
| mmHg | torr |
|---|---|
| 1 | 1 |
| 10 | 10 |
| 23.8 | 23.8 |
| 50 | 50 |
| 100 | 100 |
| 200 | 200 |
| 400 | 400 |
| 500 | 500 |
| 600 | 600 |
| 760 | 760 |
| 800 | 800 |
| 1000 | 1000 |
Why this conversion matters in chemistry
Pulmonary gas-exchange calculations cross this identity routinely. Clinical PCO₂ at 40 mmHg arterial pressure is 40 torr in the underlying physical-chemistry framework — the two units are interchangeable in any practical calculation. Both descend from mercury-column measurements: mmHg from the literal column height, torr defined as exactly 1/760 of a standard atmosphere. The two definitions match to about 0.000015%, well below the precision of any practical measurement. The identity is the everyday type cast at the boundary between clinical and physical-chemistry literature.
Formula
Where the factor comes from
Two definitions, one pressure, and neither of them refers to the other. The torr is fixed as 101325/760 Pa, tying it to the standard atmosphere and to nothing physical. The conventional millimeter of mercury is fixed as 13595.1 × 9.80665 × 10⁻³ Pa, tying it to an assigned density and an assigned gravity. Work both out and the ratio is 1.000000142, the torr being smaller by about one part in seven million. The residue is a rounding fossil. A conventional 760 mm column exerts 101325.0144 Pa, and when the atmosphere was fixed by definition the tidy integer was adopted in its place, stranding those fourteen thousandths of a pascal in the gap between the two units.
Precision and significant figures
The factor is 1 to seven significant figures, so the honest instruction is to change the label and leave the number alone. Converting adds nothing: 40 mmHg is 40 torr, not 40.000 torr, and the reading's original figures are all you have. Seeing the difference at all would mean resolving about 0.0001 mmHg at atmospheric pressure — the territory of a piston gauge in a calibration laboratory, not of anything standing on a bench. Every ordinary source of error here, from column temperature and local gravity to gauge drift and the meniscus itself, is orders of magnitude larger than the definitional gap.
Worked Examples
Standard atmospheric pressure at sea level.
Vapor pressure of water at 25 °C — the value behind humid-air calculations.
The base unit relationship — practically identical.
About a typical vacuum-distillation operating pressure.
Common mistakes
Equality is not license to add digits
Because the numbers match on any display, a value copied from mmHg into torr often gains decimals in transit — 23.8 becomes 23.800 because the spreadsheet column was formatted that way. The unit change justified none of them. Significant figures travel with the measurement, and this particular conversion contributes exactly none of its own.
The gap cannot explain a disagreement
When a value in torr and a value in mmHg refuse to match, one part in seven million is never the culprit; it is far smaller than any discrepancy you could notice. Look instead at reference level, at column temperature, at whether one figure is gauge and the other absolute, or at an instrument that has drifted since it was last calibrated.
The identity stops at these two units
It is tempting to generalize into a rule that mercury units interchange freely. They do not. Older European work reports cmHg, which is ten torr, and US instruments use inHg at 25.4 torr apiece. Only the millimeter of mercury and the torr coincide, and they coincide because the torr was defined to reproduce that column — not because mercury units share a scale.