Article — mmHg to Atm Converter
mmHg to Atm Conversion — Mercury Pressure to Atmospheres
The conversion is one constant: 1 atm equals 760 mmHg exactly. The 10th General Conference on Weights and Measures fixed this in 1954, defining one standard atmosphere as exactly 101,325 pascals or 760 mmHg. Divide mmHg by 760 to get atmospheres; multiply atmospheres by 760 to get mmHg. The factor has no measurement uncertainty.
Three pressure units dominate practical use. Medicine and physiology use mmHg (millimeters of mercury) almost exclusively — blood pressure, intraocular pressure, and intracranial pressure all report in mmHg. Chemistry uses torr (numerically identical to mmHg) for gas-law calculations. Engineering uses atm, psi, and pascal interchangeably depending on industry. This calculator handles the most common pair.
What is mmHg?
The mmHg is the pressure exerted by a column of mercury 1 millimeter tall under standard gravity (9.80665 m/s²) and at 0°C. Evangelista Torricelli invented the mercury barometer in 1643 and discovered that atmospheric pressure could support a 760-mm mercury column at sea level — the origin of the unit.
The "Hg" comes from hydrargyrum, the Latin name for mercury (literally "liquid silver"). Chemistry textbooks usually call the same unit "torr" in honor of Torricelli. The two are exactly equal: 1 mmHg = 1 torr. The choice is field convention only.
Mercury's density (13,534 kg/m³) is what makes a 760 mm column equal to 1 atm. Using water instead, the column would be 10,332 mm tall — over 10 meters. Torricelli's choice of mercury kept barometers reasonably compact. Water barometers exist but require tall outdoor towers.
mmHg to atm formula
Divide pressure in mmHg by 760 to get atm. So 152 mmHg = 0.2 atm. Multiply atm by 760 to go the other way: 0.5 atm = 380 mmHg. The factor 760 is exact by definition — there's no rounding error in the conversion itself, just in the input measurement.
Three sanity checks: 760 mmHg = 1 atm (sea level standard). 1520 mmHg = 2 atm (one extra atmosphere of pressure, like a hyperbaric chamber at 1 ATG). 380 mmHg = 0.5 atm (high altitude, half the sea-level pressure — about altitude of about 5,500 m (e.g., near Everest base camp)).
mmHg in medicine and blood pressure
Blood pressure cuffs read directly in mmHg. A "120/80" reading means systolic 120 mmHg (peak pressure when the heart contracts) over diastolic 80 mmHg (resting pressure between beats). In atm units, that's 0.158/0.105 — the same physical pressure but in an unfamiliar unit. Clinical practice never uses atm for blood pressure.
The American Heart Association sets stage-1 hypertension at 130–139 mmHg systolic. Stage 2 is 140 mmHg or higher. Crisis values above 180/120 mmHg trigger emergency intervention. Every threshold is in mmHg — converting to atm or pascals would obscure the clinical meaning.
Intraocular pressure (IOP), measured during glaucoma screening, also uses mmHg. Normal IOP is 10–21 mmHg. Above 22 mmHg is "ocular hypertension" and warrants further evaluation. Same unit as blood pressure but a different physiological measurement entirely.
Atm in chemistry and gas laws
The ideal gas law (PV = nRT) uses pressure in whatever unit you choose, with R adjusted to match. For atm, R = 0.0821 L·atm/(mol·K). For mmHg, R = 62.36 L·mmHg/(mol·K). For pascals, R = 8.314 J/(mol·K). Most general chemistry textbooks stick with atm for the standard tables of standard temperature and pressure.
IUPAC redefined STP in 1982 to 100,000 Pa (1 bar) instead of 101,325 Pa (1 atm). Most US chemistry textbooks still teach the older 1 atm definition. The two differ by 1.3 percent — not enough to matter for most calculations but enough to notice in precise work.
Pressure scale reference
Pressures span an enormous range, from below 10⁻¹² atm in laboratory ultra-high vacuum to 10⁵ atm in the deepest ocean trench. The mmHg-and-atm pair covers everyday and biological pressures cleanly; pascals or bars are better for industrial and geophysical scales.
- Mariana Trench (10,910 m deep) = 1100 atm = 836,000 mmHg
- SCUBA at 30 m depth = 4 atm = 3040 mmHg
- Hyperbaric chamber (1 ATG) = 2 atm = 1520 mmHg
- Sea level standard = 1 atm = 760 mmHg
- 10,000 ft altitude = 0.69 atm = 525 mmHg
- Everest summit (8,848 m) = 0.33 atm = 251 mmHg
- Normal blood pressure = 0.11–0.16 atm = 80–120 mmHg
- High-school lab vacuum pump = 0.001 atm = 0.76 mmHg
- Particle accelerator UHV = 10⁻¹² atm = 10⁻⁹ mmHg
mmHg, torr, and pascal
Three units, one underlying physics. The pascal is the SI base unit (1 Pa = 1 N/m²). The mmHg and torr are non-SI but accepted for medical and chemistry use. The factor 1 mmHg = 133.322 Pa comes from mercury density times gravity.
For lab work the torr is preferred in chemistry literature because it sidesteps any ambiguity about mercury column temperature corrections. The 1958 BIPM definition fixed 1 torr = 1/760 atm exactly — independent of any actual mercury barometer. So torr and mmHg agree to better than one part in 10⁷, with torr being the more strictly defined of the two.
Vacuum and altitude pressures
Vacuum systems quote pressure in mmHg (or torr) routinely. A simple mechanical pump pulls down to about 10⁻³ mmHg (0.001 torr). Diffusion pumps reach 10⁻⁶ mmHg. Turbomolecular and ion pumps drop to 10⁻⁹ mmHg (ultra-high vacuum). Particle accelerators and electron microscopes need UHV to prevent gas molecules from scattering beams.
Altitude pressure follows the barometric formula approximately. At 1500 m (5000 ft, like Denver), pressure drops to 632 mmHg (0.83 atm) — explaining altitude-adjusted cooking times and the need for pressurized aircraft cabins above about 3000 m. Mount Everest summit at 8848 m sits at 251 mmHg, one-third of sea level, requiring supplemental oxygen.
Common mmHg-atm mistakes
The first mistake is confusing absolute and gauge pressure. Tire pressure gauges read gauge pressure — the pressure above atmospheric. So a 32 psi (gauge) tire is actually 32 + 14.7 = 46.7 psi absolute = 3.18 atm = 2417 mmHg. Hyperbaric chamber depth is also given in gauge (ATG) — a "1 ATG chamber" runs at 2 atm absolute (2 ATA).
The second mistake is treating mmHg readings from different temperature conditions as identical. Mercury expands with temperature, so a "760 mmHg" reading at 0°C is slightly different in pressure from "760 mmHg" at 20°C. The 1954 CGPM definition fixes the unit at 0°C, so corrections for warm mercury are needed in precise work — typically a 0.18 mmHg correction per 1°C at the 760 mmHg range.
Mercury is a regulated environmental hazard. Most clinical settings have moved to digital pressure transducers calibrated in mmHg but using no mercury internally. New mercury sphygmomanometers (blood-pressure cuffs) are banned in many US states and most of the EU. The unit name persists; the metal does not.