How this instrument works
A millimetre of mercury is no longer measured with an actual mercury column; it is defined by arithmetic. A conventional mmHg fixes mercury's density at 13,595.1 kg/m³, gravity at the standard 9.80665 m/s², and height at one millimetre, multiplying out to exactly 133.322387415 pascals — a figure agreed by international convention rather than read off any physical barometer sitting in a laboratory.
The standard atmosphere followed a separate path to its own fixed number. In 1954 the 10th General Conference on Weights and Measures set it at exactly 101,325 pascals, a round figure chosen independently of the mercury-column definition. Divide one by the other and you get 133.322387415 ÷ 101,325 = 0.00131578966114…, a decimal that runs on forever because 101,325 factors into 3 × 5² × 7 × 193, and those larger primes never let the division terminate cleanly.
Barometric pressure work still leans on mmHg out of sheer habit: aviation altimeter settings, older mercury barometers in weather stations, and historic meteorological records spanning a century or more all use it, alongside hectopascals in newer instruments. Atmospheres appear instead wherever a round reference figure is more useful than a precise column height — gas laws, cabin pressurisation specifications, diving tables. Reconciling a century-old weather log against a modern hectopascal reading runs through exactly this ratio.
- Enter a reading into the Millimetres of mercury (mmHg) field — 760 is preloaded, the traditional sea-level figure.
- Read the Atmospheres (atm) line beneath it; it recalculates with every keystroke to full precision.
- Notice that 760 does not return a clean 1.0 — that tiny surplus is explained in the worked example below, not a display error.
- Reversing direction, multiply an atmosphere figure by 759.9998917 to recover mmHg, since one atm sits a hair under 760 mmHg rather than exactly on it.
Worked example — why a barometer reading 760 isn't quite one atmosphere
Meteorology students learn early that sea-level pressure is 760 mmHg, and aviation altimeter procedures still reference that same figure when reconciling older mercury-based readings against modern hectopascal displays. Enter 760 into Millimetres of mercury (mmHg) and Atmospheres (atm) returns 1.00000014247 — not the clean 1.0 a weather report implies, a surplus of about 0.0144 pascals.
That surplus is real, not a rounding artefact of this sheet. It exists because the mercury-column definition and the standard-atmosphere definition were fixed independently, decades apart, by two different international bodies working from different physical starting points. Torr, by contrast, was defined from the atmosphere downward as exactly 1/760 atm, so 760 torr lands on 1.0 atm precisely — a distinction that only matters in primary pressure-standards laboratories, never on an actual barometer dial.
Questions
Why doesn't 760 mmHg equal exactly 1 atm?
Because the two units were defined independently, by separate international decisions, rather than one being built directly from the other. A conventional mmHg is fixed at exactly 133.322387415 pascals, derived from mercury's density and standard gravity; one atmosphere is fixed separately at exactly 101,325 pascals. Multiplying 760 by the first figure gives 101,325.0144 pascals — a surplus of about 1.4 parts in ten million over the second.
Is this gap large enough to matter in practice?
Almost never outside metrology laboratories. On a weather report, a diving table or a chemistry gas-law problem, treating 760 mmHg as exactly one atmosphere introduces an error many times smaller than any instrument in ordinary use could detect. The distinction matters chiefly to standards bodies calibrating primary pressure references against one another, not to anyone reading a barometer.
What is the difference between mmHg and torr?
A very small one, roughly one part in seven million. Torr is defined top-down as exactly 101,325/760 pascals, while a conventional mmHg is defined bottom-up from mercury density and standard gravity. The two names get used interchangeably almost everywhere in practice, and no gauge or manometer built for ordinary use can tell them apart; only calibration certificates in metrology labs distinguish them.
Why do aviation altimeter settings still reference mmHg or inHg?
Historical continuity across a global system. Some countries set altimeters using hectopascals, others using inches of mercury, and pilots crossing between regions have long needed a reliable mental conversion between mercury-column readings and the pascal-based figures more common elsewhere — a habit that outlasted the mercury barometers instruments themselves have mostly been retired in favour of.
Is standard atmosphere the same as technical atmosphere?
No, and the two are easy to confuse. Standard atmosphere, used throughout this sheet, is exactly 101,325 pascals. Technical atmosphere, symbol at, is instead defined as one kilogram-force per square centimetre — 98,066.5 pascals, about 3.2 percent smaller — and appears on older engineering drawings and gauges, particularly ones translated from German, Japanese or Soviet-era metric practice.
Is the 0.00131578966114 factor exact or rounded?
Rounded, even though both definitions feeding it are exact. Dividing 133.322387415 by 101,325 produces a decimal that never terminates, since 101,325 carries prime factors — 7 and 193 — beyond the 2s and 5s that decimal fractions need to resolve cleanly. This sheet applies that ratio truncated at twelve significant figures, accurate to well under one part in a trillion.