How this instrument works
A millimetre of mercury began as literal geometry. In 1643 Evangelista Torricelli sealed mercury into a glass tube, inverted it over a dish, and watched his column settle near 760 mm with vacuum standing above it. Modern practice keeps that name and discards the glassware: a conventional mmHg is now fixed by arithmetic — mercury density taken as 13595.1 kg/m³, gravity as standard 9.80665 m/s², height as one millimetre — which lands on 133.322387415 Pa exactly, beholden to no real thermometer or latitude.
Standard atmosphere travelled that same road from observation to decree. It began as whatever pressure Torricelli's column represented, and in 1954 the 10th General Conference on Weights and Measures pinned it at 101325 pascals flat. Divide one definition by another and you get 133.322387415 ÷ 101325 = 0.001315789661142…, a decimal that repeats forever because 101325 factors into 3 × 5² × 7 × 193 and those sevens and 193s never clear.
Clinics kept mmHg long after laboratories moved to pascals: blood pressure at 120/80, intraocular pressure between 10 and 21, arterial oxygen tension near 95. Atmospheres survive in chemistry and diving, where 0.082057 L·atm/(mol·K) anchors gas-law homework and hyperbaric chambers run at two or three atm absolute. A respiratory physiologist reading gas tensions and a physical chemist balancing Henry's law are describing one quantity in two dialects.
- Type your reading into the Millimetres of mercury (mmHg) field — it opens at 760, sea-level pressure by tradition.
- Read the Atmospheres (atm) figure beneath it; results recompute on every keystroke, with no submit button.
- Going backwards, multiply atmospheres by 759.9998917 — one atm is a hair under 760 mmHg, never exactly 760.
- Gauges on cuffs and tyres report gauge pressure. Add local barometric pressure first if an absolute figure is what you need.
Worked example — 760 mmHg is not quite one atmosphere
Enter 760 into the Millimetres of mercury (mmHg) field — Torricelli's figure, and a sea-level number nearly every textbook prints. Atmospheres (atm) answers 1.00000014247, not a round 1. That surplus works out to 0.0144 Pa: in ordinary air, about what you shed by lifting a barometer 1.2 mm off its bench.
This gap is real, not a rounding artefact. Torr was defined as one seven-hundred-and-sixtieth of an atmosphere, so 760 torr sits exactly on 1 atm; a conventional mmHg was instead built upward from mercury density and standard gravity, landing 1.4 parts in ten million higher. Clinically it vanishes. Enter 120 for a systolic reading and you get 0.157894759 atm, where torr would give 0.157894737 — identical on any cuff ever manufactured.
Questions
Is 760 mmHg exactly one atmosphere?
No — 760 mmHg comes to 1.00000014247 atm, high by roughly 1.4 parts in ten million. One atmosphere is exactly 101325 Pa, while a conventional millimetre of mercury is exactly 133.322387415 Pa, and 760 of those total 101325.0144 Pa. A unit that does divide 760 ways evenly is torr, defined as 1/760 atm precisely. Outside primary vacuum metrology, treating 760 mmHg as one atmosphere costs you nothing at all.
What is the difference between mmHg and torr?
About one part in seven million. Torr is defined from above as exactly 101325/760 Pa = 133.3223684… Pa, whereas a conventional mmHg is built from below as 13595.1 kg/m³ × 9.80665 m/s² × 1 mm = 133.322387415 Pa. Instrument makers and clinicians use both names interchangeably, and at any pressure a hospital or workshop actually measures they are interchangeable. Calibration laboratories issuing certificates keep them apart.
Is this conversion factor exact or rounded?
Rounded, though both definitions behind it are exact. Dividing 133.322387415 by 101325 produces a decimal that repeats forever, since 101325 = 3 × 5² × 7 × 193 carries prime factors beyond 2 and 5. What this sheet applies, 0.00131578966114, is that exact ratio truncated at twelve significant figures — faithful to about one part in a trillion, which no pressure gauge ever built can resolve.
Does a blood pressure of 120/80 mmHg convert straight into atm?
Arithmetically yes: 0.15789 and 0.10526 atm. Read those carefully, though. Blood pressure is gauge pressure, measured against whatever a room already supplies, so absolute systolic pressure in a patient at sea level sits closer to 880 mmHg, or 1.158 atm. Mixing gauge and absolute figures is easily the commonest error in pressure work, and it stays invisible until somebody checks the arithmetic.
Is a technical atmosphere the same unit?
No. Standard atmosphere, used here, is 101325 Pa. Technical atmosphere, symbol at, is one kilogram-force per square centimetre — 98066.5 Pa, about 3.2% smaller — and it appears throughout older German and Soviet engineering drawings, often written ata for absolute or atü for gauge. Reading at as atm inflates a pressure by roughly 3%, which on a boiler rating is no rounding matter.
Why do American weather reports say 29.92 rather than 760?
They quote inches of mercury, not millimetres. Since an inch is exactly 25.4 mm, 29.92 inHg equals 759.968 mmHg — one column measured with a different ruler. Aviation altimeters worldwide are set in inHg or hectopascals; hospitals by contrast stayed with mmHg everywhere, which is why a chart in Lyon and a chart in Ohio both read 120/80.