On the left, a mercury column slowly falls and rises again inside a glass tube, passing a graduated scale and two red marks — systolic and diastolic. On the right, a cuff is wrapped around the arm, with pulse waves spreading from under it, each in its own rhythm, while a line of Korotkoff sounds runs beneath the symbol. A single drop of mercury creeps unhurriedly down the tube, and at the bottom a blue reading mark travels along the measuring scale from edge to edge.
Unit passport
Permitted by law for blood pressure only sheet 1/1 · rev. 2026-09Millimetre of mercury mmHg
Non-SI unit · quantity: pressure · 133.322 387 415 Pa
The unit outlived its substance
Mercury has been driven out of hospitals. The Minamata Convention ended the manufacture of mercury instruments, a European directive banned the sale of mercury sphygmomanometers and thermometers even earlier, and a glass tube with a silvery column has long vanished from the doctor’s office. Yet the unit named after the height of that column remains — not out of habit, but by the express permission of the law: the European rules on units of measurement struck the millimetre of mercury out everywhere except for the pressure of blood and other body fluids.
It is the only exception of its kind in the whole list: the other old units either disappeared entirely or were kept for whole industries, whereas here a single field of use is spelled out, and a medical one at that. The reason is not the conservatism of doctors but the cost of error: every threshold, a century of studies, every dosage and every clinical guideline is written in millimetres, and converting them to kilopascals would mean rewriting medicine from scratch for the sake of arithmetic tidiness.
The unit has long been defined by calculation rather than experiment: it is the pressure of a column of liquid with a density of thirteen thousand five hundred and ninety-five point one kilograms per cubic metre and a height of one millimetre under standard gravity. Not a drop of real mercury is needed — the number is enough.
In international texts the symbol is written mmHg, as one word, where Hg comes from the Latin hydrargyrum; English prose also allows “mm Hg” with a space. The capital H and the small g must never be swapped, because they form the symbol of a chemical element, not of a unit; run-together forms with periods, such as “mm.Hg”, are wrong as well.
The torr and the millimetre of mercury differ in the seventh significant figure, because the torr is defined as an atmosphere divided by seven hundred and sixty, while the millimetre is defined through the density of mercury and standard gravity. For medicine the difference is irrelevant, and for vacuum engineering too, but metrological tables keep it. Even more often the millimetre of mercury is confused with the millimetre of water: mercury is thirteen and a half times heavier.
01 · Definition
liquid column · cuff · Korotkoff soundsThe unit is defined as the pressure exerted by a column of liquid one millimetre high with a density of thirteen thousand five hundred and ninety-five point one kilograms per cubic metre in a field of standard gravity. The density is that of mercury at zero degrees, and the acceleration is the agreed nine point eight zero six six five, so the number comes out exact rather than measured.
This is the main subtlety: a real mercury column would depend on both temperature and latitude, since mercury expands when heated and gravity is stronger at the pole than at the equator. Old manometers therefore needed corrections, whereas the present definition is free of them — it simply fixes a number of pascals, and mercury survives in it only as a memory.
Blood pressure is still measured the way Riva-Rocci and Korotkov devised: the cuff is inflated above systolic pressure, then air is let out slowly while listening. As long as the artery is squeezed shut, there is no blood flow and no sound; as soon as the cuff pressure falls just below systolic, blood breaks through in spurts and a tapping appears in the stethoscope — the first sound. When the cuff stops obstructing the flow altogether, the tapping vanishes, and that is diastolic.
From this follows an unpleasant property of the method that people rarely think about: pressure can be read only at the moments when the heart beats. Between two beats the cuff manages to drop several millimetres, and whatever happened in that gap is hidden from the observer — which is why the air must not be let out faster than two or three millimetres per second: the measurement starts to err in both directions at once.
Why you must not rush the deflation
On the left the cuff lets out air and the mercury in the tube falls past the scale, and each heartbeat answers with a flash at the stethoscope — but only in the interval between diastolic and systolic pressure, where blood breaks through the squeezed artery. On the right the same thing is unrolled in time: the deflation line runs downward, the beats are marked with dots, and a grey band shows the audible zone. Speed up the deflation or lower the pulse, and the gap between beats in millimetres grows, so the first sound is caught already below the true systolic and the last one still above the true diastolic — and the pressure ends up recorded too low on the upper number and too high on the lower one.
02 · Conversion
pressure · clinic · liquid columnsOne pressure in three conversations
The first tab spreads the entered pressure across the units. The second reads the same number as a patient’s upper pressure and says what it means by the clinical thresholds. The third shows how tall a column of various liquids would balance that pressure — and from this very comparison the mercury tube was once born.
A rule to remember: one millimetre of mercury is about one eighth of a kilopascal, and one hundred and twenty millimetres are sixteen kilopascals.
The clinical categories here are given for pressure measured at the doctor’s office; for home and 24-hour measurements the thresholds are about five millimetres lower, because many people’s pressure rises from the setting of the office itself. A single measurement decides nothing at all: the diagnosis is made over several visits.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
mercury · aneroid · automatic · catheterFour ways to find out the pressure
The instrument that gave the unit its name
The mercury sphygmomanometer is good because it shows the quantity directly: the height of the column is the pressure, and it needs no calibration beyond checking the zero and the cleanliness of the tube. That is why for a century and a half it served as the reference instrument against which all others were checked, and why its disappearance proved painful — the comparison standard had to be replaced with verified electronic manometers.
A needle instead of a column
An aneroid passes the flexing of an elastic capsule to a needle through levers and a toothed sector, so it shows not the column height itself but a mechanical model of it, which drifts from the truth over time. The spring ages, the gearing loosens after falls, and all the while the scale keeps looking convincing — which is why such instruments must be verified regularly and after every drop onto the floor. On the other hand they contain no mercury at all, and it is they that took the place of the departed glass tubes.
A monitor that does not hear the sounds
A home blood pressure monitor has neither a microphone at the artery nor ears: it follows the pressure oscillations in the cuff itself and finds the moment when the pulsations are largest — that is the mean arterial pressure. The upper and lower numbers are not measured but calculated from empirical fractions of this maximum, and the manufacturer fits those fractions on a sample of volunteers. Hence the discrepancies between devices, the requirement of clinical validation for every model, and the futility of arguing which of two devices is “more accurate”.
Pressure taken from inside
In intensive care, a catheter in the radial artery passes the pulsation to a transducer membrane, and the monitor draws the curve beat by beat without waiting for a cuff. Here the physics of the liquid column returns: the transducer must be raised exactly to the level of the right atrium, because every ten centimetres of height offset add or subtract about seven and a half millimetres of mercury — the very same arithmetic Torricelli used.
05 · Writing rules
mmHg · mm Hg · a fraction with a slashTwo numbers with a slash
A blood pressure reading looks like a fraction but is not one: these are two separate measurements, the upper and the lower, and they must not be treated as a numerator and a denominator.
The first mistake is harmless in conversation and dangerous in a document where kilopascals may stand next to it. The second mangles the symbol: Hg is the symbol of a chemical element, with a capital H and a small g, and neither letter case nor periods may be changed. The third substitutes the wrong unit, and the pressure comes out seventy-five times higher than the real one. The fourth attributes a rate to a unit of pressure, although what is meant is the cuff deflation rate. The fifth calculates mean pressure as the half-sum, whereas diastole lasts longer than systole and the formula weights the lower pressure twice. The sixth forgets that thresholds are a convention adopted from study outcomes rather than a natural boundary, which is why different societies still argue about one hundred and thirty versus one hundred and forty.
06 · Neighbouring units
pascal · centimetre of water · torrThe neighbours of the millimetre of mercury are other ways of expressing the same pressure: through force per area, through a column of water and through a fraction of an atmosphere. In medicine they coexist in one chart, since each body fluid has its own historical unit, and they cannot be swapped.
The unit of the system to which the millimetre is tied by an exact number. In physiology its only inconvenience is that familiar thresholds turn into fractional sixteen and eleven kilopascals.
A measure of small pressures: venous, cerebrospinal, in the breathing circuit. Where the count runs to a few millimetres of mercury, a water column gives a larger and more convenient scale.
A unit of vacuum engineering named after Torricelli and differing from the millimetre in the seventh significant figure — a difference that matters only in a metrological table.
Standing nearby among the Simetrium passports are horsepower and calorie — units that the system was supposed to displace and could not, because behind them stands not physics but an accumulated body of familiar numbers.
07 · Historical section
archive · from the tube to the lawHow mercury entered medicine and left it
{title}
{text}
Mercury is thirteen and a half times heavier than water, and only because of that does an instrument for measuring blood pressure fit on a table instead of requiring a stepladder.
The only exception written into law
The European directive on units of measurement is simple: there is a list of what is permitted, and everything not on it is withdrawn from use. The millimetre of mercury stands apart in that list, because it carries a reservation about its field of use — the pressure of blood and of other body fluids — and the unit is admitted for nothing else. No other quantity has such a targeted reservation, and it appeared not at the request of metrologists but at the insistence of medical societies.
The argument was practical, not sentimental: diagnostic thresholds, dosages, the results of decades-long studies and every clinical guideline are expressed in millimetres, and converting them to kilopascals would mean that one hundred and forty becomes eighteen point seven. The number loses its recognisability, the doctor loses the feel for it, and the gain comes down to a neater table of units.
Most curious of all, the substance that gave the unit its name has meanwhile been banned: the Minamata Convention ended the manufacture of mercury instruments, and the glass tubes vanished from doctors’ offices. The unit remains in perfect working order, since its definition has long required no mercury — only numbers — and medicine goes on measuring in millimetres of a column that no longer exists anywhere.
Catalogue of quantities
pressure and physiologySI base units
highlighted: kilogram, metre and second — the pascal is built from themWhich pressure is where in the body
kilopascals and water column are calculated from the first column| Measurement site | mmHg | kPa | cmH₂O | How it is measured |
|---|---|---|---|---|
| {name} | {mm} | {kpa} | {cm} | {how} |
The third column explains why venous and cerebrospinal pressure are measured in water rather than mercury: the values there are so small that in millimetres of mercury the entire physiological norm would fit into a dozen divisions, whereas a water column stretches the same norm over a hundred and makes the difference visible. The unit was chosen not for rigour but for the convenience of the scale, and in that sense medicine has stayed true to the habit that once gave birth to the mercury column itself.