How this instrument works
The pascal is the SI unit of pressure — one newton spread over one square metre — and it took the name of Blaise Pascal at the General Conference on Weights and Measures in 1971. One pascal is tiny, near what a sheet of paper exerts lying flat, so working pressures get quoted in kilopascals: one standard atmosphere is 101.325 kPa under its 1954 definition, a passenger car tyre sits around 220 kPa, and a severe North Atlantic storm can drop barometers to 96 kPa.
Millimetres of mercury go back to Evangelista Torricelli, who in 1643 inverted a mercury-filled tube into an open dish and watched its column settle near 760 mm, leaving vacuum above. Height makes an awkward basis for any unit, since mercury thins as it warms and gravity varies with latitude, so metrologists froze both quantities. The conventional millimetre of mercury is now defined as what a 1 mm column of fluid whose density is exactly 13595.1 kg/m³ exerts under standard gravity, 9.80665 m/s². Multiply those three numbers and you land on 133.322387415 Pa exactly.
Neither unit has displaced its rival. Millimetres of mercury remain the language of the sphygmomanometer, of intraocular and intracranial pressure, and arterial blood gases across North America; European measurement law keeps mmHg legal precisely for blood pressure and other body fluids. Kilopascals rule almost everywhere else — Canadian weather bulletins quote barometric pressure as 101.3 kPa, Australian and Canadian tyre placards are stamped in kPa, and wards throughout Britain, Europe and Australasia record a normal arterial carbon dioxide tension as 4.7 to 6.0 kPa where an American chart would say 35 to 45 mmHg.
- Enter your reading in the Kilopascals (kPa) field; it opens at 100 kPa, close to one atmosphere.
- Read the answer on the Millimetres of mercury (mmHg) line — it updates on every keystroke.
- Starting from hectopascals or millibars? Divide by ten first: a 1013 hPa barometer is 101.3 kPa.
- To run this backwards, divide your mmHg figure by 7.50061575846.
- Gauge stays gauge and absolute stays absolute; nothing is added or removed for ambient air.
Worked example — 100 kPa certified in mmHg
One calibration lab holds its pressure balance at 100 kPa absolute and has to issue the certificate in an older unit, because the customer's dial gauge is still marked in millimetres of mercury. Enter 100 in the Kilopascals (kPa) field; the Millimetres of mercury (mmHg) line returns 750.061575846.
Check that against sea level. Standard atmospheric pressure, 101.325 kPa, comes out as 759.99989 mmHg — just under the round 760 that the torr gives by definition, since one torr is one seven-hundred-and-sixtieth of an atmosphere while the conventional millimetre of mercury is pinned instead to fixed density and fixed gravity. That missing 0.00011 mmHg is the whole gap between two units people treat as identical.
Questions
Is one kilopascal exactly 7.50061575846 mmHg?
No — exactness sits on the other side of this equation. One conventional millimetre of mercury is defined as precisely 133.322387415 Pa, so a kilopascal works out to 1000 ÷ 133.322387415 = 7.500615758456563…, a decimal that never terminates. The twelve-figure multiplier used here is that value correctly rounded, and what remains is under one part in a hundred billion — far below the resolution of any pressure instrument you are likely to be holding.
Is one millimetre of mercury identical to one torr?
Not quite, though outside a metrology lab you can treat them as interchangeable. One torr is defined as one seven-hundred-and-sixtieth of the standard atmosphere, or 133.3223684… Pa, whereas the conventional millimetre of mercury is 133.322387415 Pa. They differ by roughly 1.4 parts in ten million, which on a 120 mmHg blood pressure amounts to 0.000017 mmHg. It matters when you are writing a calibration certificate or comparing vacuum standards, and essentially never otherwise.
Why do some blood gas reports use kPa instead of mmHg?
Because those health systems record clinical measurements in SI units throughout. Normal arterial oxygen tension reads 11 to 13 kPa in Britain, Ireland, much of Europe and Australasia, against 83 to 98 mmHg on a United States chart; carbon dioxide runs 4.7 to 6.0 kPa versus 35 to 45 mmHg. Identical measurements, different clothes. At a bedside, multiplying kPa by 7.5 converts quickly enough to be safe.
Does this convert gauge pressure or absolute pressure?
Both, because a reference level travels with your number rather than with its unit. Blood pressure, tyre pressure and most workshop gauges read above ambient air, while barometric and blood gas figures are absolute. Convert 220 kPa of tyre pressure and you get roughly 1650 mmHg gauge, not absolute. If you want an absolute figure, add local barometric pressure — near 101 kPa at sea level — before typing anything in.
Can I just multiply by 7.5?
For clinical and bedside work, yes. Using 7.5 leaves your answer low by about 82 parts per million, which across one full atmosphere of 760 mmHg is six hundredths of one millimetre — invisible on any cuff or dial. Hold on to the full multiplier for calibration reports, instrument specifications, and anywhere a discrepancy in the fourth significant figure would be queried.
Why does my mercury manometer disagree with this?
Because a real column is not a conventional unit. That definition assumes mercury whose density is exactly 13595.1 kg/m³ — its value at 0 °C — standing in gravity of exactly 9.80665 m/s². Warm the mercury to 20 °C and it thins by roughly 0.36%, so its column must stand that much taller to balance one pressure; move from equator toward pole and local gravity climbs about half a percent, pushing the other way. Laboratory manometers apply both corrections before anyone records a reading.