Kilopascals to mmHg

Where Clinical Monitoring Still Reports Millimetres of Mercury

Physiology textbooks and international standards documents increasingly state pressures in kilopascals, while the instruments at the bedside — the tonometer in an eye clinic, the intracranial monitor on an intensive care unit, the transducer on a central line — display millimetres of mercury. A student reading a European paper in kPa and then looking at a monitor in mmHg is looking at the same physiology twice, in two dialects.

The factor: 1 kPa = 7.50061683 mmHg. An intraocular pressure written as 2.00 kPa in a metric text is therefore 15.0 mmHg on a tonometer readout — right in the middle of the published statistical range.

Educational reference only — this is not medical advice. Everything here describes what these measurements are and what ranges the literature publishes. It is educational only, it is not a diagnosis, and it must never be used to judge a reading, guide treatment or replace a qualified clinician.

The Measurements Behind the Numbers

Tonometry

Intraocular pressure is inferred from how the cornea resists flattening. Goldmann applanation is the long-standing reference method, and its scale is engraved in millimetres of mercury.

Intracranial monitoring

A catheter or parenchymal probe reports the pressure inside a rigid skull shared by brain tissue, blood and cerebrospinal fluid — the quantity neurocritical care guidelines are written around.

Central venous pressure

A transduced central line reports pressure in the great veins near the right atrium, referenced to a fixed anatomical level so that readings can be compared over time.

Derived pressures

Cerebral perfusion pressure is not measured at all: it is one pressure subtracted from another, which only works if both are expressed in the same unit before the arithmetic.

Converting a Textbook kPa Figure to the Monitor's Units

The usual reason to be here is a source document that uses SI units and a display, chart or exam question that does not.

Type the kilopascal value from the source

Enter 2, 1.33, 0.67 or whatever the paper prints; the millimetres of mercury appear beside it immediately. A decimal comma works as well as a point, which matters when the figure was copied from a continental European journal.

Set it beside the published range

Ranges quoted in the literature are broad and method-dependent, so treat the converted value as a position on a scale rather than a verdict. The table further down lists the commonly cited bands in both units.

Move it into your notes or dataset

The copy button on each field puts the number alone on the clipboard — no unit, no spacing — which is what a statistics package or a revision table wants. Ctrl + C inside the field behaves the same way.

Reverse it when the source is already in mmHg

The swap arrows (↔) turn the page into mmHg → kPa for the return trip, which is what you need to write a chart value into an SI-only report. That direction multiplies by 0.13332237, so 15 mmHg comes back as 2.00 kPa.

Monitored Pressures and Their Published Ranges

Ranges widely cited in clinical reference texts, shown in the unit the instrument displays and in kilopascals. They describe typical published values for adults and vary with method, posture, reference level and the source consulted — they are here to give the converted numbers a scale, nothing more.

Measurement	How it is obtained	Typical published range (mmHg)	Same range (kPa)
Intraocular pressure	Goldmann applanation tonometry	10–21 mmHg	1.33–2.80 kPa
Intraocular pressure, population mean	Applanation tonometry	about 15.5 mmHg	about 2.07 kPa
Intracranial pressure, adult supine	Intraventricular or parenchymal probe	5–15 mmHg	0.67–2.00 kPa
Intracranial pressure, threshold cited in guidelines	Continuous monitoring	above 22 mmHg	above 2.93 kPa
Central venous pressure	Transduced central line at the reference level	2–8 mmHg	0.27–1.07 kPa
Cerebral perfusion pressure, commonly quoted target	Calculated, not measured	60–70 mmHg	8.00–9.33 kPa
Intra-abdominal pressure, normal adult	Bladder pressure measurement	5–7 mmHg	0.67–0.93 kPa
Intra-abdominal hypertension, defined threshold	Bladder pressure measurement	12 mmHg and above	1.60 kPa and above

The spread is worth noticing. Everything a venous or intracranial catheter reports lives inside the first three kilopascals, while a perfusion figure is several times larger — which is exactly why a monitor built for one of them cannot simply be pointed at another.

What This Page Offers a Reader Working Across Both Units

Two fields, either as the source

Whichever unit the document uses, type into that box and the other keeps pace, so a whole range can be walked through end to end without resetting anything.

Turn around for an SI write-up

One press of the swap arrows converts in the mmHg → kPa direction, the one you want when a chart figure has to be reported in kilopascals.

Centimetres of water are on the menu

Both unit lists are searchable and hold all 26 pressure units, including cmH₂O — the scale older manometer-based venous measurements were recorded on.

Clean values for a study table

Results carry up to eight decimals and copy as the number by itself, so converted figures paste straight into a spreadsheet column without editing.

Questions About mmHg in Clinical Measurement

Why is eye pressure reported in millimetres of mercury?

Applanation tonometry was calibrated in mmHg long before SI units reached the clinic, and the entire body of ophthalmic research, equipment scales and teaching material grew up around those numbers. Changing the unit now would mean re-anchoring decades of published data, so the kilopascal appears mostly in metrology documents and in the occasional metric textbook.

What does it mean to zero a transducer before a pressure is read?

The transducer is opened to the air at a fixed anatomical reference level and told that this is zero, so everything it then reports is pressure relative to atmosphere at that height. Because a column of fluid adds roughly 0.74 mmHg per centimetre, a transducer left a few centimetres above or below the reference level shifts every reading — a unit conversion cannot rescue a measurement referenced to the wrong place.

Which of these pressures appear in kilopascals in the literature?

Kilopascals turn up most often in physiology teaching texts, in device and metrology standards, and in journals that enforce SI throughout. Reviews of cerebral autoregulation, for instance, are commonly written with both — a 50–150 mmHg span appearing as 6.7–20 kPa. Bedside displays and clinical guidelines have largely stayed with millimetres of mercury.

Is cerebral perfusion pressure measured or worked out?

Worked out. It is the difference between the arterial pressure driving blood into the head and the intracranial pressure resisting it, so no single sensor reports it. That makes unit consistency more than cosmetic: subtracting a value in kilopascals from one in millimetres of mercury produces a number that means nothing at all.

Why is abdominal pressure measured through the bladder?

The bladder wall is compliant enough that, with a small instilled volume, it behaves as a passive membrane and passes the surrounding abdominal pressure to a transducer on an existing catheter. It gives an indirect reading without an extra puncture, which is why the published thresholds — 12 mmHg, or 1.60 kPa, and upward — are defined for that technique.