One millimetre of mercury is 0.133322387415 kilopascals, so a kilopascal is 7.5006 mmHg. The unit is a survival from mercury manometers, and it persists for one reason above all others: blood pressure.
About the units
A millimetre of mercury is the pressure exerted by a one-millimetre column of mercury under standard gravity, with mercury at a defined density of 13,595.1 kg/m³. It is a unit of pressure defined by a physical arrangement rather than by force over area, which puts it in the same family as inches of water and metres of head. Mercury sphygmomanometers have largely been retired for environmental reasons, and modern blood pressure monitors are electronic — but they still report in mmHg, because a century of clinical thresholds, guidelines and research is expressed in that unit and rebasing it would be dangerous.
The exact factor
The exact value is 133.322387415 Pa per mmHg. The related unit Torr is defined slightly differently, as exactly 1/760 of a standard atmosphere, giving 133.3223684 Pa — a difference of one part in seven million, which is irrelevant clinically and matters only in precision vacuum work. Reference values: a blood pressure of 120/80 mmHg is 16.0/10.7 kPa, and the hypertension threshold of 140/90 is 18.7/12.0 kPa.
Where you meet this conversion
Blood pressure, universally and in every country including fully metric ones. Intracranial and intraocular pressure use the same unit, as do blood gas partial pressures — arterial oxygen at 95 mmHg, carbon dioxide at 40. A handful of countries, notably in Scandinavia and parts of Eastern Europe, have used kilopascals in clinical practice, which is why the conversion is worth having: a Swedish laboratory report in kPa and an international guideline in mmHg describe the same measurement.