Blood Pressure Figures and Their kPa Equivalents
Almost every blood pressure number you will ever see — on a home monitor, in a textbook, on a clinic printout — is written in millimetres of mercury. Yet the SI unit of pressure is the pascal, and a handful of journals, national standards and physiology courses print the same readings in kilopascals. If you are comparing a paper with a chart, or working through a physiology exercise, the two scales have to line up.
Why the Numbers Look the Way They Do
An instrument that set the habit
Two numbers, two moments
Where kilopascals turn up
A number with conditions attached
Converting a Reading Pair Step by Step
A blood pressure value is two numbers, so it takes two passes — the systolic figure and then the diastolic one.
Start with the upper figure
Type the systolic number into the mmHg box — 118, 132, 145 — and the kilopascal value appears while you type. If your keyboard produces a comma for decimals, that is accepted too, and any spaces you type are ignored.
Repeat for the lower figure
Overtype the same box with the diastolic number. Nothing has to be cleared or reset first, so a page of paired readings goes through quickly.
Take the plain number away
The copy button beside each field puts the digits alone on the clipboard, without a unit label, which is what a spreadsheet column or a homework answer box wants. Pressing Ctrl + C inside a field does the same thing.
Go the other way for a kPa source
Reading a paper that quotes 17.3 kPa? The ↔ button turns the pair around and runs kPa → mmHg. The multiplier that way is 7.500617, so 17.3 kPa is 129.8 mmHg.
Published Category Boundaries in Both Units
The bands below are the adult categories set out in the 2017 ACC/AHA high blood pressure guideline, which is one of several classification schemes in use — European and WHO documents draw the lines differently. They are reproduced here purely to show how familiar cut-off numbers read once they are expressed in kilopascals, rounded to one decimal.
| Category (2017 ACC/AHA) | Systolic (mmHg) | Systolic (kPa) | Diastolic (mmHg) | Diastolic (kPa) |
|---|---|---|---|---|
| Normal | under 120 | under 16.0 | and under 80 | and under 10.7 |
| Elevated | 120–129 | 16.0–17.2 | and under 80 | and under 10.7 |
| Stage 1 | 130–139 | 17.3–18.5 | or 80–89 | or 10.7–11.9 |
| Stage 2 | 140 or above | 18.7 or above | or 90 or above | or 12.0 or above |
| Crisis threshold | above 180 | above 24.0 | and/or above 120 | and/or above 16.0 |
Two things stand out once the right-hand columns exist. The kilopascal figures are roughly one seventh of the millimetre ones, so the whole familiar range compresses into a narrow band between about 10 and 25 — and the round numbers stop being round. Nobody would have chosen 18.7 kPa as a boundary; it is 140 mmHg wearing a different coat.
What Helps When Working Through a Reading List
Systolic then diastolic, no resetting
Both boxes stay live and editable, so a long column of paired values goes through one overtype at a time.
Whichever unit the source uses
The reverse button matters here because papers and charts do not agree: one gives you mmHg, the next gives kPa, and both directions are one click apart.
Beyond the two units on the tab
Searchable lists on both sides carry every pressure unit in the app, useful when a physiology text quotes centimetres of water or an atmosphere fraction instead.
Round it yourself, not before
Results carry up to eight decimals, so you decide whether the answer is 15.9986842 or simply 16 — the tool never rounds the intermediate value for you.
Questions About Blood Pressure Units
Why is blood pressure still written in mercury when mercury instruments have gone?
Continuity of data. Every reference range, every clinical trial endpoint and every classification threshold accumulated since the 1890s is expressed in mmHg, and renumbering them would break comparability with a century of evidence. Modern aneroid and oscillometric devices contain no mercury at all — they are calibrated to report the number a mercury column would have shown.
What do the two figures in a reading such as 120/80 represent?
The first is systolic pressure, the highest arterial pressure reached as the left ventricle ejects blood. The second is diastolic pressure, the lowest point between beats while the ventricle refills. In kilopascals the same pair reads 16.0 / 10.7, and the 40 mmHg gap between them — 5.3 kPa — is the pulse pressure.
How is mean arterial pressure calculated from a systolic and diastolic pair?
The usual teaching approximation is the diastolic value plus one third of the pulse pressure, because at resting heart rates the cardiac cycle spends about twice as long in diastole as in systole. For 120/80 that is 80 + 40/3 ≈ 93.3 mmHg, or 12.4 kPa. The approximation drifts when the heart rate is high and diastole shortens.
Which countries or publications actually print blood pressure in kPa?
It is a publishing convention rather than a bedside one. Where national measurement law requires SI units, medical texts and journals often give kPa first with mmHg in parentheses — Chinese literature is the most familiar example — and physiology teaching sometimes uses kPa to keep pressure consistent with the rest of a problem set. Cuffs and monitors themselves are still marked in mmHg virtually everywhere.
Why do cuff size and arm position matter to the recorded number?
Because the cuff has to transmit its pressure evenly to the artery. Standard guidance sizes the bladder at roughly 40 % of arm circumference in width and about 80 % in length; a bladder that is too small has to be inflated harder and tends to read high, an oversized one the reverse. Arm height matters for a plain hydrostatic reason: every 10 cm the cuff sits below heart level adds around 7.7 mmHg — close to 1 kPa — to the column of blood being measured.
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