Which one is correct to use: mmHg or mm Hg? Should it be closed up, spaced out or both are acceptable? The AMA manual has both instances.

For example, "The trial compared outcomes associated with 'lower' (<110 mmHg) or 'higher' (<150 mm Hg) systolic blood pressure targets in 200 participants."


According to https://en.wikipedia.org/wiki/Millimeter_of_mercury#cite_note-2 even though this is not an SI unit, mmHg is the correct usage, not spaced out. I think mm Hg is still quite readable.

Other combinations of two units such as kWh for kilowatt hour also seem to use this pattern.

  • I think there is a distinction to be made, though. Units that are stuck together like kilowatt-hours are usually indicative of multiplication, that is "kilowatts x hours," since it's measuring how many kilowatts you're using per hour times the number of hours. mm Hg, on the other hand, is read "millimeters (of) mercury," and is merely indicative of a reading on a scale, as opposed to "mercury-millimeters." There's no combination of units, which is why I'm inclined to think that mm Hg spaced out would be more correct, at least in a scientific sense. – Schlockading Jul 10 '15 at 7:04
  • Yes, it does come down to reading a single measure. But the mercury component indicates the density of this chemical. If you mesured the same pressure with a column of water, it would be much higher. So there is still a multiplier. – Kim Ryan Jul 11 '15 at 0:31

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