Millimeter Mercury to Meter Water (4 °C)

Clinical medicine is deeply conservative about units because misreadings kill people. Doctors, nurses, and patients worldwide have memorized "120/80 is normal" in mmHg. Converting to kPa (16.0/10.7) would require retraining millions of clinicians and rewriting every guideline. The WHO considered the switch and decided the risk of transcription errors during transition outweighed the elegance of SI compliance. So mmHg stays — likely for decades more.

The top number (systolic) is the peak pressure when the heart contracts and pushes blood into the arteries — typically 90–120 mmHg. The bottom number (diastolic) is the lowest pressure between beats when the heart relaxes — typically 60–80 mmHg. A reading of 140/90 mmHg or above is classified as hypertension. The gap between the two (pulse pressure) also matters: a wide gap above 60 mmHg may signal stiff arteries.

In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish of mercury, and watched the column drop to about 760 mm. The empty space above was the first laboratory vacuum. The height of the mercury column became the measurement of atmospheric pressure — 760 mmHg at sea level. Nearly 400 years later, we still use his column height as a pressure unit in medicine and vacuum science.

For all practical purposes, they are identical — 1 torr = 1/760 atm ≈ 133.322 Pa, and 1 mmHg ≈ 133.322 Pa. The difference is about 0.00015% and arises from the torr being defined from the atmosphere while mmHg is defined from mercury density. Medicine uses mmHg; vacuum physics uses torr. They are interchangeable in any real-world measurement.

Intraocular pressure (glaucoma screening): normal is 10–21 mmHg, above 21 is suspicious. Partial pressure of oxygen in arterial blood (PaO₂): normal is 80–100 mmHg. Central venous pressure: 2–6 mmHg. Intracranial pressure: normal below 15 mmHg, dangerous above 20 mmHg. Carbon dioxide in blood (PaCO₂): 35–45 mmHg. The unit pervades clinical monitoring far beyond the blood pressure cuff.

Because pump engineers think in terms of how high the pump can lift water. A pump rated at 30 mH₂O can push water 30 meters straight up — no conversion needed to figure out if it can reach the tenth floor. The unit also makes friction-loss calculations intuitive: if a 100-meter horizontal pipe run has 5 mH₂O of friction loss, you subtract that directly from the pump's head rating.

Exactly 1 meter. That is the beauty of this unit — depth in meters of fresh water equals gauge pressure in mH₂O (seawater is about 2.5% denser, so 1 m depth = ~1.025 mH₂O). A 10-meter pool exerts 10 mH₂O at the bottom, which is why your ears hurt at the deep end. Divers experience roughly 10 mH₂O of additional pressure for every 10 meters of descent.

Municipal water mains deliver 20–60 mH₂O (roughly 2–6 bar or 30–85 psi) at the meter. A gravity-fed rooftop tank 10 meters above the tap provides about 10 mH₂O — barely enough for a decent shower, which is why booster pumps are common in buildings with rooftop storage. High-rise buildings need pressurisation systems because gravity alone cannot push water above about 60 mH₂O without boosting.

10.33 mH₂O ≈ 1 atmosphere ≈ 1.013 bar. For quick math: 10 mH₂O ≈ 1 bar (error about 2%). This rule of thumb is used constantly in plumbing and fire protection: a building with a water tank 40 m above ground level has roughly 4 bar of static pressure at the base. Multiply meters by 0.1 and you have bar — close enough for pipe sizing.

Water is densest at 3.98 °C, which gives a reproducible standard: at 4 °C, a 1-meter column of water exerts exactly 9,806.38 Pa. At 20 °C the density drops by ~0.2%, and at 80 °C by ~2.8%. For pump and plumbing work the difference is trivial, but calibration laboratories and instrument manufacturers specify 4 °C to maintain traceability across measurements worldwide.