Blood Pressure Units: mmHg Explained for Patients

Why We Still Measure Blood Pressure with a Unit from the 1890s

Mercury thermometers are banned in most hospitals. Mercury sphygmomanometers -- the blood pressure devices with a glass column of liquid mercury -- have been phased out across the EU since 2009 and are disappearing elsewhere. Yet every doctor on the planet still reports blood pressure in "mmHg," millimeters of mercury. A reading of 120/80 mmHg refers to how high a mercury column would rise under that pressure, even though the actual mercury is long gone from the exam room.

The persistence of mmHg is not inertia. Decades of clinical trials, drug dosing guidelines, and diagnostic thresholds are all calibrated to these numbers. Switching to kilopascals would mean retraining millions of clinicians and risking dangerous misinterpretations during the transition. So mmHg endures -- and understanding what it actually means makes you a more informed patient.

What mmHg Measures

mmHg stands for "millimeters of mercury." One mmHg is the pressure exerted by a 1-millimeter column of mercury at 0 degrees Celsius under standard gravity.

The technical conversions:

• 1 mmHg = 133.322 pascals (Pa)
• 1 mmHg = 0.133322 kilopascals (kPa)
• 760 mmHg = 1 atmosphere (standard atmospheric pressure at sea level)

For quick conversions, try our mmHg to Pascal converter. You can also convert to other pressure units with our mmHg to atmosphere converter.

How Mercury Blood Pressure Devices Worked

A Brief History

The story starts earlier than you might think. In 1733, English clergyman Stephen Hales made the first blood pressure measurement by inserting a glass tube directly into a horse's artery and watching how high the blood rose. The horse survived, reportedly.

The milestones that led to modern measurement:

1881 - Samuel von Basch created the first sphygmomanometer that did not require cutting into the patient.

1896 - Italian physician Scipione Riva-Rocci introduced the mercury column design with an inflatable cuff -- the same concept used for over a century afterward.

1905 - Russian surgeon Nikolai Korotkoff developed the auscultatory method: using a stethoscope to listen for specific sounds (now called Korotkoff sounds) that indicate systolic and diastolic pressure.

Why Mercury Became the Standard

Mercury hit a sweet spot of physical properties:

1. Density: At 13.6 times denser than water, a short column measures significant pressure
2. Visibility: The silvery liquid reads clearly against a graduated scale
3. Consistency: Mercury does not compress, so readings are reliable
4. Low maintenance: Properly maintained devices rarely need recalibration

What Replaced Mercury

Environmental and toxicity concerns drove the switch. Today's clinical settings use:

• Aneroid sphygmomanometers (mechanical dial gauges)
• Automated oscillometric devices (digital monitors)

Both still display results in mmHg. The unit outlived the substance.

Systolic and Diastolic: What the Two Numbers Mean

A reading of 120/80 mmHg (spoken "120 over 80") has two components:

Systolic Pressure (Top Number)

The pressure in your arteries when your heart contracts and pumps blood out. This is the peak pressure your vessels experience -- the "push" phase. It reflects how hard your heart is working and the condition of your large arteries.

Diastolic Pressure (Bottom Number)

The pressure between heartbeats, when your heart is resting and refilling. This is the minimum pressure -- the "rest" phase. It reflects resistance in your blood vessels and peripheral artery health.

Why Both Matter

Either number being too high signals cardiovascular risk. Research published in the New England Journal of Medicine (the SPRINT trial) found that targeting a systolic pressure below 120 mmHg reduced major cardiovascular events by 25% compared to the standard target of 140 mmHg. Diastolic readings still carry diagnostic weight, especially in younger adults.

Blood Pressure Categories

The American Heart Association defines these ranges:

Normal: Healthy range. Worth maintaining with diet and exercise.

Elevated: The warning zone. Lifestyle changes -- cutting sodium, increasing activity, managing stress -- can prevent progression.

Stage 1: A doctor may recommend medication depending on your overall cardiovascular risk profile.

Stage 2: Usually requires medication alongside lifestyle changes.

Hypertensive Crisis: Seek immediate medical attention, especially with symptoms like chest pain, shortness of breath, or vision changes.

Converting mmHg to Other Pressure Units

Some countries use kilopascals (kPa) for blood pressure. France, Portugal, and parts of Scandinavia report readings in kPa. The conversion:

mmHg to kPa: Divide by 7.5 kPa to mmHg: Multiply by 7.5

Our Pascal to mmHg converter handles these conversions in either direction. For converting to other pressure units like bar or psi, see our mmHg to bar converter.

Quick Reference

Full Reading Conversion

120/80 mmHg in kPa:

• Systolic: 120 / 7.5 = 16.0 kPa
• Diastolic: 80 / 7.5 = 10.7 kPa
• Result: 16.0/10.7 kPa

Why Most Countries Stick with mmHg

Despite kPa being the "proper" SI unit, mmHg dominates for practical reasons:

1. Clinical data: Decades of drug trials, dosing protocols, and treatment thresholds reference mmHg values
2. International communication: Medical literature overwhelmingly uses mmHg
3. Familiarity: Both clinicians and patients recognize what "140/90" means
4. Granularity: mmHg provides finer gradations for clinical decisions

Countries using kPa adopted it as part of broader metrication efforts, but they remain the minority in medical practice.

Getting Accurate Readings

Measurement technique matters more than most people realize. The difference between a relaxed reading and a stressed one can be 20-30 mmHg -- enough to cross from "normal" into "Stage 1 hypertension."

Before Measuring

• Skip caffeine, smoking, and exercise for 30 minutes beforehand
• Empty your bladder (a full bladder can raise readings by 10-15 mmHg)
• Sit quietly for 5 minutes first
• Do not talk during the measurement

Positioning

• Back supported, feet flat on floor, legs uncrossed
• Arm supported at heart level on a table
• Cuff placed 2-3 cm above the elbow crease
• Cuff bladder should cover 80% of upper arm circumference (wrong size means wrong reading)

Best Practices

• Take 2-3 readings, 1-2 minutes apart, and average them
• Measure at the same time each day
• Keep a written log with dates and notes
• Use a validated monitor (check the STRIDE BP validation list)

Common Pitfalls

• White coat syndrome: Anxiety in medical settings inflates readings by 10-30 mmHg in some patients
• Arm position: Too high reads low; too low reads high
• Wrong cuff size: Too small reads high; too large reads low
• Recent meals: Eating can temporarily affect readings

Combined Reference Table

The Bottom Line

mmHg persists as the blood pressure unit because the cost of switching outweighs the benefit. What matters for your health is not the unit -- it is knowing your numbers, measuring correctly, and acting on readings that fall outside the normal range. If your doctor says "130 over 85," you now know exactly what that means, how it compares to healthy targets, and why it is still reported in millimeters of a metal that is no longer in the room.

If you have concerns about your blood pressure, consult a qualified healthcare professional who can provide guidance based on your complete medical history.