Blood Pressure Units
Understanding mmHg Measurements
Learn the BasicsBlood pressure readings like "120/80" are given in millimeters of mercury (mmHg)—a unit dating back to early mercury barometers. Understanding what these numbers mean, why this unit persists in medicine, and how to interpret readings is essential for monitoring cardiovascular health.
Why Millimeters of Mercury?
Historical Origin
The first blood pressure measurements in the 1700s used mercury manometers—U-shaped tubes of mercury where blood pressure pushed the mercury column up. The height in millimeters became the standard unit.
Why It Persists
- Continuity: Decades of medical research uses mmHg
- Precision: Convenient numbers for typical readings (40-200)
- Clinical familiarity: All healthcare providers understand mmHg
- International standard: Used worldwide in medicine
Converting to other units would require rewriting medical guidelines, retraining clinicians, and replacing all existing documentation.
Blood Pressure Categories
| Category | Systolic (mmHg) | Diastolic (mmHg) | |
|---|---|---|---|
| Normal | Less than 120 | and | Less than 80 |
| Elevated | 120-129 | and | Less than 80 |
| High BP Stage 1 | 130-139 | or | 80-89 |
| High BP Stage 2 | 140+ | or | 90+ |
| Hypertensive Crisis | 180+ | and/or | 120+ |
Based on American Heart Association guidelines (2017).
Converting mmHg to Other Units
While medicine uses mmHg exclusively, you may need conversions for scientific or engineering contexts:
| BP Reading | mmHg | kPa | PSI | bar |
|---|---|---|---|---|
| Normal | 120/80 | 16.0/10.7 | 2.32/1.55 | 0.160/0.107 |
| High Stage 1 | 135/85 | 18.0/11.3 | 2.61/1.64 | 0.180/0.113 |
| High Stage 2 | 150/95 | 20.0/12.7 | 2.90/1.84 | 0.200/0.127 |
Conversion Formulas
- mmHg to kPa: Multiply by 0.1333
- mmHg to PSI: Multiply by 0.01934
- mmHg to bar: Multiply by 0.001333
Understanding the Numbers
Systolic Pressure (Top Number)
The pressure in arteries when your heart beats and pumps blood. It's the higher number because this is the maximum pressure in your circulatory system.
Diastolic Pressure (Bottom Number)
The pressure in arteries between heartbeats, when the heart rests and refills with blood. It's lower because this is the minimum pressure.
Pulse Pressure
The difference between systolic and diastolic (normally 40-60 mmHg). A very wide or narrow pulse pressure can indicate health issues.
Factors Affecting Blood Pressure
| Factor | Effect on BP |
|---|---|
| Exercise (immediate) | Increases temporarily |
| Exercise (long-term) | Decreases |
| Sodium intake | Increases |
| Stress | Increases |
| Caffeine | Increases temporarily |
| Alcohol (excess) | Increases |
| Age | Typically increases |
| Body position | Varies by position |
| Time of day | Lower at night, higher in morning |
Home Blood Pressure Monitors
Modern digital monitors display readings in mmHg, the same unit used clinically. Key features to look for:
- Validated accuracy: Look for clinically validated devices
- Proper cuff size: Too small or large gives inaccurate readings
- Memory function: Stores readings for tracking
- Averaging feature: Calculates average of multiple readings
Upper arm monitors are generally more accurate than wrist monitors.
Conclusion
Blood pressure is universally measured in millimeters of mercury (mmHg) due to historical convention and clinical practicality. Normal blood pressure is below 120/80 mmHg. Understanding both numbers—systolic (heart beating) and diastolic (heart resting)—helps you monitor cardiovascular health. While you can convert to other pressure units, medical contexts always use mmHg.