This article is for general educational purposes only and is not a substitute for medical advice. If you have concerns about your blood pressure, consult a qualified healthcare provider.
Why blood pressure uses mmHg
Blood pressure is measured in millimetres of mercury (mmHg) — a unit that dates back to the invention of the mercury sphygmomanometer by Scipione Riva-Rocci in 1896. A cuff was inflated to stop blood flow; as pressure was released, the column of mercury fell and the researcher listened for the pulse to resume. The height of mercury when the first pulse appeared was the systolic pressure; when all pulses disappeared, the diastolic.
Modern monitors use digital pressure sensors rather than mercury (which is now banned or restricted in medical devices in many countries), but the mmHg unit persists for consistency with over a century of clinical data.
To convert between mmHg and other pressure units (kPa, Pa, bar, PSI, inHg), use the Pressure Converter. For reference: 1 mmHg ≈ 0.133 kPa and 1 mmHg ≈ 0.01934 PSI.
The two numbers: systolic and diastolic
A blood pressure reading is written as systolic / diastolic (e.g., “120/80 mmHg”, read as “120 over 80”):
Systolic pressure (the higher number): the pressure in your arteries when the heart muscle contracts and pumps blood out. This is the peak pressure in the arterial cycle.
Diastolic pressure (the lower number): the pressure in your arteries between heartbeats, when the heart is at rest and refilling. This is the minimum pressure during the cycle.
Both numbers matter. You can have normal systolic and high diastolic pressure (isolated diastolic hypertension), or high systolic with normal diastolic (isolated systolic hypertension — more common in older adults as arterial walls stiffen). Either pattern carries cardiovascular risk.
Blood pressure categories (AHA/ACC 2017 guidelines)
The American Heart Association and American College of Cardiology revised the blood pressure guidelines in 2017:
| Category | Systolic | Diastolic | |
|---|---|---|---|
| Normal | < 120 mmHg | AND | < 80 mmHg |
| Elevated | 120–129 mmHg | AND | < 80 mmHg |
| High blood pressure Stage 1 | 130–139 mmHg | OR | 80–89 mmHg |
| High blood pressure Stage 2 | ≥ 140 mmHg | OR | ≥ 90 mmHg |
| Hypertensive crisis | > 180 mmHg | AND/OR | > 120 mmHg |
| Low blood pressure | < 90 mmHg | AND | < 60 mmHg |
Elevated blood pressure (120–129 systolic, below 80 diastolic) is a new category introduced in 2017. It doesn’t require medication but indicates lifestyle changes are needed to prevent progression to hypertension.
Hypertensive crisis (≥180/≥120) is a medical emergency. Seek immediate care if blood pressure reaches this level, especially with symptoms like severe headache, chest pain, shortness of breath, or vision changes.
Why mmHg is specifically used for clinical ranges
Blood pressure ranges (like 120/80) are defined in mmHg because all clinical research, drug trials, and intervention thresholds have been established using mmHg. Translating into other units for clinical contexts would introduce confusion.
However, knowing the SI equivalents is useful in physics or engineering contexts:
- 120 mmHg ≈ 16.0 kPa ≈ 2.32 PSI ≈ 0.158 atm
- 80 mmHg ≈ 10.7 kPa ≈ 1.55 PSI ≈ 0.105 atm
What affects blood pressure readings
Blood pressure is not a fixed number — it fluctuates throughout the day in response to:
- Physical activity: Rises significantly during exercise, drops during rest
- Stress and anxiety: The sympathetic nervous system raises BP (this drives “white coat hypertension”)
- Sleep: BP dips 10–20% during sleep (“nocturnal dipping”) — absence of this dip is associated with higher cardiovascular risk
- Posture: Standing from lying down can cause a brief drop (orthostatic hypotension)
- Temperature: Cold causes vasoconstriction and temporarily raises BP
- Caffeine: Can temporarily raise BP by 5–10 mmHg
- Sodium intake: Chronic high-sodium diet is associated with sustained higher BP
- Time of day: BP follows a circadian rhythm — lowest in early morning, rising steeply before waking, with a second peak mid-afternoon
How to get an accurate reading at home
- Sit quietly for 5 minutes before measuring
- Sit with your back supported, feet flat on the floor, arm resting at heart level
- Do not talk, exercise, or use caffeine/nicotine for 30 minutes before measuring
- Take 2–3 readings 1–2 minutes apart and average them
- Measure at the same time each day (morning before medication; evening before bed)
- Keep a log of readings with dates and times for your provider
A validated upper-arm automatic monitor is more accurate than wrist monitors, which are sensitive to wrist position.
The difference between pulse pressure and mean arterial pressure
Pulse pressure is systolic minus diastolic: for 120/80, that’s 40 mmHg. A wide pulse pressure (≥60 mmHg) indicates arterial stiffness and is associated with increased cardiovascular risk, particularly in older adults.
Mean arterial pressure (MAP) ≈ diastolic + (systolic − diastolic)/3. It represents the average pressure driving blood through the body. A MAP below 60 mmHg indicates inadequate organ perfusion. For 120/80: MAP = 80 + 40/3 ≈ 93 mmHg.
How to convert blood pressure to other pressure units
If you need to work with mmHg alongside other pressure units in a physics, engineering, or altitude context, use the Pressure Converter. It converts between Pascal, kPa, MPa, bar, mbar, PSI, atm, mmHg, Torr, inHg, and inH₂O — all from a single value. Note: while mmHg in blood pressure is measured at body temperature (37°C), the standard technical definition of 1 mmHg is based on 0°C mercury — for clinical purposes, the difference is negligible.