Understand what systolic and diastolic blood pressure numbers mean, how to measure accurately, and what ranges indicate for your health.
A Brief History of Blood Pressure Measurement
The story of blood pressure measurement spans centuries. English physician William Harvey described blood circulation in 1628, establishing that the heart pumps blood through the body in a continuous loop. However, measuring the actual force of that blood against artery walls came much later.
In 1733, English clergyman and scientist Stephen Hales performed the first direct blood pressure measurement by inserting a brass pipe into the artery of a horse and measuring how high the blood rose in a glass tube — an invasive and impractical technique for humans.
The modern era began in 1896 when Italian physician Scipione Riva-Rocci invented the mercury sphygmomanometer — an inflatable arm cuff connected to a mercury column. Then in 1905, Russian physician Nikolai Korotkoff discovered that by listening through a stethoscope to the sounds made as the cuff deflates, you could measure both the systolic and diastolic pressures without breaking the skin. This non-invasive technique, refined with electronic sensors today, remains the foundation of every blood pressure measurement you receive.
What the Two Numbers Mean
Systolic (top number): The pressure in your arteries when the heart muscle contracts and pumps blood outward. This is the peak pressure in each heartbeat cycle.
Diastolic (bottom number): The pressure in your arteries between heartbeats, when the heart muscle is relaxing and refilling. This is the resting pressure between beats.
mmHg: Millimetres of mercury — the unit used to express blood pressure, a legacy of the original mercury column devices.
Blood Pressure Categories (AHA Guidelines)
| Category | Systolic | Diastolic | Action |
|---|---|---|---|
| Normal | < 120 | < 80 | Maintain healthy habits |
| Elevated | 120 – 129 | < 80 | Lifestyle changes recommended |
| High — Stage 1 | 130 – 139 | 80 – 89 | Consult a doctor; possible medication |
| High — Stage 2 | ≥ 140 | ≥ 90 | Medical treatment likely needed |
| Hypertensive Crisis | ≥ 180 | ≥ 120 | Seek emergency care immediately |
Health Risks of High Blood Pressure (Hypertension)
- 1
Heart attack and coronary artery disease: Persistently high pressure damages the inner lining of arteries, making them stiff and narrow. This forces the heart to work harder and can trigger a complete blockage leading to heart attack.
- 2
Stroke: High blood pressure is the single largest risk factor for stroke. It can cause arteries in the brain to rupture (haemorrhagic stroke) or become blocked by clots (ischaemic stroke).
- 3
Kidney damage (hypertensive nephropathy): The kidneys rely on a network of tiny blood vessels. Chronic high pressure thickens and narrows these vessels, impairing the kidney's ability to filter waste, eventually leading to chronic kidney disease.
- 4
Heart failure: The heart compensates for high pressure by enlarging and thickening its walls. Over time this makes the heart muscle stiff and less efficient, leading to heart failure — when the heart can no longer pump enough blood to meet the body's needs.
- 5
Vision loss (hypertensive retinopathy): High pressure damages the blood vessels in the retina, causing them to leak or thicken. This can lead to blurred vision, bleeding in the eye, and in severe cases, blindness.
- 6
Aortic aneurysm: Chronically elevated pressure can weaken the walls of the aorta — the body's main artery — causing it to bulge (aneurysm). A ruptured aortic aneurysm is life-threatening.
Health Risks of Low Blood Pressure (Hypotension)
- 1
Dizziness and fainting (syncope): When blood pressure drops suddenly — especially when standing up quickly (orthostatic hypotension) — blood flow to the brain temporarily decreases, causing lightheadedness or fainting.
- 2
Shock: Severely low blood pressure deprives organs of oxygen and nutrients. This can lead to organ damage and is a medical emergency requiring immediate treatment.
- 3
Falls and injury: Sudden drops in blood pressure, particularly in older adults, dramatically increase the risk of falls, which can cause fractures and head injuries.
- 4
Heart and brain complications: Prolonged inadequate blood flow can cause heart attack, stroke, or kidney failure even at low pressure readings, particularly in people with pre-existing conditions.
Tips for Accurate Home Measurement
Sit quietly for 5 minutes before measuring — physical activity temporarily raises blood pressure.
Avoid caffeine, alcohol, and tobacco for at least 30 minutes before measuring.
Keep your arm at heart level and supported; let it rest on a table rather than holding it up.
Take two or three readings one minute apart and record the average.
Measure at the same time each day — ideally morning and evening — for consistent tracking.
Use a validated upper-arm monitor; wrist devices are less reliable for most people.
Do not talk during the measurement — speaking can raise readings by up to 10 mmHg.
How to Read Your Blood Pressure
Know the Two Numbers
A blood pressure reading shows two numbers, e.g. 120/80 mmHg. The top number (systolic) is the pressure when your heart beats. The bottom number (diastolic) is the pressure between beats.
Prepare Correctly Before Measuring
Sit quietly for at least 5 minutes before taking a reading. Avoid caffeine, exercise, and smoking for 30 minutes beforehand. Sit with your back supported, feet flat on the floor, and your arm at heart level.
Place the Cuff Correctly
Wrap the cuff snugly around your bare upper arm, one finger-width above the elbow crease. The tube should run down the inside of your arm. A poorly positioned cuff can give inaccurate readings.
Take Multiple Readings
Take two or three readings one minute apart and use the average. Blood pressure naturally varies throughout the day, so a single measurement may not be representative.
Look Up Your Category
Compare your reading against the AHA table above. If your reading is consistently in the Elevated, Stage 1, or Stage 2 range — or if you see any reading ≥ 180/120 — contact a healthcare professional.
Try our free tool
Blood Pressure Chart
Frequently Asked Questions
Q: Which arm should I use?
A: Use the arm your doctor recommends, or measure both arms on your first check. If there is a consistent difference, always use the arm with the higher reading.
Q: What causes a high reading at the doctor's office?
A: This is called "white coat hypertension" — anxiety raises blood pressure temporarily. Home readings are often more accurate. Use a validated home monitor and keep a log.
Q: How often should I check my blood pressure?
A: If your levels are normal and you have no risk factors, once a year at check-ups is sufficient. If you have high blood pressure or are on medication, your doctor may recommend daily monitoring.