Medical disclaimer: This article is for educational purposes only. Always speak to a qualified healthcare professional about your specific readings and health circumstances.
What blood pressure actually measures
Blood pressure is the force your blood exerts against the walls of your arteries as your heart pumps it around your body. Think of it like water pressure in a hosepipe: if the pressure is too high for too long, the walls of the hose — or in this case, your arteries — are placed under strain.
High blood pressure (hypertension) is often called "the silent killer" precisely because it rarely causes noticeable symptoms. You can have dangerously elevated blood pressure for years without feeling unwell, which is why regular monitoring matters enormously — especially as a family, since hypertension has a significant genetic component.
The two numbers explained
Every blood pressure reading gives you two numbers separated by a forward slash, such as 120/80. They measure two different moments in your heart's cycle:
- Systolic (the top number) — the pressure in your arteries when your heart contracts and pushes blood outward. This is the higher of the two numbers and the one clinicians tend to watch most closely for cardiovascular risk.
- Diastolic (the bottom number) — the pressure when your heart is resting between beats and refilling with blood. This reflects how relaxed your arterial walls are between contractions.
Both numbers are measured in millimetres of mercury (mmHg), a unit inherited from the mercury-column devices used in early blood pressure measurement.
Normal ranges — and what they mean
The following ranges are based on current clinical guidelines from NHS England and major cardiovascular bodies. They apply to adults at rest; blood pressure naturally rises during physical activity, stress, or cold temperatures.
- Optimal: Below 120/80 mmHg
- Normal: 120–129 / less than 80 mmHg
- Elevated: 130–139 / 80–89 mmHg (previously called pre-hypertension)
- High (Stage 1): 140–159 / 90–99 mmHg
- High (Stage 2): 160+ / 100+ mmHg
- Hypertensive crisis: 180+ / 120+ mmHg — seek medical attention immediately
Blood pressure also tends to rise gradually with age. A reading that would be considered normal for a 40-year-old may warrant monitoring in a 25-year-old. Context matters, which is why logging readings over time — rather than treating each result in isolation — gives you and your doctor far more useful information.
When to speak to a doctor
A single elevated reading is not necessarily cause for alarm. Blood pressure fluctuates throughout the day based on activity, caffeine, stress, and even the position you are sitting in. One high reading at the GP surgery — often elevated simply from the anxiety of being there (known as "white coat hypertension") — does not mean you have hypertension.
Speak to your GP if you have consistently elevated readings over several weeks, if your diastolic regularly exceeds 90, if you experience headaches, visual disturbances, chest pain, or shortness of breath alongside elevated readings, or if there is a family history of cardiovascular disease.
A reading above 180/120 is a hypertensive crisis. If accompanied by symptoms such as chest pain, back pain, difficulty breathing, or changes in vision, call emergency services immediately.
Tracking blood pressure at home
Home monitoring is increasingly recommended by clinicians because it provides a more accurate picture than occasional surgery readings. When measuring at home: rest for five minutes before reading, sit with your back supported and your feet flat on the floor, keep your arm at heart height, and avoid caffeine or strenuous activity in the 30 minutes before.
Take two readings, two minutes apart, at the same time each day (morning and evening is ideal). Record every reading with the date and time. Over a few weeks, you will have a genuinely useful dataset that your doctor can use to make informed decisions — far better than a single clinic reading once every six months.
A dedicated health records app that logs vitals with timestamps makes this process seamless, and gives you the kind of longitudinal view that single-visit measurements simply cannot provide.