Blood pressure is one of the most important indicators of cardiovascular health, yet most people don't fully understand what the two numbers mean or when they should act on the results. High blood pressure (hypertension) affects around 1 in 3 UK adults and significantly raises the risk of heart attacks, strokes, and kidney disease — yet it causes almost no symptoms until serious damage has occurred. This guide explains exactly how to read, understand, and act on a blood pressure reading.
What the Two Numbers Mean
Blood pressure is always given as two numbers: systolic/diastolic — for example, 120/80 mmHg.
- Systolic pressure (top number): The pressure in your arteries when your heart beats and pumps blood out. This is the higher number and represents peak arterial pressure.
- Diastolic pressure (bottom number): The pressure when your heart is resting between beats, as it fills with blood. This represents baseline arterial pressure.
- mmHg: Millimetres of mercury — the unit used to measure pressure in blood pressure monitors.
Blood Pressure Categories (NHS / NICE Guidelines)
| Category | Systolic | Diastolic | What It Means |
|---|---|---|---|
| Optimal | Below 120 | Below 80 | Ideal for cardiovascular health |
| Normal | 120–129 | 80–84 | Normal range — maintain with lifestyle |
| High-normal | 130–139 | 85–89 | Monitor closely — lifestyle changes recommended |
| Stage 1 Hypertension | 140–159 | 90–99 | Medical review recommended |
| Stage 2 Hypertension | 160–179 | 100–109 | Medical treatment usually required |
| Stage 3 / Severe | 180+ | 110+ | Urgent medical attention needed |
| Low (Hypotension) | Below 90 | Below 60 | May cause dizziness; investigate if symptomatic |
| Hypertensive Crisis | 180+ | 120+ | Call 999 / emergency care immediately |
Which Number Matters More?
Both numbers are important, but their relative significance changes with age:
- In people under 50, diastolic pressure is more predictive of cardiovascular risk.
- After 50, systolic pressure becomes more important, as arteries naturally stiffen.
- Isolated systolic hypertension (high systolic, normal diastolic) is the most common type in people over 65.
Either number in the hypertensive range warrants clinical attention, regardless of the other.
Why Blood Pressure Varies
Blood pressure is not static — it fluctuates throughout the day based on activity, stress, temperature, and posture. Normal variation includes:
- Time of day: Typically lower at night during sleep, rising sharply in the morning (the "morning surge") — this is when most heart attacks and strokes occur.
- Exercise: Systolic pressure rises significantly during exercise (up to 200+ mmHg) — this is normal and healthy.
- Emotion and stress: "White coat hypertension" — elevated readings caused purely by the anxiety of being measured at a GP surgery — affects up to 20% of patients.
- Posture: Standing from sitting can cause a brief drop (orthostatic hypotension).
- Arm position: The cuff must be at heart level for an accurate reading.
How to Measure Blood Pressure Accurately at Home
Home monitoring is often more accurate than single clinic readings and is recommended by NICE for confirming a diagnosis of hypertension. Follow these steps:
- Rest for 5 minutes before measuring — sitting quietly, no phone use.
- Use a validated upper-arm monitor (wrist monitors are less accurate). Validated models are listed on the British Hypertension Society website.
- Sit with back supported, feet flat, arm at heart level resting on a table.
- Don't talk, eat, or have caffeine in the 30 minutes before measuring.
- Take two readings 1–2 minutes apart. Discard the first; record the second.
- Measure at the same time each day (morning before medication, evening before bed) for 7 consecutive days.
- Ignore day 1 readings when calculating your average (they're often higher).
The average of your home readings is what your doctor uses to guide treatment decisions.
Lifestyle Factors That Reduce Blood Pressure
For most people with Stage 1 hypertension, lifestyle changes can reduce blood pressure by 5–15 mmHg — often enough to avoid medication:
- Salt reduction: Each 1g/day reduction in salt intake reduces systolic BP by approximately 3–5 mmHg. The UK average intake is 8g/day; the target is 6g/day.
- Exercise: 150 minutes of moderate aerobic exercise per week reduces systolic by 5–8 mmHg on average.
- Weight loss: Each 1 kg of weight lost reduces systolic by approximately 1 mmHg.
- Alcohol reduction: Heavy drinking significantly raises BP; reducing to within recommended limits (14 units/week) improves readings.
- DASH diet: Rich in fruits, vegetables, whole grains, and low-fat dairy — shown to reduce systolic by 8–14 mmHg.
- Stopping smoking: Each cigarette causes a temporary spike; long-term smoking damages arterial walls.
Summary
Blood pressure is expressed as systolic/diastolic mmHg. Optimal is below 120/80. Consistent readings above 140/90 constitute hypertension and warrant medical review. Single readings can be misleading — use home monitoring over 7 days for a reliable picture. Lifestyle changes (salt, exercise, weight) can reduce blood pressure meaningfully for most people before medication is needed.