What Do Blood Pressure Numbers Mean?

Blood pressure is the force of blood pushing against artery walls, measured in millimetres of mercury and written as two numbers. It is among the most useful measurements in medicine precisely because raised blood pressure produces no symptoms at all until it has already done damage.

The two numbers

Systolic, the top and higher number, is the pressure when the heart contracts and pushes blood out.

Diastolic, the bottom and lower number, is the pressure between beats, when the heart is relaxed and refilling.

A reading of 120/80 means systolic 120, diastolic 80, and is spoken as 120 over 80.

The ranges

Reading	Category
Below 90/60	Low
90/60 to 120/80	Ideal
120/80 to 140/90	Raised, sometimes called pre-hypertension
140/90 or above	High blood pressure
180/120 or above	Very high, needs urgent attention

A reading qualifies for a category if either number falls into it. Someone at 135/95 has high blood pressure on the diastolic number alone.

Home readings are interpreted slightly lower than clinic readings, with 135/85 at home treated as roughly equivalent to 140/90 in a surgery, because the clinical setting itself tends to raise the figure.

Why it matters

Sustained high blood pressure damages arteries slowly and silently. It is a major risk factor for stroke, heart attack, heart failure, kidney disease, vascular dementia and sight loss.

The relationship is continuous rather than a threshold. Risk begins rising well below the diagnostic cut-off, which is why the raised category exists even though it is not usually treated with medication.

The important point is that it produces no symptoms. Headaches, nosebleeds and dizziness are not reliable signs, and someone can have a reading of 170/100 and feel entirely well. The only way to know is to measure.

What makes a reading falsely high

A great deal, which is why single readings are not used for diagnosis.

• White coat effect. The clinical setting raises blood pressure in a substantial proportion of people, sometimes by 20 points or more.
• Caffeine in the previous half hour.
• Talking during the measurement, which can add several points.
• A full bladder, which can add up to 10.
• Crossed legs, an unsupported arm, or an arm below heart level.
• The wrong cuff size. A cuff too small for the arm gives a falsely high reading, and this is a common error.
• Recent exercise, cold, stress or having rushed to the appointment.

For a reliable reading: sit quietly for five minutes, feet flat, back supported, arm resting at heart level, no talking, and take two or three readings a minute apart, discarding the first.

When a diagnosis is made

A single raised reading is not enough. UK guidance is to confirm with either ambulatory monitoring, where a device takes readings over 24 hours, or a week of home readings taken twice daily.

That approach exists specifically because clinic readings alone over-diagnose, and because the reverse also occurs: masked hypertension, where readings are normal in the surgery and high in ordinary life.

What lowers it without medication

The evidence here is strong, and the effects are additive.

Reducing salt. Most people eat well above the recommended limit, and the majority of it comes from processed food rather than the salt cellar. Cutting intake produces measurable reductions within weeks.

Losing excess weight. One of the most effective single changes, with each kilogram lost associated with a small but consistent fall.

Regular aerobic exercise. 150 minutes a week of moderate activity lowers readings independently of weight loss.

Moderating alcohol. Regular heavy drinking raises blood pressure substantially, and reducing it lowers readings within weeks.

Stopping smoking. Each cigarette raises pressure acutely, and smoking damages arteries in ways that compound the risk.

More potassium, from vegetables, fruit, beans and nuts, which partly offsets the effect of sodium.

Together these can produce reductions comparable to a single medication, which is why they are recommended first for raised readings and alongside treatment for high ones.

Low blood pressure

Readings below 90/60 are only a problem if they cause symptoms: dizziness on standing, fainting, blurred vision, unusual fatigue. Many healthy, fit people run naturally low and feel fine.

Symptomatic low blood pressure is worth investigating, since it can be caused by dehydration, medication, or an underlying condition.

How often to check

Adults should have blood pressure checked at least every five years, and more often if it has been raised, if there is a family history, or once over 40. Checks are available free at GP surgeries and many pharmacies.

Home monitors are inexpensive and reasonably accurate if validated, used correctly and fitted with the right cuff size. A record of home readings over a week is more useful to a doctor than any single measurement taken in a surgery.

Pulse pressure and what the gap means

The difference between the two numbers is called pulse pressure, and it carries information of its own.

A reading of 120/80 has a pulse pressure of 40, which is typical. A wide gap, such as 160/70, generally indicates stiffening of the large arteries, which happens naturally with age and more quickly with untreated high blood pressure.

Isolated systolic hypertension, where the top number is raised while the bottom stays normal, is the most common pattern in people over 60 for exactly this reason. It is treated, because the raised systolic figure carries the risk regardless of what the diastolic number does.