What do the two blood pressure numbers actually mean?
Blood pressure is written as two figures because the pressure in your arteries changes with every heartbeat.
The top number is systolic pressure — the peak, measured when the heart contracts and ejects blood into the arteries.
The bottom number is diastolic pressure — the trough, measured when the heart relaxes between beats and refills. It reflects the resistance in the arteries while the heart is not pumping.
They are measured in millimetres of mercury (mmHg), a unit inherited from the original mercury column instruments.
What the numbers indicate:
Systolic rises with age as arteries stiffen, and is generally the stronger predictor of cardiovascular risk in older adults.
Diastolic reflects peripheral resistance in the small arteries.
The gap between them is the pulse pressure. A wide gap can indicate arterial stiffening.
General categories used in UK guidance, though thresholds differ between countries and guidelines — the US lowered its definition of hypertension in 2017, so figures you read may not be directly comparable:
Around 90/60 to 120/80 is usually considered the healthy range.
120/80 to 140/90 is often described as elevated or pre-hypertensive.
140/90 or above on repeated readings generally indicates hypertension. Home monitoring thresholds are set slightly lower.
Why single readings mislead. Blood pressure varies continuously with activity, stress, caffeine, temperature, time of day and posture. White coat hypertension — readings elevated specifically in a clinical setting — is common enough that home or 24-hour ambulatory monitoring is often used to confirm a diagnosis.
Why it matters. Hypertension is largely symptomless, which is why it is called a silent condition, while substantially raising the risk of stroke, heart attack, kidney disease and vascular dementia over time.
Any reading causing concern, or a very high reading with symptoms, should be discussed with a clinician — this is general information only.