What do the two numbers in a blood pressure reading mean?
Systolic and diastolic, what each one is measuring, the NHS thresholds and why the clinic figure and the home figure are different, and why one reading proves nothing.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 4 min
- Safety
- caution
Short answer
The top number is systolic — the pressure in your arteries when the heart squeezes. The bottom is diastolic — the pressure that remains between beats. NHS guidance treats a reading as high blood pressure at 140/90 or higher when checked by a healthcare professional, or 135/85 or higher when checked at home. Those are two figures for the same condition, because people read lower at home.
It is the most common number anyone is given about their own body and hardly anyone is told what either half of it measures. Understanding the two numbers makes the readings legible, and understanding why the home and clinic thresholds differ prevents the most common misreading of all.
Safety
Step by step
- Understand the top number.Systolic pressure: the pressure in your arteries at the moment the heart contracts and pushes blood out. It is the higher of the two and it is the one that moves most with stress, activity and age.
- Understand the bottom number.Diastolic pressure: the pressure that remains in the arteries between beats, while the heart is relaxing and refilling. It is the lower number and it reflects the resting resistance in the circulation.
- Learn the two thresholds.NHS guidance: usually considered high blood pressure at 140/90 or higher when checked by a healthcare professional, and 135/85 or higher when checked at home. The home figure is lower because people read lower at home.
- Do not compare a home reading against the clinic figure.This is the commonest misreading. A home reading of 138/88 is not comfortably below 140/90 — it is above the home threshold of 135/85. Use the figure that matches where the reading was taken.
- Treat a single reading as one data point.Blood pressure varies through the day and rises with stress, pain, caffeine, a full bladder, talking, and a rushed arrival at the surgery. Diagnosis is normally made from repeated readings or a 24-hour ambulatory monitor, not from one number.
- Measure properly if you are measuring at home.Sitting, back supported, feet flat, arm supported at heart height, cuff on bare skin, five minutes of rest first, no caffeine or exercise beforehand, and no talking during it. Take two or three readings a minute apart and use the later ones.
- Use the free checks that exist.Pharmacies in England now offer free blood pressure checks. NHS advice for adults aged 40 or over is to have a check if they have not had one for more than five years, and the NHS Health Check covers it for eligible adults.
- Know what low means.Low blood pressure matters when it causes symptoms — dizziness, light-headedness on standing, fainting, blurred vision. A low number in someone who feels entirely well is usually not a problem, and it is the symptoms that make it one.
Tips
- Buy a monitor validated for clinical accuracy and use an upper-arm cuff rather than a wrist one. Cuff size matters: too small a cuff reads high.
- Write the readings down with the date and time, or use the monitor's memory. A pattern over a fortnight is what a clinician can use; a single alarming figure is not.
- Blood pressure is usually silent. "I would feel it if it were high" is the belief that keeps hypertension undiagnosed, and it is not true.
Common mistakes
- Repeating the measurement until you get a reading you like — Readings drift downwards with repetition as you relax, so taking six and keeping the lowest produces a number that means nothing. Follow a fixed protocol and record what it gives.
- Stopping medication because the readings are now normal — The readings are normal because of the medication. Stopping is a decision to take with the prescriber, never on the strength of a good week.
- Measuring over a sleeve — It affects the reading. The cuff goes on bare skin, on a supported arm at heart height.
If it doesn't work
One very high reading at the pharmacy
Cause: Could be genuine, could be white coat effect, could be the circumstances — Fix: The pharmacist will advise, and may suggest repeat or home readings, or refer you. Do not panic on one number and do not dismiss it either.
Readings much higher at the surgery than at home
Cause: White coat hypertension is a recognised effect — Fix: Say so, and ask about home or ambulatory monitoring. It is exactly why the home threshold exists.
Feeling dizzy on standing up
Cause: Possible postural drop in blood pressure, which can be caused by medication — Fix: Mention it to the GP practice or the pharmacist, particularly if you take blood pressure medication. Falls caused by this are common and the treatment is often adjustable.
Questions people ask
What is a normal reading?
There is a range rather than a single value, and what matters clinically is where you sit relative to the thresholds and what else is going on. The NHS publishes the thresholds for high blood pressure; interpreting your own numbers is a conversation with a clinician.
Does it matter which arm?
Readings can differ between arms. Clinicians measure both initially and then use the arm with the higher reading. If you have been told which arm to use, use that one.
Do smartwatches measure it accurately?
Wrist and wearable devices vary in accuracy and are not generally equivalent to a validated upper-arm monitor. For anything that matters clinically, use a proper cuff.
Is it different across the UK?
The physiology and the thresholds are the same. What differs is the services around it — free pharmacy checks and NHS Health Checks are organised differently in each nation.