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Understanding readings

What do systolic and diastolic blood pressure numbers mean?

A plain-language guide to the top and bottom numbers in a blood pressure reading, plus how to record them accurately.

By BpBook team4 min read

A blood pressure reading has two numbers, usually written like 120/80 mmHg. The first is systolic pressure and the second is diastolic pressure. They describe different moments in the heartbeat, so both belong in your record.

Systolic pressure: the top number

Systolic pressure is the pressure in your arteries when the heart contracts and pumps blood. It is written first. For example, the systolic value in 120/80 is 120 mmHg.

Diastolic pressure: the bottom number

Diastolic pressure is the pressure between beats, while the heart relaxes and fills. It is written second. In 120/80, the diastolic value is 80 mmHg.

The two values can move in different ways. That is why recording only one number loses useful context. Add the pulse if your monitor displays it, along with the date, time and a note about anything unusual.

How should you use the numbers?

A single home reading is a snapshot, not a diagnosis. Blood pressure changes with activity, stress, sleep, caffeine and posture. Use the categories and thresholds discussed by your healthcare professional rather than trying to diagnose yourself from an app or calculator.

For a quick summary, you can use the blood pressure average calculator, but keep the individual readings too. Patterns are easier to review when the original values and circumstances are available.

Write a reading down correctly

  • Keep the numbers in the same order: systolic/diastolic.
  • Include the unit if you are sharing a result: mmHg.
  • Record the time and which arm you used.
  • Note if you had recently exercised, smoked, had caffeine or felt unwell.
  • Follow the same resting and cuff routine each time.

If a reading worries you, repeat it only after resting as instructed and contact your healthcare professional for advice. If you feel seriously unwell or believe you may be experiencing a medical emergency, seek urgent local help.

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Sources

Sources last checked 23 September 2026.