A blood pressure reading such as 120/80 has two numbers: the top one (systolic) is the pressure in your arteries when the heart beats, and the bottom one (diastolic) is the pressure between beats. Both are measured in millimetres of mercury, written mmHg, and read aloud as "120 over 80". Normal is below 120 and below 80; high starts at 130 or 80 in US guidance and at 140 or 90 in European and NHS guidance.
What does the top number (systolic) mean?
The systolic number is the peak pressure each heartbeat pushes into the arteries. It is the number that rises most with age and the one guidelines use first to define hypertension. A systolic reading of 130 or more counts as high in the US categories; 140 or more is high everywhere. Systolic pressure also swings the most during the day, so it is the number that looks alarming after coffee, a brisk walk or a stressful call, and the reason a single reading proves little.
What does the bottom number (diastolic) mean?
The diastolic number is the lowest pressure in the arteries while the heart refills between beats. It reflects the resistance in the small vessels. Diastolic pressure tends to rise until about age 50 and then drift down as the large arteries stiffen. A diastolic of 80 or more is high in US guidance, 90 or more in European and NHS guidance. A diastolic below 60 in an older adult is worth a conversation with a clinician, especially if it comes with light-headedness.
Which number matters more?
Both count, and a reading is classified by whichever number is higher. Over the age of about 50, the systolic number predicts heart and stroke risk more strongly, and "isolated systolic hypertension" (a high top number with a normal bottom number) is the most common form in older adults. Under 50, a high diastolic number is more often the first sign of a problem. The gap between the two, called pulse pressure, also carries information: a gap above about 60 mmHg in an older adult suggests stiff arteries and is something your doctor will note.
Blood pressure categories chart
| Category | Systolic (mmHg) | Diastolic (mmHg) | |
|---|---|---|---|
| Normal | below 120 | and | below 80 |
| Elevated (ACC/AHA) | 120–129 | and | below 80 |
| Hypertension stage 1 (ACC/AHA) | 130–139 | or | 80–89 |
| Hypertension stage 2 (ACC/AHA) | 140 or higher | or | 90 or higher |
| Hypertension (ESC 2024, NHS clinic) | 140 or higher | or | 90 or higher |
| Hypertension (NHS home average) | 135 or higher | or | 85 or higher |
| Severe hypertension / hypertensive emergency (AHA) | higher than 180 | and/or | higher than 120 |
The US and European systems disagree only about the band between 130/80 and 140/90: Americans call it stage 1 hypertension, Europeans call it "elevated" and treat it according to overall risk. Whichever table your doctor uses, the reading that matters is the average of several days of home readings, not the highest number you have ever seen.
What if only one of my numbers is high?
It still counts. A reading of 145/78 is stage 2 hypertension in the US table and hypertension in the European one because the systolic number crosses the line, even though the diastolic number is fine. Likewise 118/92 is hypertension on the diastolic number alone. Write both numbers every time; a log that only records "the top one" cannot be classified.
When is a reading an emergency?
If a reading is higher than 180/120 mm Hg, the American Heart Association says to wait at least one minute and measure again. If it is still that high and you have no symptoms, the AHA calls it severe hypertension: contact your healthcare professional immediately. If it comes with chest pain, shortness of breath, back pain, numbness, weakness, a change in vision or difficulty speaking, the AHA calls it a hypertensive emergency: call 911, or your local emergency number.
How should I write the numbers down?
Always as systolic/diastolic with the date and time, and note the context: before or after medication, morning or evening, sitting for five minutes or not. Add the pulse if your monitor shows it; it is the third number on most displays and is not part of the blood pressure. A photo of the monitor works as well as a written line if the digits are clear: the Monitor2Log diary reads the display and puts systolic, diastolic and pulse into the right columns, and you confirm each row before it is saved.
Sources
- American College of Cardiology / American Heart Association, hypertension guideline (2017, updated 2025).
- European Society of Cardiology, 2024 guidelines for elevated blood pressure and hypertension.
- NHS, high blood pressure (hypertension): diagnosis; NICE NG136.
- American Heart Association, "Understanding blood pressure readings".
General information only, not medical advice. A single reading never replaces a conversation with your clinician.