Check if your blood pressure is normal. AHA classification.
Guide to Blood Pressure
What is blood pressure?
Blood pressure is the force with which blood pushes against the walls of arteries during the cardiac cycle. It is measured in millimeters of mercury (mmHg) and expressed as two values: systolic (upper) - when the heart contracts and pumps blood, and diastolic (lower) - when the heart rests between beats. Normal blood pressure is around 120/80 mmHg, though these values may vary depending on age and health status.
Blood pressure categories
- Normal - below 120/80 mmHg, optimal range for health
- Elevated - 120-129 / below 80 mmHg, requires monitoring
- Hypertension stage 1 - 130-139 / 80-89 mmHg, requires lifestyle changes
- Hypertension stage 2 - 140/90 mmHg or higher, requires treatment
- Hypertensive crisis - above 180/120 mmHg, medical emergency
How to measure blood pressure correctly?
Take measurements at rest, after a few minutes of relaxation. Avoid caffeine, cigarettes, and physical exercise before measuring. Sit with your back supported, arm at heart level. Take at least two measurements one minute apart and record the average. Taking readings at different times of day provides a better picture - in the morning and evening.
Factors affecting blood pressure
Blood pressure is influenced by genetics, high-sodium diet, lack of physical activity, obesity, stress, smoking, and excessive alcohol consumption. Controlling blood pressure requires lifestyle changes, and in some cases, medication. Regular monitoring is crucial, especially with a family history of hypertension.
A single reading calls a normal 135 systolic 'high' more than a quarter of the time
Blood pressure moves between readings, so one measurement is a sample rather than a value. Taking a within-person spread of 8 mmHg between readings as an illustration, someone whose true systolic is 135 will read above 140 on about 26.6% of single measurements. Average three and that falls to 14.0% — which is why guidelines ask for repeated readings rather than one.
How it works
- Records systolic and diastolic readings and averages them the way guidelines intend.
- Shows how much the uncertainty narrows as you add readings.
- Places the average against published categories rather than reading a single measurement as a verdict.
standard error = within-person SD ÷ √(number of readings) 95% band ≈ ±1.96 × standard error with an illustrative SD of 8 mmHg: 1 reading → ±15.7 mmHg 3 readings → ±9.1 mmHg 5 readings → ±7.0 mmHg
Worked example
The measurement noise around a true systolic of 135 mmHg, at different numbers of readings.
- 1 reading → 95% band ±15.7 mmHg
- 2 readings → ±11.1 mmHg
- 3 readings → ±9.1 mmHg
- 5 readings → ±7.0 mmHg
- 7 readings → ±5.9 mmHg
Averaging narrows the band by the square root of the count, so three readings cut it 1.73 times and five cut it 2.24 times. The single reading that crosses a threshold is frequently noise, and the fix is repetition rather than a better machine.
Reading the result
- The 8 mmHg spread above is an illustration, not your number. Within-person variability differs by individual, time of day, recent activity, caffeine and simply being in a clinic — the shape of the arithmetic holds regardless, but the width of your own band does not come from this page.
- The threshold itself is not settled. European guidance has generally treated 140/90 as the hypertension boundary while the 2017 American guideline moved it to 130/80, so the same average is above the line under one and below it under the other. Which categories your result is compared against is a clinical question.
- Readings taken at home over several days generally represent you better than one taken in a clinic, where the setting itself raises many people's pressure. That effect is well enough recognised to have a name, and it is a reason a single high reading is a prompt to measure again rather than a conclusion.
- This page does arithmetic on numbers you supply and nothing else. It cannot diagnose, and a persistently raised average — or any single very high reading — is a matter for a clinician rather than a calculator.
Common questions
- How many readings should I average?
- More than one, and guidelines typically ask for several across separate occasions. The gain shrinks as you add more: going from one reading to three narrows the band by 1.73 times, but from three to five only narrows it a further 1.29 times.
- My reading was high once — is that hypertension?
- One reading above a threshold is not a diagnosis. With a true value of 135 and ordinary variability, single readings land above 140 more than a quarter of the time by chance alone. Repeat the measurement over several days and take the result to a clinician rather than acting on one number.