Blood pressure is among the most measured things in medicine and among the least understood by the people being measured.

What the two numbers are

The upper figure, systolic, is the pressure in the arteries when the heart contracts. The lower, diastolic, is the pressure between beats when the heart relaxes and fills.

Both matter. Systolic pressure becomes the stronger predictor of cardiovascular risk with increasing age, as arteries stiffen. In younger adults diastolic carries relatively more weight.

The units are millimetres of mercury, a legacy of the original measurement device.

The thresholds

These have changed over time and differ between guideline bodies, which causes reasonable confusion.

Broadly, readings around 120 over 80 or below are considered optimal. Elevated ranges above that lead into categories variously described as high-normal, elevated, or stage one hypertension depending on which guideline you're reading.

Some guidelines lowered their thresholds for defining hypertension in recent years, which reclassified a large number of people overnight. That decision was based on trial evidence suggesting benefit from treating at lower levels, and it was also contested, partly on grounds of overdiagnosis and treatment burden.

What matters practically is that risk is continuous. There isn't a level below which pressure is harmless and above which it's dangerous. Thresholds are decision points for intervention, chosen on the balance of benefit and harm, not natural boundaries.

Why single readings mislead

The most important practical point. Blood pressure varies substantially — across the day, with activity, with stress, with position, with recent caffeine, with a full bladder.

A single reading in a clinic is a poor estimate of a person's usual pressure. It's a snapshot of a variable quantity taken in an environment that tends to raise it.

White coat hypertension — elevated readings in clinical settings but normal at home — affects a meaningful proportion of people. Masked hypertension, the reverse, also exists and is arguably more concerning since it goes undetected.

This is why guidelines increasingly recommend confirming a diagnosis with either ambulatory monitoring over 24 hours or a series of home readings, rather than acting on clinic measurements alone.

Measuring at home properly

If you're taking readings yourself, the technique matters considerably.

Use a validated upper-arm device. Wrist monitors are less reliable, and validation lists are published by hypertension societies.

Sit quietly for five minutes first, feet flat on the floor, back supported, arm supported at heart level. Don't talk during the measurement.

Avoid caffeine, exercise and smoking for at least half an hour beforehand. Empty your bladder first — a full bladder raises readings measurably.

Take two or three readings a minute apart and record all of them. Take them at consistent times, typically morning and evening.

And record everything rather than selecting good ones. The pattern across a week is what's informative, not any individual number.

Cuff size, which nobody checks

An underappreciated source of error. A cuff too small for the arm produces falsely high readings; too large produces falsely low ones.

The error can be substantial — large enough to change a diagnosis. Standard cuffs supplied with home monitors don't fit everybody, and larger cuffs are available separately.

Measuring your upper arm circumference and checking it against the cuff's stated range takes a minute and is worth doing once.

What actually lowers it

The interventions with reasonable evidence, roughly by effect size.

Weight loss, where there's excess weight. One of the larger available effects.

Dietary sodium reduction. Effects vary between individuals — some people are considerably more salt-sensitive than others — and average effects are meaningful. Most sodium in modern diets comes from processed food rather than the salt cellar.

Increased potassium, primarily through fruit and vegetables, with a caution for anyone with kidney disease or on certain medications.

Regular aerobic exercise. Consistent modest effects.

Reduced alcohol, where consumption is high.

Dietary patterns emphasising vegetables, fruit, whole grains and low-fat dairy have been specifically trialled and show effects comparable to some medications.

Where medication is indicated, it's highly effective, and the evidence for treating hypertension in reducing stroke and cardiovascular events is among the strongest in medicine.

Why it matters

Hypertension is largely asymptomatic, which is precisely the problem. People feel entirely well while sustained elevated pressure damages arteries, the heart, the kidneys and the brain over years.

The absence of symptoms is also why adherence to treatment is poor — taking a daily medication for a condition you can't feel, with side effects you can, is a difficult proposition.

Which is worth knowing if you're finding it hard to keep taking something. That reaction is entirely normal and it's a conversation worth having with your doctor rather than simply stopping, since alternatives with different side effect profiles usually exist.

This article provides general information. Blood pressure management should be discussed with a qualified healthcare professional who knows your medical history.