Understanding Blood Pressure

Understanding blood pressure

What the two numbers mean, how to read your own, and how to measure it properly. Explained in plain English, at your own pace.

1 · The basics

What is blood pressure?

Your heart is a pump. Each beat pushes blood out into your arteries (the stretchy tubes that carry blood away from the heart). Blood pressure is how hard that blood presses against the artery walls. It isn't one steady number: it rises with every beat and eases off in between. That's why a reading has two numbers.

120/80mmHg

120 is the systolic pressure (say “sis-TOL-ik”): the peak, while your heart squeezes.

80 is the diastolic pressure (say “dy-a-STOL-ik”): the low point, while your heart rests and refills between beats.

You say it “120 over 80.” The top number always comes first.

What does “mmHg” mean?

mmHg means “millimetres of mercury.” The first blood pressure gauges measured how high the pressure could push a column of liquid mercury up a glass tube. A pressure of 120 mmHg could push mercury 120 millimetres (12 cm) high. Modern monitors don't use mercury, but the unit stuck.

2 · Try it

Read your number

Move the sliders, or type a reading, to see which category it falls into. The upper categories are called high blood pressure, or hypertension (the medical word for blood pressure that stays higher than it should). When the two numbers land in different categories, the higher category is the one that counts.

Normal
118/76 mmHg

One reading isn't a diagnosis. Blood pressure changes from minute to minute. Doctors look at the average of several readings, taken properly, on different days.

Blood pressure category chart Systolic on the vertical axis from 80 to 200, diastolic on the horizontal axis from 50 to 130, divided into the five American Heart Association categories. A dot marks your reading.

    Source: American Heart Association blood pressure categories for adults. These come from the 2017 ACC/AHA guideline and were kept unchanged in the 2025 AHA/ACC guideline. “Higher category counts” is also the guideline's rule.

    3 · The pipes

    Why the pipes matter

    Blood pressure isn't only about the heart. It also depends on the arteries it pumps into. Healthy arteries are soft and springy. They stretch a little with each beat and soak up some of the push. If they become stiff (less stretchy, which happens gradually with age) or narrowed (for example by plaque, a fatty build-up on the inner wall), the same heartbeat creates more pressure.

    Peak pressure: lowerSame heartbeat each time. Only the artery changes.

    An illustration of the idea, not a medical model. The gauge shows “lower” to “higher”, not real numbers.

    Think of a garden hose

    Put your thumb over the end and the water has a smaller opening to get through. Pressure builds up behind your thumb. A narrowed artery works the same way.

    A soft, new hose swells a little with each surge from the tap and smooths it out. An old, stiff hose can't give, so each surge hits harder. A stiff artery mostly raises the top number.

    This is one reason everyday habits matter. Many of the factors in the next section affect the arteries as well as the heart.

    4 · Everyday life

    What moves the numbers

    Lots of ordinary things nudge blood pressure up or down, some for a few minutes and some over months. Tap a card to see which way it tends to push.

    5 · At home

    How to measure it properly

    Small things like crossed legs, a full bladder or a chat can shift a reading. Use this checklist each time. Tap an item to see where it applies on the picture.

    A person sitting correctly to measure blood pressure: back against the chair, feet flat, arm resting on a table at heart level, cuff on the upper arm.

      In the 30 minutes before: no caffeine, exercise or smoking, and empty your bladder first.

      Why one reading isn't the whole story

      Your blood pressure moves all day. It goes up when you climb stairs, laugh, feel rushed or have a coffee, and drops when you relax or sleep. A single number is a snapshot of one moment.

      Some people also read higher at the clinic simply because they feel a bit nervous there. This is called the white coat effect.

      That's why doctors look at several readings, averaged over days, often including readings taken at home.

      Illustration only, not real data. Each dot is one possible reading. Any one of them alone could mislead.

      6 · Words

      Glossary

      Every medical word used on this page, in plain English.