How to Read Your Blood Pressure Numbers Correctly

Blood pressure is the force your blood puts on the walls of your arteries as it moves around your body. It is measured in millimetres of mercury, written as mmHg, which is a leftover from the old mercury gauges. You always get two numbers because the pressure rises and falls with each heartbeat.

• Systolic (the top number) is the higher figure. It is the pressure at the moment your heart contracts and pushes blood out. This is the number doctors watch most closely, especially as you get older.
• Diastolic (the bottom number) is the lower figure. It is the pressure while your heart relaxes and refills between beats. It never drops to zero, because your arteries stay under some pressure at all times.

Different countries draw the lines in slightly different places, but the categories below follow the widely cited American College of Cardiology and American Heart Association guidance. Treat them as a map, not a diagnosis, because your doctor interprets your numbers alongside your age, history and other risks.

Notice the word "or" in the high categories. You only need one number over the line to fall into that category. Someone with a reading of 135 over 78 has stage 1 high blood pressure on the systolic number alone, even though their diastolic is fine.

This is the part most people get wrong. Blood pressure is not a fixed value like your shoe size. It moves constantly, all day, in response to stress, movement, caffeine, sleep, temperature and even the act of having your pressure taken. A single number is a snapshot of one moment, and that moment might not be typical of you.

One famous version of this is white coat effect, where the stress of being in a clinic pushes your reading up, so it reads higher there than it does at home. The opposite, masked high blood pressure, also exists, where clinic readings look fine but home readings are high. This is exactly why doctors diagnose high blood pressure from a pattern of readings across time and settings, never from one number in one room.

If you measure at home, small things make a real difference. Sloppy technique can add or subtract 10 mmHg or more, which is enough to move you between categories. Here is how to take a reading that is actually worth recording.

1. Settle first. Avoid caffeine, exercise and smoking for 30 minutes before. Sit quietly for 5 minutes before you measure. Empty your bladder, because a full one nudges the reading up.
2. Sit properly. Back supported, feet flat on the floor, legs uncrossed. Rest your arm on a table so the cuff sits at roughly the level of your heart.
3. Use the right cuff, in the right place. Put the cuff on bare skin, not over a sleeve. A cuff that is too small reads falsely high. Most upper-arm monitors are more reliable than wrist ones.
4. Stay quiet and still. Do not talk, scroll your phone, or move during the reading. Talking alone can raise the number by several points.
5. Take two or three readings. Wait a minute between each, then record the average rather than the single lowest or highest. Measure at the same times of day to compare like with like.

If your numbers sit in the elevated or high range, the good news is that blood pressure responds well to changes you control. None of this replaces the plan your doctor makes with you, but these are the levers with the strongest evidence behind them.

Regular activity is one of the most reliable. Both endurance exercise, like brisk walking or cycling, and resistance training lower blood pressure, with typical reductions of roughly 5 to 7 mmHg after training in people with high readings. Cutting back on salt, losing excess weight, drinking less alcohol, sleeping better and managing stress all pull in the same direction, and they stack.