How to Measure Your Blood Pressure at Home, Properly

A single reading taken in a clinic, after you have rushed to get there and sat in a waiting room, is among the least reliable numbers in medicine. Readings taken at home predict strokes and heart attacks better than clinic readings do — which is why guidelines increasingly rely on them.

That only holds if they are taken correctly. Most are not, and the errors are large enough to change whether someone is diagnosed with high blood pressure at all.

First: buy the right machine

An upper-arm monitor. Not a wrist monitor, and not a smartwatch.

Wrist devices are extremely sensitive to the position of the wrist relative to the heart, and give inconsistent results. Watches and rings that claim to measure blood pressure are not accurate enough for clinical decisions, whatever the marketing says.

Check that the model is clinically validated. The British and Irish Hypertension Society publishes a list of validated monitors on its website — checking a model against it before buying takes two minutes and rules out a good deal of what is sold online. A validated monitor costs relatively little; an unvalidated one is worse than none, because it produces numbers people act on.

Cuff size is the commonest error of all

A cuff that is too small gives falsely high readings — sometimes by a substantial margin. Enough to have someone started on medication they did not need.

Measure around the middle of your upper arm with a tape measure, and check that figure against the range printed on the cuff. Many monitors are sold with a standard cuff that does not fit larger arms, and a large cuff usually has to be bought separately. If your arm circumference is near the top of the printed range, size up.

How to take a reading

Before

• No caffeine, exercise or smoking for 30 minutes
• Empty your bladder — a full bladder genuinely raises the reading
• Sit quietly for five minutes first. Not two. This is the step most often skipped and it matters

Position

• Sit in a chair with your back supported
• Both feet flat on the floor, legs uncrossed. Crossing your legs raises the reading
• Arm supported on a table, cuff at the level of your heart. An unsupported arm held up raises it; an arm hanging down lowers it
• Cuff on a bare arm, not over a sleeve, and not with a sleeve rolled tightly above it

During

• Do not talk, and do not look at your phone. Talking during a measurement adds several points
• Stay still and breathe normally

How many

Take two or three readings, one minute apart. Discard the first, and average the rest. The first reading is almost always the highest, and treating it as the answer is a reliable way to worry yourself unnecessarily.

The seven-day protocol

This is what is actually used to make decisions, and it is worth doing properly if a diagnosis is being considered.

1. Twice a day — morning and evening, at roughly the same times. Morning readings before medication and before breakfast
2. Two readings each time, a minute apart
3. Continue for seven days
4. Discard the whole of the first day — it is consistently unrepresentative
5. Average everything from days two to seven

Write them down, or use the monitor's memory. Bring the whole set, not a summary — the spread is informative in itself.

What the numbers mean at home

Home thresholds are lower than clinic thresholds, because the clinic setting itself raises readings. This surprises people who compare the two.

• Home average below 135/85 — generally considered normal
• Home average 135/85 or above — the threshold at which high blood pressure is generally diagnosed and treatment considered

The equivalent clinic figure is 140/90. So a home reading of 138/88 is not "fine because it is under 140" — it is above the home threshold.

White coat and masked hypertension

White coat effect — high in clinic, normal at home — is common and is exactly why home monitoring exists.

Masked hypertension is the reverse: normal in clinic, high at home. It is less well known, carries real risk, and is only ever found by measuring at home.

Two things worth doing once

• Measure both arms the first time. A consistent difference between arms is worth knowing about, and from then on you use the arm with the higher reading
• Take a reading standing up after sitting, particularly if you are older or on medication and ever feel dizzy on standing. A significant drop is worth reporting

What not to do

• Do not check repeatedly through the day. Blood pressure varies constantly and normally; frequent checking produces anxiety, which raises the reading, which produces more checking. It is a genuinely common trap
• Do not react to a single high reading. One number means very little
• Do not stop or adjust medication based on home readings without discussing it — a good result usually means the treatment is working, not that it is no longer needed

When a reading is an emergency

Seek urgent medical help for a reading of 180/120 or above together with any of: chest pain, breathlessness, a severe headache, visual disturbance, weakness or numbness on one side, difficulty speaking, or confusion.

A very high reading with no symptoms at all still needs assessment promptly, but is not usually an emergency — sit quietly and repeat it before acting.

What else belongs in the picture

Blood pressure is one component of cardiovascular risk rather than the whole of it. Alongside it, it is worth knowing your cholesterol, HbA1c and kidney function, and having a urine test for protein.

And if your blood pressure is high and difficult to control, obstructive sleep apnoea is a common and treatable underlying cause that is regularly missed — particularly if you snore and wake unrefreshed.