Reading blood pressure numbers properly
What systolic and diastolic actually measure, why Indian and American thresholds differ, and how to take a reading that means something.
Last reviewed · 1,251 words
In short
- The top number is the pressure when the heart contracts; the bottom is the pressure between beats. Both matter, and the higher category governs.
- Indian guidelines call hypertension 140/90. American guidelines since 2017 call it 130/80, which reclassified a large share of the population overnight.
- A single reading means very little. Diagnosis requires readings on separate occasions, and home readings predict outcomes better than clinic ones.
- Technique changes the number by 10 to 20 mmHg. A full bladder, crossed legs, an unsupported arm or talking all raise it.
- Blood pressure is usually symptomless, which is why it is measured rather than felt.
Blood pressure is the force of blood against artery walls, written as two numbers in millimetres of mercury.
Systolic, the higher number, is the pressure while the heart contracts. Diastolic, the lower, is the pressure between beats when the heart is filling.
120/80 is read as "120 over 80".
The categories, and why sources disagree
Two major sets of thresholds are in use, and the difference is not small.
Indian and European guidelines:
| Category | Systolic | Diastolic | |
|---|---|---|---|
| Optimal | Below 120 | and | Below 80 |
| Normal | 120–129 | and/or | 80–84 |
| High normal | 130–139 | and/or | 85–89 |
| Grade 1 hypertension | 140–159 | and/or | 90–99 |
| Grade 2 hypertension | 160–179 | and/or | 100–109 |
| Grade 3 hypertension | 180+ | and/or | 110+ |
American guidelines since 2017:
| Category | Systolic | Diastolic | |
|---|---|---|---|
| Normal | Below 120 | and | Below 80 |
| Elevated | 120–129 | and | Below 80 |
| Stage 1 | 130–139 | or | 80–89 |
| Stage 2 | 140+ | or | 90+ |
A reading of 135/85 is high normal under Indian guidelines and stage 1 hypertension under American ones. The 2017 American revision reclassified roughly 30 million additional people as hypertensive without anyone's blood pressure changing.
Neither is wrong. The evidence shows risk rising continuously from about 115/75 upward, with no natural cut-off, so where the line is drawn is a judgement about when to intervene rather than a fact about biology.
Indian clinical practice follows 140/90 for diagnosis in most settings, with earlier intervention for those who have diabetes or existing cardiovascular disease.
Where the two numbers fall in different categories, the higher category applies. 125/95 is hypertension, on the diastolic reading alone.
Taking a reading that means something
Technique moves the number by 10 to 20 mmHg, which is larger than most treatment effects.
Before: no caffeine, exercise or smoking for 30 minutes. Empty your bladder — a full one adds 10 to 15 mmHg. Sit quietly for five minutes.
Position: back supported, feet flat on the floor, legs uncrossed. Crossing the legs adds 2 to 8 mmHg. The arm supported at heart level — an unsupported arm adds up to 10.
Cuff: on bare skin, not over a sleeve. The bladder of the cuff should cover 80% of the arm's circumference. A cuff that is too small reads high, by 10 to 40 mmHg, and this is the most common equipment error in people with larger arms.
During: do not talk. Talking adds 10 to 15 mmHg.
Take two or three readings a minute apart and average the second and third. The first is almost always the highest.
Both arms, the first time. A consistent difference above 10 to 15 mmHg between arms is worth reporting; afterwards, use the higher arm.
White coat and masked hypertension
White coat hypertension is a raised reading in a clinic and a normal one at home, affecting 15% to 30% of people. It is not entirely benign — it carries some increased long-term risk — but it does not usually need treatment.
Masked hypertension is the reverse: normal in the clinic, raised at home. It is less recognised and more dangerous, because it goes untreated.
Both are reasons why home readings predict cardiovascular outcomes better than clinic readings, and why guidelines increasingly recommend home or 24-hour ambulatory monitoring before diagnosing hypertension.
The standard home protocol is twice a day, morning and evening, for seven days, discarding day one and averaging the rest. A home average above 135/85 corresponds roughly to a clinic reading of 140/90.
What moves it
Salt. Indian diets average 8 to 11 g a day against a WHO recommendation of 5. Reducing intake lowers systolic pressure by 4 to 6 mmHg in those who are salt-sensitive, which includes a large share of South Asians. Most of the salt comes from pickles, papad, packaged snacks and restaurant food rather than from the salt shaker.
Weight. Roughly 1 mmHg per kilogram lost, up to a point.
Exercise. 150 minutes a week of moderate activity lowers systolic pressure by 5 to 8 mmHg. Isometric exercise — wall sits, handgrip holds — has shown notably strong effects in recent trials.
Alcohol. Each daily drink above one adds roughly 1 mmHg.
Potassium. Fruit, vegetables and pulses raise potassium, which lowers blood pressure. The DASH dietary pattern combines this with reduced sodium and lowers systolic pressure by 8 to 14 mmHg — comparable to a single medication.
Sleep and stress. Both matter; untreated sleep apnoea is a common and reversible cause of resistant hypertension.
When to act immediately
A reading of 180/120 or above with symptoms — chest pain, breathlessness, severe headache, visual change, weakness or difficulty speaking — is a medical emergency. Go to hospital.
180/120 or above without symptoms is a hypertensive urgency. Rest five minutes, repeat the reading, and contact a doctor the same day.
Below 90/60 with symptoms — dizziness, fainting, confusion, cold clammy skin — also needs attention. Low readings without symptoms are usually fine and often good.
Hypertension is otherwise symptomless. Headaches, nosebleeds and flushing are not reliable signs, and waiting to feel something is why it is called the silent killer. India has an estimated 220 million people with hypertension, of whom fewer than half know.
Choosing and using a monitor
Upper arm monitors are the ones to buy. Wrist and finger monitors are more position-sensitive and less accurate, and their readings vary with how the wrist is held.
Look for clinical validation. Organisations including the British and Irish Hypertension Society and STRIDE BP publish lists of validated devices, and a monitor that is not on one is unverified whatever the packaging claims.
Cuff size is the most common error. Measure the circumference of your upper arm at the midpoint and buy the cuff that fits it. A standard cuff on a large arm overestimates by 10 to 40 mmHg, which is enough to produce a diagnosis that is not real.
Check it against a clinic device once a year, and replace the cuff when the material stiffens.
Automatic monitors can struggle with atrial fibrillation and other irregular rhythms, giving erratic readings. Some flag it; where it is suspected, a manual measurement is needed.
Reading a series rather than a number
Blood pressure varies continuously — through the day, with activity, with stress, with sleep. Readings taken an hour apart can differ by 20 mmHg in a healthy person.
It follows a daily rhythm: lowest during sleep, rising sharply on waking, peaking in the late morning and again in the early evening. The morning surge is why cardiovascular events cluster in the first hours after waking.
Recording a series makes this legible in a way a single reading never can. Log the date, time, both numbers and the pulse, and note anything unusual — poor sleep, illness, a stressful day, a missed dose.
A pattern to look for: readings that are consistently high in the morning and normal in the evening suggest the current medication is not covering the full 24 hours, which is a common and easily corrected problem. That finding only exists if the readings are timed and recorded.
Bring the log to appointments. A week of home readings is more useful to a clinician than any single measurement taken in their room.
What this calculator assumes
- Standard adult categories. Children, pregnancy and some chronic conditions use different thresholds entirely.
- Readings taken with correct technique. A poorly taken reading is not made meaningful by categorising it.
- The higher of the two numbers determines the category.
- Trends across many readings are the information; any single reading is not.
- This is not a diagnosis. Hypertension is diagnosed by a clinician from readings on separate occasions, usually with home or ambulatory monitoring.