📚 What's in This Guide
A single blood pressure reading can be misleading — nerves, caffeine, a full bladder, or simply the anxiety of being measured (so-called "white coat" effect) can all push a one-off number higher than your true baseline. That's why NICE guideline NG136 builds its entire diagnostic approach in the UK around averages taken over several days, not single readings.
This calculator follows that protocol properly: enter two consecutive readings, one minute apart, morning and evening, for up to 7 days — the same structure NICE specifies — and it automatically excludes day 1, calculates your true average, works out your Mean Arterial Pressure, checks how consistent your readings are, flags any invalid or incomplete data, and classifies the result against the actual NICE NG136 stages — separately for home/ABPM and clinic thresholds, since they aren't the same number.
1. The NICE Home Monitoring Protocol
For home blood pressure monitoring (HBPM), NICE recommends:
- Take two consecutive readings, at least 1 minute apart, each time you measure — not one.
- Measure morning and evening, ideally at the same times each day.
- Continue for at least 4 days, ideally for 7 days.
- Discard the readings from day 1 and average all the remaining readings.
Followed exactly, that's 4 readings a day (2 morning + 2 evening) for 7 days — 28 readings in total, 24 of which count once day 1 is excluded. This calculator's input table matches that structure directly: two systolic/diastolic pairs for morning, two for evening, each day. Day 1 is excluded because early readings tend to run higher — you're still getting used to the cuff, the routine, and the act of measuring itself. Averaging it in would skew your result upward. The "Exclude Day 1" toggle applies that rule automatically to whatever you've entered, visually greying out day 1's row so you can see exactly which readings are being counted, and the calculator also tells you how many complete monitoring days (all 4 readings entered) it actually has before treating a result as protocol-complete.
2. Why Averaging Matters More Than Any Single Reading
Blood pressure naturally varies through the day and between days — with activity, stress, temperature, posture, and even the time since your last coffee. A single high reading is common and does not, on its own, mean you have hypertension. NICE's diagnostic approach exists specifically to filter out that noise: a clinic reading of 140/90 mmHg or higher triggers further monitoring (ABPM or HBPM), not an immediate diagnosis — the diagnosis is confirmed (or ruled out) by the averaged result.
3. The NICE NG136 Hypertension Stages
Most calculators reduce this to a single "normal vs high" line. NICE actually defines four bands, and the numbers differ depending on whether you're looking at a clinic reading or a home/ABPM average:
| Stage | Clinic Reading | Home / ABPM Average |
|---|---|---|
| Not hypertensive | Below 140/90 mmHg | Below 135/85 mmHg |
| Stage 1 hypertension | 140/90 – 159/99 mmHg | 135/85 – 149/94 mmHg |
| Stage 2 hypertension | 160/100 – 179/119 mmHg | 150/95 mmHg or higher |
| Severe hypertension | 180 systolic or 120 diastolic or higher | Same-day clinical assessment — not typically an HBPM finding alone |
Severe hypertension (clinic ≥180/120 mmHg) needs same-day medical assessment, especially with symptoms like new confusion, chest pain, or visual disturbance, or signs of organ damage — this calculator flags that range explicitly rather than just showing a number.
4. Why Home and Clinic Thresholds Are Different
NICE is explicit that home and ambulatory (ABPM) readings run about 5 mmHg lower than clinic readings on average, largely because of the white-coat effect of being measured by a clinician. That's why the home/ABPM diagnostic threshold (135/85) is lower than the clinic threshold (140/90) — they're designed to represent the same underlying blood pressure level, measured two different ways. Using the clinic threshold on a home average (or vice versa) will systematically misclassify people, which is why this calculator asks which context your readings came from before classifying them.
5. Mean Arterial Pressure & Reading Consistency
Mean Arterial Pressure approximates the average pressure in your arteries across a full heart cycle, weighted toward diastole (since the heart spends more time relaxed than contracted). A MAP of roughly 70-100 mmHg is generally considered adequate to perfuse major organs; this is supplementary context, not a diagnostic figure in its own right.
This calculator also reports the standard deviation of your systolic and diastolic readings — a measure of how consistent your numbers are. A tight spread suggests a stable baseline; a wide spread can indicate white-coat effect, measurement technique issues, or genuine variability worth mentioning to your GP.
6. Low Blood Pressure
There's no single official UK diagnostic threshold for low blood pressure the way there is for hypertension — unlike high blood pressure, low blood pressure is generally only considered clinically significant when it comes with symptoms such as dizziness, light-headedness, blurred vision, or fainting. A commonly used reference point is a reading below roughly 90/60 mmHg. This calculator flags readings in that range so you know to mention it to a healthcare professional if you're also experiencing symptoms.
7. What to Look for in a BP Calculator
| Feature | Why it matters | This calculator |
|---|---|---|
| Actually implements the discard-day-1 rule | Most calculators just mention it in text without applying it to your data | Included |
| Separate home/ABPM vs clinic thresholds | Using the wrong threshold systematically misclassifies results | Included |
| Full NG136 staging (not just normal/high) | Stage 1 vs Stage 2 vs Severe carries different clinical urgency | Included |
| Mean Arterial Pressure & reading consistency (SD) | Extra context beyond a single average number | Included |
| Explicit low blood pressure flag | Most calculators only look for high readings | Included |
| Runs entirely in your browser, nothing stored | Health data stays private | Included |
8. Methodology & Limitations
- The averaging protocol, staging thresholds, and treatment targets follow NICE guideline NG136, "Hypertension in adults: diagnosis and management" (2019, amended 2022).
- This calculator does not diagnose hypertension or hypotension. A diagnosis requires clinical assessment, confirmation of technique and equipment, and consideration of your individual history — not just a number against a table.
- Standard deviation is calculated as a sample standard deviation across whichever readings are currently included (i.e., after any day-1 exclusion).
- Mean Arterial Pressure uses the standard simplified formula (DBP + ⅓ pulse pressure), which is an approximation most accurate at normal resting heart rates.
- The age-band toggle changes only the displayed treatment target, not the diagnostic stage — NICE's staging thresholds themselves are not age-adjusted for standard adults; the 80+ treatment target is a management decision based on falls risk, not a different diagnostic cutoff.
- This tool is for informational purposes only and does not replace measurement with a validated monitor, correct technique, or assessment by a GP or other qualified healthcare professional.
9. Frequently Asked Questions
First-day readings tend to run higher because people are less used to the monitor and the measurement routine, an effect sometimes described alongside white-coat hypertension. NICE's home blood pressure monitoring (HBPM) protocol asks you to take two readings, one minute apart, morning and evening, for at least 4 days (ideally 7), then average all the readings excluding the first day.
Under NICE NG136, a clinic reading below 140/90 mmHg, or a home/ABPM average below 135/85 mmHg, is below the diagnostic threshold for hypertension. Home and ambulatory readings are naturally about 5 mmHg lower than clinic readings, which is why the two thresholds differ.
Stage 1 hypertension is a clinic reading of 140/90 to 159/99 mmHg (home/ABPM average 135/85 to 149/94). Stage 2 hypertension is a clinic reading of 160/100 to 179/119 mmHg (home/ABPM average 150/95 or higher). Severe hypertension is a clinic systolic of 180 mmHg or higher, or diastolic of 120 mmHg or higher, which needs same-day medical assessment.
There is no single official diagnostic threshold for low blood pressure the way there is for high blood pressure, but a reading below roughly 90/60 mmHg is commonly used as a reference point. Low blood pressure is generally only a concern if it comes with symptoms such as dizziness, light-headedness, or fainting.
No. This tool averages the numbers you enter and shows where that average sits against published NICE NG136 reference ranges. It cannot diagnose hypertension, does not know your medical history, and is not a substitute for assessment by a GP or other healthcare professional.
This calculator is for informational purposes only and is not medical advice or a diagnosis. If you have symptoms, contact NHS 111 or, in an emergency, call 999. See Section 8 for methodology and sources.