Methodology

Pulse pressure

pulse pressure = systolic − diastolic

Exact, and always a whole number. For a reading of 142/85 mm Hg the pulse pressure is 57 mm Hg. There is no estimation involved.

Estimated mean arterial pressure

estimated MAP = (systolic + 2 × diastolic) ÷ 3

This is an estimate, not a measurement. The formula assumes a normal heart rate and a normal-ish arterial waveform. Other estimators exist and give slightly different numbers, and a true mean arterial pressure is something an arterial line measures directly rather than something a formula produces from two cuff numbers.

The equivalent form diastolic + (systolic − diastolic) ÷ 3 is algebraically identical. This site uses the first form and rounds to one decimal place before display, so the number shown is the number tested.

The order the categories are evaluated in

This is the part most calculators get wrong. In the 2025 AHA/ACC Guideline for the Prevention, Detection, Evaluation and Management of High Blood Pressure in Adults, the two lower bands are defined with and — both numbers must be below the threshold — while the two stages are defined with or, so either number alone is enough. The bands therefore overlap, and the order of evaluation decides the answer.

if systolic ≥ 140 or diastolic ≥ 90    → stage 2
else if systolic ≥ 130 or diastolic ≥ 80  → stage 1
else if systolic ≥ 120                     → elevated
else                                       → normal

A reading of 119/85 mm Hg sits in the stage 1 row, because its diastolic number reaches 80 even though its systolic number does not reach 120. Evaluating from the top down is what makes that come out right.

The guideline’s classification has exactly four levels, and this site does not add a fifth. Readings above 180/120 mm Hg are handled by a separate fixed line that is the same for every reader; it is not a category and it is not personalised.

What the calculator refuses

A reading is accepted only if both numbers are whole numbers, systolic is between 60 and 300 mm Hg, diastolic is between 30 and 200 mm Hg, and systolic is higher than diastolic. Anything else returns a message asking you to check the numbers — never a category and never a statement about health. A transposed pair is a typo, not a finding.

Where the numbers you enter come from

This site does not measure anything. It has no transducer, no sensor and no connection to any device; it accepts two numbers you read off a monitor you already own. That is a meaningful distinction rather than a technicality: 21 CFR 870.1130 covers noninvasive blood pressure measurement systems, which derive pressures “through the use of transducers placed on the surface of the body.” bpmeter has no transducer and derives nothing from a signal.

On the device you do use, the 2025 AHA/ACC Guideline for the Prevention, Detection, Evaluation and Management of High Blood Pressure in Adults says:

“Reliance on cuffless devices, including smartwatches, for accurate blood pressure measurements should be avoided until these devices demonstrate greater precision and reliability.”

That is also why this site will never pair with a watch, a Bluetooth cuff or a health app, and why the browser is instructed to refuse Bluetooth access outright. The full reasoning is on its own page, and the privacy page explains the mechanism.

How a reading is taken changes what it says. The measurement guide collects the American Heart Association’s published instructions, quoted and dated.

Changing any of this

The category thresholds live in a single committed data file with a source URL and a checked date on every row. A row without that provenance does not render. Changing a formula bumps the calculation version and updates this page in the same change; changing a threshold bumps the table version. Neither happens because a number looked wrong — only because the guideline changed. Every source, with dates.