In one sentence
The pressure in your arteries at the two ends of each heartbeat, systolic as the heart contracts and diastolic as it relaxes, measured at the upper arm with a cuff and written as the pair, in millimeters of mercury.
Why it matters
It is among the most studied numbers in medicine, and the relationship runs all the way down. In one million adults with no vascular disease at the start, from 61 prospective studies and 12.7 million person-years, each 20 mm Hg of usual systolic pressure (or 10 mm Hg diastolic) at ages 40 to 69 was associated with more than a twofold difference in the rate of death from stroke and a twofold difference from ischemic heart disease, with no evidence of a threshold down to at least 115/75 mm Hg; the proportional differences were about half as steep at ages 80 to 89, but the absolute differences were larger (Lewington 2002). The same analysis found that, for a single measurement, the average of the two pressures was slightly more informative than either alone. It is also one of the few core measures that answers to what you do within weeks: in the Look AHEAD trial of 5,145 adults with type 2 diabetes and excess weight, four years of intensive lifestyle intervention lowered systolic pressure by 5.3 mm Hg against 3.0 in the comparison group, alongside 6.2 percent weight loss against 0.9 (Wing 2010).
What moves it
Minute to minute: posture, talking, a full bladder, cold, pain, caffeine and nicotine in the previous half hour, and the act of being measured, which is why the first reading of a session runs high and is set aside. Across the day: it is lowest asleep, rises steeply on waking, and runs higher in the evening in many people. Day to day: a short night, alcohol the evening before, a salty meal, and a hard session, after which it runs lower for some hours. Over weeks and months: body weight, salt, alcohol, aerobic exercise, sleep, and the season, since it runs higher in winter. Medicines, in both directions, which is why the log matters.
How to measure it well
A validated upper-arm cuff of the right size; wrist cuffs and the pressure estimates some watches and rings offer are different measurements, and VT cannot tell them from a cuff's in Apple Health or Health Connect, so connect only an upper-arm cuff and keep the series to it. Sit for five minutes first, back supported, feet flat, arm resting at heart level, bladder empty, no talking; three readings a minute apart, the first set aside; morning before food and medicines, and evening; the same arm every time. For a baseline, a seven-day block with the first day discarded; after that, once a week at the same time, and a block again whenever something changes. The full routine is the how-to, Blood pressure: how and how often.
How Vitality Tracker uses it
Every reading is kept with its systolic, diastolic and time, the pulse when you type it, and the app it came from when it is synced; when you type three readings in a sitting, VT sets the first aside. The first reading of a session can be set aside, as the measurement guidelines advise; the day's figure is the average of the readings kept. That figure feeds the arteries area of the Vitality Score at any age, as 60 percent of the area, counting in full for seven days and still used for fourteen. From the age of 50 the same readings become the three pressure indices (next entry). Beside each reading VT shows its category, from Optimal to Very high, colored amber from 130/85 and red from 140/90 mm Hg; these describe the reading, not a diagnosis. The European home-measurement guideline's figure for a raised home average is 135/85 mm Hg.
How far to trust the number
Measured, and well: a cuff validated to the international protocol agrees with reference measurement to within a few mm Hg on average. The larger variation is yours. Two readings a minute apart can differ by 5 to 10 mm Hg, and the same person at different hours by more, which is why one reading is never the number, the day's average is, and the week's average is the one to read. A cuff that is too small reads high; an arm hanging below heart level reads high; crossed legs read high. Keep to one cuff, one arm, one routine, and the series is comparable with itself.
Sources
- Lewington S, et al. Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies. Lancet 2002;360:1903–13. doi:10.1016/S0140-6736(02)11911-811911-8>)
- Look AHEAD Research Group, Wing RR. Long-term effects of a lifestyle intervention on weight and cardiovascular risk factors in individuals with type 2 diabetes mellitus: four-year results of the Look AHEAD trial. Arch Intern Med 2010;170:1566–75. doi:10.1001/archinternmed.2010.334
- Stergiou GS, et al. 2021 European Society of Hypertension practice guidelines for office and out-of-office blood pressure measurement. J Hypertens 2021;39:1293–302. doi:10.1097/HJH.0000000000002843 (record only; the home threshold is the guideline's figure)
- Vitality Tracker, How it works (the blood-pressure measures); the engine catalogue of 15 October 2026 (what each reading stores).